Notes for Quarter I - Quia



Notes for Quarter 4

Information Posted on 6/07/06

Notes for Chapter 22 – The Nature of Light

Section 1 – What is Light?

While other types of waves, such as sound or water, require a medium to travel, light does not. Light is an electromagnetic wave – a wave than can travel through empty space or matter and consists of changing electric and magnetic fields. As Figure 1, p. 632 shows, the electric and magnetic fields are at right angles – or are perpendicular – to each other. The fields are also perpendicular to the direction of the wave motion.

Electric fields surround every charged object, and pull oppositely charged objects towards it and repel like-charged objects. A magnetic field surrounds every magnet. Because of magnetic fields, paper clips and iron filings are pulled toward magnets.

An electromagnetic (EM) wave can be produced by the vibration of an electrically charged particle. When the particle vibrates, the electric field around it also vibrates. As the electric field starts vibrating, a vibrating magnetic field is created. It is the vibration of an electric field and a magnetic field together that produces an EM wave that carries energy released by the original vibration of the particle. The transfer of energy as electromagnetic waves is radiation.

In the near vacuum of space, the speed of light is about 300,000 km/s. Light travels more slowly in air, glass, and other types of matter. Although light travels quickly, it takes about 8.3 minutes for the sun’s light to reach the Earth. It takes this much time because the Earth is 150,000,000 km away from the sun. The EM waves from the sun are the major source of energy on Earth, and serve as the means by which plants and animals survive.

Section 2 – The Electromagnetic Spectrum

The entire range of electromagnetic (EM) waves is called the electromagnetic spectrum (Fig. 1, p. 636). As can be seen from this diagram, the spectrum is divided into regions according to the length of the waves. Also, there is no sharp division between one kind of wave and the next. Let’s take a look at the seven main parts of the EM spectrum, going from left to right on the diagram:

Radio waves: These EM waves cover a wide range, and have some of the longest wavelengths (longer than 30 cm.) and lowest frequencies. They are used for broadcasting radio signals (Fig. 2, p. 637) and television signals.

Microwaves: These EM waves have wavelengths between 1 mm and 30 cm. Microwaves are sent and received by cell phones, radar, and between Earth and satellites in orbit. And they are also created in microwave ovens to heat foot (Fig. 3, p. 638).

Infrared: These EM waves vary between 700 nanometers (nm) and 1 mm. We feel infrared waves as heat on our skin on a sunny day. Almost all things give off infrared waves, including people! Infrared waves are invisible (as are all EM waves except visible light), but some devices can detect infrared waves and generate a picture (Fig. 5, p. 639).

Visible Light: These EM waves cover a very narrow range on the spectrum that humans can see. They have wavelengths between 400 nm and 700 nm. Some of the sun’s energy that reaches Earth is visible light. This visible light is white light, and contains all wavelengths of visible light combined. We see these different wavelengths of visible light as different colors (Fig. 6, p. 640). The colors of the visible spectrum can be easily remembered by the memory aid ROYGBV.

Ultraviolet: This type of EM wave is also produced by the sun, with wavelengths varying between 60 nm and 400 nm. Ultraviolet (UV) light affects your body in both good and bad ways. Too much exposure to UV light can cause sunburn, skin cancer, wrinkles, and eye damage. On the positive side, small amounts of UV light are essential for skin cells to obtain vitamin D.

X-rays: These EM waves have wavelengths between 0.001 nm and 60 nm. They can pass through many materials, and are therefore useful in the medical field for example, to examine broken bones (Fig. 9, p. 642). This penetrating ability also allows x-rays to be used as security devices in airports and other public buildings.

Gamma rays: These EM waves are the most energetic of all, having wavelengths less than 0.1 nm. They can penetrate most materials very easily, and are used widely in the medical field and other areas. For example, gamma rays are used to kill cancerous tumors and kill bacteria in foods.

Section 3 – Interactions of Light Waves

Reflection happens when light waves bounce off an object. When you look in a mirror, you are seeing light reflected twice – first from you and them from the mirror. The law of reflection states that the angle of incidence is equal to the angle of reflection. Look at Figure 1, p. 644. The beam of light traveling toward the mirror is the incident beam. The beam of light reflected off of the mirror is called the reflected beam. The dotted line perpendicular to the mirror is the normal. The angles mentioned above are created between this normal and the incident and reflected beams.

Since a mirror’s surface is smooth, light reflects off all points of the mirror at the same angle – this is called regular reflection. A wall’s surface, however, is rough. Light beams hit the wall’s surface and reflect at many different angles – this is called diffuse reflection (Fig. 2, p. 645). This second type of reflection explains why you can’t see your reflection in a wall.

Objects that produce visible light such as the sun are called luminous. Visible objects that are not light sources, but rather reflect light that strikes them, are called illuminated. The moon and various objects around you are illuminated objects.

The transfer of energy by light waves to particles of matter is called absorption. A beam of light shining through the air becomes dimmer the farther it travels because it is absorbed by particles. Scattering is an interaction of light with matter that causes light to change direction. Scattering is what makes the sky blue – shorter wavelength light is scattered more than longer wavelength light.

Refraction is the bending of a wave as it passes at an angle from one substance to another. Refraction of light happens because the speed of light varies as it travels from one substance, or medium, to another. Light travels 300,000 km/s in a vacuum, but will travel more slowly through air or other mediums such as glass. We can experience refraction in a number of ways; for example, a straw in a glass of water appears bent because light is moving from the medium of air to water (see also Fig. 6, p. 648). White light passing through a prism can be separated into its component colors because of refraction also (Fig. 7, p. 648).

Diffraction is the bending of waves around barriers or through openings. Light waves can’t diffract much around large obstacles such as buildings (which is why we can’t see around corners). It must pass through a narrow opening, around sharp edges, or through a small barrier (Fig. 8, p. 649).

Interference is a wave interaction that happens when two or more waves overlap. When two waves combine by constructive interference, the resulting wave has a greater amplitude, or height, than the individual waves had. When waves combine by destructive interference, the resulting wave has a smaller amplitude than the individual waves had. Diffracted light waves can cause both types of interference (Fig. 9, p. 650).

Section 4 – Light and Color

When light strikes any form of matter, it can interact with the matter in three different ways-it can be reflected, absorbed, or transmitted. Transmission is the passing of light through matter. Look at Figure 1, p. 652. When you look out of a window on a sunny day, you see objects beyond because light is transmitted through the glass. You can see your reflection because light is reflected off the glass. Finally, the glass feels warm to the touch because some light is absorbed by the glass.

Matter such as air, glass, and water through which visible light is easily transmitted is transparent. Matter such as wax paper that transmits light but also scatters light is said to be translucent. Matter that does not transmit any light is said to be opaque. You can’t see through opaque objects such as things made of metal and wood.

We nonetheless see opaque objects because some of the light falling on them is absorbed, and some is reflected. Only the reflected light reaches your eyes and it detected. A red rose absorbs all colors except red. It is red wavelengths of light that are reflected off of the rose and into your eyes, and you see the rose as being red. Objects appearing white have all colors reflected, and objects appearing black have all colors absorbed.

The primary colors of light are red, green, and blue. When colors of light combine, you see different colors. This is called color addition. When two colors of light are added together, you see a secondary color of light. These secondary colors of light are cyan, magenta, and yellow. See Figure 5, p. 655 for how these secondary colors of light are formed. The colors on a television screen are produced by color addition of the primary colors of light.

The situation changes when mixing colors of pigment. A pigment is a material that gives a substance its color by absorbing some colors of light and reflecting others. Chlorophyll is the pigment that gives plants their green color, and melanin is the pigment that gives your skin its color. When you mix pigments together, more colors of light are absorbed or taken away. So mixing colors of pigment is called color subtraction. The primary pigments are yellow, cyan, and magenta. Secondary pigments are green, red, and blue. Figure 6, p. 656 shows how the pigments combine to produce many different colors.

Information Posted on 5/15/06

Notes for Chapter 20 – The Energy of Waves

Section 1 – The Nature of Waves

A wave is any disturbance that transmits energy through matter or empty space. Examples of waves include light waves, microwaves, sound waves, and water waves. Energy can be carried away from its source by a wave: just drop a rock in a pond – waves from the rock’s splash carry energy away from the splash.

However, it is important to understand that the material through which the wave travels does not move with the energy – see Fig. 1, p. 574. As a wave travels, it does work on everything in its path. For example, waves in a pond do work on anything floating on the water’s surface – this is why leaves and other objects move up and down. And it is this fact that things move on the water’s surface that tells you that the waves are transferring energy.

Most waves transfer energy by the vibration of particles in a medium – a substance through which a wave can travel. A medium (plural media) can be a solid, liquid, or gas. When a particle vibrates (moves back and forth, as shown in Fig. 2, p. 575), it can pass its energy to a particle next to it. The second particle will vibrate like the first particle does. In this way, energy is transmitted through a medium.

Waves that need a medium are called mechanical waves. Sound waves and water waves are examples of mechanical waves. Electromagnetic waves, however, do not need a medium to transfer energy through, and can even go through matter, such as air, water, and glass. For example, energy that reaches us from the sun goes through the vacuum of space. Besides visible light, other types of electromagnetic waves include the other parts of the electromagnetic spectrum: gamma rays, X-rays, ultraviolet light, infrared, microwaves, and radio waves.

All waves transfer energy by repeated vibrations. However, waves can differ in many ways. They can be classified based on the direction in which the particles of the medium vibrate compared with the direction in which the waves move. The two main types of waves are transverse waves and longitudinal waves. Transverse waves are waves in which the particles of the medium move in an up-and-down motion (like the points on the rope in Fig. 5, p. 577). Longitudinal waves are waves in which the particles of the medium vibrate parallel to the direction of wave motion (like a spring pushed back and forth as shown in Fig. 6, p. 578). When one end of the spring is pushed on, and coils crowd together, and compression waves are created. When the spring is pulled back, rarefaction waves are created. Sound waves are also longitudinal waves (Fig. 7).

When waves form at or near the boundary between two media, a transverse and longitudinal wave can combine to form a surface wave. Here, the particles of the medium move in circles rather than up and down.

Section 2 – Properties of Waves

By examining properties of waves, such as their height, waves can be compared and described. The amplitude of a wave is related to its height. A wave’s amplitude is the maximum distance that the particles of a medium vibrate from their rest position (Fig. 1, p. 580). As demonstrated in class, shaking one end of a rope creates taller waves (larger wave amplitude). A wave with a larger amplitude also carries more energy than one with a smaller amplitude does.

Another property of waves is wavelength – the distance between any two crests or compressions next to each other in a wave. The distance between two troughs or rarefactions next to each other is also a wavelength. A wave with a shorter wavelength carries more energy than a longer wavelength does. This was also demonstrated in class with the rope, because more energy was put into the rope when it was shaken rapidly, creating waves with smaller wavelengths.

The number of waves produced in a given amount of time is the frequency of the wave (Fig. 3, p. 582). Frequency is usually expressed a unit called hertz (Hz). One hertz equals one wave per second (1 Hz = 1/s). Again, we can return to the example of the rope to better understand frequency and its relationship with energy – when the rope was shaken quickly – taking more energy, high-frequency waves were made. So higher frequency waves carry more energy than lower frequency waves.

Wave speed is the speed at which a wave travels. Wave speed (v) can be calculated using wavelength (λ, the Greek letter lambda), and frequency (f), by using the wave equation:

v = λ x f

Three of the basic properties of a wave are related to one another in the wave equation – wave speed, frequency, and wavelength. If you know any two of the properties, you can use the equation to find the other.

Frequency and wavelength are inversely related: the higher the frequency, the shorter the wavelength, and the lower the frequency, the longer the wavelength. For example, a sound wave traveling underwater at 1,440 m/s that has a frequency of 360 Hz will have a wavelength of 4.0 m, but a sound wave with twice the frequency will have a wavelength half as big, and so will be 2.0 m in length.

Section 3 – Wave Interactions

Reflection happens when a wave bounces back after hitting a barrier (Fig. 1, p. 584). All waves – water, sound, and light waves, can be reflected. We can see objects thanks to their interaction with light. Of course, not all waves are reflected when they hit a barrier. It this was the case, then the world would appear white! Additionally, if all waves reflected off of your sunglasses, then you wouldn’t see anything! Sometimes waves are transmitted, or passed through, a substance.

Refraction is the bending of a wave as the wave passes from one medium to another at an angle. For example, a light wave passing at an angle into a new medium such as water is refracted because the speed of the wave changes in the water (Fig. 2, p. 585). Refraction was demonstrated in class with the pencil placed in a beaker of water, where it appeared bent. Sometimes, as in sunlight, colors are refracted by different amounts. When this happens, the light is dispersed, or spread out, into its separate colors. This is how rainbows occur – raindrops split the sun’s white light into its component colors.

Diffraction is the bending of waves around a barrier or through an opening. If the barrier or opening is larger than the incoming wave’s wavelength, there is only a small amount of diffraction. If the barrier or opening is same size or smaller than the incoming wave’s wavelength, the amount of diffraction is large (Fig. 3, p. 586).

The result of two or more waves overlapping and forming a single wave is called interference. Depending on how the waves interact with each other results in either constructive or destructive interference (Fig. 4, p. 587). When the crests of one wave overlap the crests of another wave or waves (the troughs also overlapping) the result is constructive interference. When waves combine in this way, the energy carried by the waves is also able to combine – so the resulting wave has a larger amplitude than the original waves had. Destructive interference happens when the crests of one wave and the troughs of another wave overlap. The new wave has a smaller amplitude than the original waves had.

In a standing wave (Fig. 5, p. 588) certain parts of the wave are always at the rest position because of total destructive interference between all the waves. Other parts have a larger amplitude because of constructive interference. The frequencies at which standing waves are made are called resonant frequencies. When an object vibrating at or near the resonant frequency of a second object causes the second object to vibrate, resonance occurs. A resonating object absorbs energy from the vibrating object and vibrates as well (Fig. 6, p. 588).

Information Posted on 5/5/06

Notes for Chapter 18 – Electromagnetism

Section 1 – Magnets and Magnetism

A magnet is any material that attracts iron or things made of iron. Magnets have three properties: a north and south magnetic pole, magnetic force, and a magnetic field.

Each end of a magnet is a magnetic pole (north and south). Magnetic poles are points on a magnet that have opposite magnetic qualities. Magnetic poles are always found in pairs – no magnets that have only a north pole, or only a south pole, exist. In fact, cutting a magnet in half will produce two new magnets, each with a north and south pole (Fig. 7, p. 514). One end of a magnet always points to the north - this is shown in a compass, which contains a freely rotating magnet (Fig. 2, p. 511).

When two magnets are brought close together, the magnets each exert a magnetic force on the other, due to the spinning electric charges in the magnets. The magnetic force between magnets depends on how the poles of the magnets line up – like poles repel, and opposite poles attract (Fig. 3, p. 511).

A magnetic field exists in the region around a magnet in which magnetic forces can act. The shape of a magnetic field can be shown with lines drawn from the magnet’s north pole to its south pole (Fig. 4, p. 512). These lines that map out the magnetic field are called magnetic field lines. The closer the lines are together, the stronger the magnetic field is. Since the magnetic field on a magnet is strongest at the poles, it is observed that the lines around a magnet are closets together at the poles.

Whether or not a material is magnetic or not depends on its atoms. As electrons move around, it makes, or induces, a magnetic field. This gives the atom a north and south pole. In non-magnetic materials such as copper and aluminum, the magnetic fields of the individual atoms cancel each other out. But in materials such as iron, nickel, and cobalt, groups of atoms are in tiny areas called domains. The atom’s north and south poles in a domain line up and make a strong magnetic field (Fig. 5, p. 513).

Dropping a magnet or hitting it too hard can move the domains, and the magnet can become demagnetized. Another thing that can demagnetize a magnet is high temperature (the atoms in a hotter material vibrate faster, and this causes the domains to no longer line up, causing a loss of magnetism). It is possible to make a magnet, as Fig. 6, p. 513 shows - an iron nail is magnetized by rubbing it in one direction with one pole of a magnet. The domains in the nail become aligned, and it’s now capable of picking up the paper clip.

Some magnets are made of iron, cobalt, or nickel (or a mixture of those metals). These type of magnets, called ferromagnets, have strong magnetic properties (Fig. 8, p. 514). Another kind of magnet is an electromagnet, a magnet made by an electric current. Magnets can also be described as temporary or permanent. Temporary magnets are made of materials that are easy to magnetize, but lose their magnetization easily. Permanent magnets are difficult to magnetize, but once they are, they tend to keep their magnetic properties for a long period of time.

The Earth itself is one giant magnet, and in fact behaves as if it had a giant bar magnet running down its center. The poles of this imaginary magnet are located near Earth’s geographic poles (Fig. 9, p. 515). A compass needle points north because the magnetic pole of Earth that is closest to the geographic North Pole is a magnetic south pole. So the needle points to the north because its north pole is attracted to a very large magnetic south pole.

What generates the Earth’s magnetic field is the movement of electric charges in the Earth’s core, which is made of mostly of iron and nickel. The rotation of the Earth causes the liquid in the core to flow. Electric charges move, and a magnetic field is made. It is this magnetic field that helps to create the colorful aurora (Fig. 10, p. 516).

Section 2 – Magnetism from Electricity

Danish physicist Hans Christian Oersted discovered the relationship between electricity and magnetism in 1820. From his experiments, he concluded that an electric current produces a magnetic field. His work, along with that of other scientists, gave us insights into electromagnetism – the interaction between electricity and magnetism.

The magnetic fields generated by the electric wires in Oersted’s experiments were not strong enough to be very useful. However, a magnetic field can be strengthened with the use of two devices - a solenoid and an electromagnet. A solenoid is a coil of wire that produces a magnetic field when carrying an electric current (Fig. 2, p. 519). An electromagnet is made up of a solenoid wrapped around an iron core. The magnetic field of an electromagnet can be hundreds of times stronger than that of a solenoid. These devices can be used in practical ways, as Figures 3 & 4 show, p. 520-521.

Just as a current-carrying wire causes a magnet to move, a magnet can also cause a current-carrying wire to move (Fig. 5, p. 521). This property is useful in electric motors – devices that change electrical energy into mechanical energy.

A galvanometer uses an electromagnet to measure electric current, and is sometimes found in equipment used by electricians, such as ammeters and voltmeters (Fig. 7, p. 523).

Section 3 – Electricity from Magnetism

Just as a magnetic field can be generated from an electric current, so can an electric current be generated from a magnetic field. Joseph Henry and Michael Faraday conducted experiments that proved this (Figs. 1 & 2, p. 524-525). The process that they discovered, in which an electric current is made by changing a magnetic field, is called electromagnetic induction. As these two figures show, the current produced can be increased if the magnet is moved through the wire coil at a faster rate – the faster movement of the magnet causes its magnetic field to change faster, thereby inducing, or creating, a greater electric current.

So Faraday’s experiments showed that an electric current is made when a magnet moves in a coil of wire or when a wire moves between the poles of a magnet (Fig. 3, p. 526).

An electric generator uses electromagnetic induction to change mechanical energy into electrical energy (Figs. 4 & 5, p. 526-527). The electric current produced by the generator shown in Fig. 5 changes direction each time the coil make a half turn. Because the current changes direction, it is an alternating current.

The energy that generators convert into electrical energy comes from different sources. For example, in a nuclear power plant, it’s thermal energy from a nuclear reaction. The energy boils water into steam, which turns a turbine. This turbine turns the magnet of the generator, which makes an electric current and generates electricity.

Another device that uses the principle of electromagnetic induction is a transformer. A transformer increases or decreases the voltage of alternating current. A simple transformer is made up of two coils of wire wrapped around an iron ring. These two cold are called primary and secondary. The number of loops in the primary and secondary coils of a transformer determines whether it increases or decreases the voltage, as shown in Fig. 7, p. 528). A step-up transformer increases voltage and decreases current. A step-down transformer decreases voltage and increases current. The overall amount of energy going into and out of the transformer, however, does not change.

The electric current that brings electricity into your home is usually transformed three times (Fig. 8, p. 529). As the diagram shows, the power plant steps up the voltage thousands of times. It’s then stepped down at a local power distribution center, and stepped down yet again at a transformer near your house.

Information Posted on 4/26/06

Chapter 17 – Introduction to Electricity

Section 1 – Electric Charge and Static Electricity

Charge is a physical property – an object can have a positive charge, a negative charge, or no charge at all. Charge is most easily understood by learning how charged objects interact with each other. Charged objects exert a force – either a push or pull – on other charged objects. The law of electric charges states that like charges repel, or push away, and opposite charges attract (Fig. 2, p. 474). Without the attraction between protons and electrons in an atom, atoms would fly apart.

The force between charged objects is an electric force. The size of this force depends on two things: the amount of charge on each object (the greater the charge, the greater the electric force is), and the distance between charges (the closer the charges are, the greater the electric force). Charged objects are affected by electric force because charged objects have an electric field around them. An electric field is the region around a charged object in which an electric force is exerted on another charged object. A charged object in the electric field of another charged object is attracted or repelled by the electric force acting on it.

There are three ways to charge an object-see Fig. 3, p. 476: (1) friction happens when electrons are ‘wiped’ from one object to another, (2) charging by conduction happens when electrons move from one object to another by direct contact, and (3) induction, which happens when charges in an uncharged metal object are rearranged without direct contact with a charged object.

Just as with energy and mass, charges are not created or destroyed when you charge something by any of the above methods – the number of protons and electrons stays the same; electrons are simply moved from one atom to another. Because of this, charges are said to be conserved, just as mass and energy are in interactions. You can use an object called an electroscope to see if something is charged (Fig. 4, p. 477).

Most materials are either conductors or insulators, based upon how easily charges move in them. An electrical conductor is a material in which charges move easily (most metals are good conductors). An electrical insulator is a material in which charges cannot move easily (plastic, wood, and rubber are examples of insulators).

Static electricity is the electric charge at rest on an object. The charges of static electricity do not move away from the object that they are in – so the object keeps its charge. The loss of static electricity as charges move off an object is called electric discharge.

Lightning is another example of electrical discharge, and one that poses a potential danger. A lightning rod is often mounted atop a building. Since it is joined to Earth by a conductor, such as a wire, it is said to be grounded, providing a path for electric charges to move to Earth.

Section 2 – Electric Current and Electrical Energy

Electrical energy is the energy of electric charges. In most of the things using electrical energy, the charges flow through wires. This flow of charges is called electric current. Electric current is the rate at which charges pass a given point. The higher the current is, the greater the number of charges that pass the point each second. Electrical current is expressed in units of amperes (often shortened to amps). When used in equations, the symbol fir current is the letter I.

When a switch is flipped (as on a flashlight), an electric field is set up in the wire at the speed of light. The electric field causes the free electrons in the wire to move. The energy of each electron is transferred instantly to the next electron (Fig. 1, p. 482). You can think if the electric field as a command to the electrons to charge ahead.

There are two kinds of electric current – direct current (DC), and alternating current (AC). As Fig. 2, p. 483 shows, in direct current, the charges flow in the same direction, whereas in alternating current, the charges continually shift from flowing in one direction to flowing in the reverse direction. The electric current from outlets in your home is AC. The electric current from the batteries found in a camera, for example, is DC.

Voltage is defined as the potential difference between two points in a circuit, and is expressed in volts (V). Here is a way to envision how voltage works: imagine that you’re on a bike at the top of a hill. You can roll down the hill because of the difference in height between the two points. The “hill” that causes charges in a circuit to move is voltage. Voltage is a measure of how much work is needed to move a charge between two points. You can think of voltage as the amount of energy released as a charge moves between two points in the path of a current. The higher the voltage is, the more energy is released per charge.

Resistance is another factor that determines the amount of current in a wire. Resistance is defined as the opposition to the flow of an electric charge, and is expressed in ohms. When used in equations, the symbol for resistance is the letter R. You can think of resistance as “electrical friction” – the higher the resistance of a material is, the lower the current in the material. Good conductors such as copper have low resistance. Poor conductors, such as iron, have higher resistance. The resistance in insulators such as plastic and rubber is so high that electric charges cannot flow in them.

The thickness and length of a wire also affects its resistance, as shown in the model in Fig. 5, p. 486. A thick pipe allows more electric charges to pass through. A short pipe has less resistance than a long pipe does because the charges do not have to travel as far. Resistance also depends on temperature. The atoms of a material vibrate faster at a high temperature, and get in the way of the flowing electric charges. So material at a high temperature has high resistance, and material at a low temperature has low resistance. Materials at a very low temperature are called superconductors, and very little energy is wasted when electric charges move in them.

To generate electrical energy, cells change chemical or radiant energy into electrical energy. Batteries are made of one or more cells. As Fig. 7, p. 487 shows, cells can contain a mixture of chemicals called an electrolyte, which allows charges to flow. Cells also contain electrodes made from conducting materials. The electrode is the part of the cell where charges enter or exit. There are two kinds of cells; wet and dry cells. A car battery is made of several wet cells that use sulfuric acid as the electrolyte. The electrolytes in dry cells are solid or paste-like. Small radios and flashlights use dry cells.

Thermocouples convert thermal energy into electrical energy. Joining wires of two different metals into a loop makes a simple thermocouple, like the one shown in Fig. 9, p. 488. The temperature difference within the loop causes charges to flow through the loop – the greater the temperature difference, the greater the current. Photocells convert light energy into electrical energy. When light shines on a photocell, electrons in the silicon atoms gain enough energy to move between atoms. This energy is then able to move through a wire to provide electrical energy.

Section 3 – Electrical Calculations

Georg Ohm, a German school teacher, was instrumental in discovering how current, voltage, and resistance are related. In studying the resistance of materials, he measured the current that resulted from different voltages applied to a piece of metal wire. As the graph on the left of Fig. 1, p. 490 shows, Ohm found that the ratio of voltage (V) to current (I) is a constant for each material. This ratio is the resistance (R) of the material. When the voltage is expressed in volts (V) and the current is in amperes (A), the resistance is in ohms (Ω). The following equation is called Ohm’s law because of the work that Ohm did:

V

R = ---------- , or V = I X R

I

As the graph on the right of Fig. 1 shows, as the resistance goes up, the current goes down, and as the resistance decreases, the current increases.

The rate at which electrical energy is changed into other forms of energy is electric power. The unit for power is the watt (W), and the symbol for power is the letter P. Electrical power is calculated by using the following equation:

power = voltage x current, or P = V x I

Light bulbs have power rating measured in watts (typically 60 W, 75 W, or 120 W). Another common unit of power is the kilowatt (kW). One kilowatt is equal to 1,000 W. Kilowatts are typically used to express high values of power, such as that needed to heat a house.

The amount of electrical energy used in a home depends on the power of the electrical devices in the house and length of time that those devices are on. The equation for determining electrical energy is:

electrical energy = power x time, or E = P x t

Different amounts of electrical energy are used each day in a home. Electric companies usually calculate electrical energy by multiplying the power in kilowatts by the time in hours. So the unit of electrical energy is usually kilowatt-hours (kWh). If 2,000 W of power are used for 3 h, then 6 kWh of energy were used. Power companies use meters, such as the one shown in Fig. 3, p. 492. You can reduce your electric bill by taking practical steps, such as running a fan as much as possible (rather than the air conditioner, and turning off lights when they are not in use.

Section 4 – Electric Circuits

Just like a roller coaster, an electric circuit always forms a loop – it begins and ends in the same place. Because a circuit forms a loop, a circuit is a closed path. So, an electric circuit is a complete, closed path through which electric charges flow. As Fig. 1, p. 494 illustrates, all circuits need 3 parts: an energy source, wires, and a load. Loads such as a light bulb or radio are connected to the energy source by wires. The function of loads is to change electrical energy into other forms of energy such as thermal, light, or mechanical energy. As the load changes electrical energy into other forms, they offer some resistance to electric circuits.

Sometimes a circuit contains a switch (Fig. 2, p. 495). The function of the switch is to open and close a circuit, and is made of two pieces of conducting material, one of which can be moved. In order for charges to flow through a circuit, the switch must be closed, or ‘turned on.’ If a switch is open, or ‘off,’ the loop of the circuit is broken. Light switches, power buttons on radios, and keys on computers open and close circuits.

A circuit can be a series circuit or a parallel circuit. One main difference between these two types of circuits is the way in which the loads are connected. A series circuit is a circuit in which all parts are connected in a single loop (Fig. 3, p. 496). There is only one path for charges to follow, so charges moving through a series circuit must flow through each part of the circuit. All of the loads in a series circuit share the same current. A parallel circuit is a circuit in which loads are connected side by side. Charges in a parallel circuit have more than one path on which they can travel. Unlike a series circuit, the loads in a parallel circuit do not have the same current.

In homes, several circuits connect all of the lights, appliances, and outlets. The circuits branch out from a breaker box or a fuse box that acts as “electrical headquarters” for the building. Each branch receives a standard voltage, which is 120 V in the U.S.

In a circuit failure, broken wires or water cause a short circuit. In this case, charges do not go through one or more loads in the circuit. The resistance decreases, so the current increases. This leads to the wires heating up, causing the circuit to fail. Safety features such as fuses and circuit breakers help to prevent electrical fires.

A fuse is a thin strip of metal; charges in the circuit flow through this strip. If the current is too high, the metal strip can melt, as shown in Fig. 5, p. 498. As a result, the circuit is broken, and charges stop flowing.

A circuit breaker is a switch that automatically opens if the current is too high. A strip of metal in the breaker warms up, bends, and opens the switch, which opens the circuit, stopping the flow of charges. Open circuit breakers can be closed by flipping a switch after the problem has been fixed.

Information Posted on 4/6/06

Chapter 16 – Atomic Energy

Section 1 - Radioactivity

French scientist Henri Becquerel’s key experiment with fluorescent minerals was perhaps the first insight into the nature of radioactivity. The mineral that he was working with had given off energy that had passed through paper that was wrapped around the photographic plate that he had placed the mineral on. Becquerel concluded that the energy had come from uranium, an element contained in the mineral he was working with. This energy that Becquerel observed is called nuclear radiation – high energy particles and rays given off by the nuclei of some atoms.

Marie Curie, working with Becquerel, named the process by which some nuclei give off nuclear radiation radioactivity, also called radioactive decay. During radioactive decay, an unstable nucleus gives off particles and energy. There are three kinds of radioactive decay: alpha decay, beta decay, and gamma decay.

Alpha decay is the release of an alpha particle from a nucleus. It has a mass number of 4 and a charge of 2+. Recall that the mass number is the sum of protons and neutrons in the nucleus of a helium atom. So an alpha particle is in fact helium-4. Many large radioactive nuclei give off alpha particles and become nuclei of different elements – see, for example, how radium-226 decays into radon-222 and an alpha particle (Fig. 2, p. 449).

It is important to understand that mass and charge are always conserved in radioactive decays. Just like in chemical equations, where the masses of the reactant and product sides must be equivalent, the decay products must have the same mass (and charge) as the nuclei did prior to decay. Look again at Figure 2: the mass numbers before and after the decay are 226. Charge is also conserved – both before and after the decay, the charge is 88+.

Beta decay is the release of a beta particle from a nucleus. A beta particle can be either an electron (with a charge of 1-), or a positron (with a charge of 1+). Again, as Figure 3, p. 450 shows, both mass and charge are conserved. Also, notice that, as in alpha decay, beta decay involves the original nucleus decaying into a nucleus of a different element. There are two types of beta decay. This is because not all isotopes of an element decay in the same way. Recall that isotopes are atoms that have the same number of protons as other atoms of the same element do, but different numbers of neutrons. In one type of beta decay, a neutron breaks into a proton and electron – this occurs when carbon-14 decays, shown in Fig. 3. In the second type, a proton breaks into a positron and a neutron – this occurs when carbon-11 decays.

Gamma decay involves the release of gamma rays from a nucleus. Energy is also given off during alpha decay and beta decay. Some of this energy is in the form of very high energy light called gamma rays. Gamma rays – since they are light – have no mass or charge. Therefore, gamma decay alone doesn’t cause one element to change into another element – this can only happen with alpha or beta decay.

These three different forms of nuclear radiation have different abilities to penetrate, or go through, matter. This different is due to their mass and charge, as Figure 4, p. 451 shows. Alpha particles, since they’re made up of 2 protons and 2 neutrons, have the most mass and charge of any type of decay. Because of this, they are able to be stopped, or absorbed, before penetrating far. Beta particles, being electrons or positrons, have a 1- or 1+ charge and almost no mass. Because of this, they are more penetrating than alpha particles. Since gamma rays have no mass or charge, they are the most penetrating, and can cause damage deep within matter. Alpha particles, although only slightly penetrating, can cause the most damage since they’re the most massive.

Each radioactive isotope has its own rate of decay, called half-life. A half-life is the amount of time it takes one-half of the nuclei of a radioactive isotope to decay. The table on p. 453 shows the half-lives of several isotopes. As you can see, there can be a wide range of half-lives: uranium-238 has a half-life of 4.5 billion years! Oxygen-21, on the other hand, has a half-life of 3.4 seconds! Look at Figure 6 on p. 453 to see how the process works – after one half-life, one-half of the original sample has decayed, and the other half is unchanged. After two half-lives, only one fourth of the original sample now remains unchanged. Remember, a half-life involves half of the sample of the isotope – that’s why, after three half-lives, there’s only one-eighth of the original sample that remains. Let’s look at an example: suppose we have 20 grams of nitrogen-13, which has a half-life of 10 minutes. In 20 minutes, how much of the original sample will we have? Well, after 10 minutes (one half-life), we’ll have half of the original sample – 10 grams. In another 10 minutes (20 minutes total now), two half-lives have completed, and so we have now half of 10 grams, or 5 grams – one-fourth of the original 20 gram sample. Scientists can use some radioactive isotopes to provide accurate ages (dates) for a variety of objects, from fossilized remains to the age of ancient meteorites.

Radioactivity, besides being used to determine the ages of different objects, has a number of uses in healthcare and in industry. In your own home, your smoke detector may have a small amount of radioactive americium. Radioactive materials can be used to treat illnesses, including cancer. They can also be used in a number of industrial settings, such as testing the thicknesses of metal sheets, and even to power space probes.

Section 2 – Energy from the Nucleus

There are ways that the energy of the nucleus can be harnessed for constructive - and destructive – purposes. Nuclear fission is the process in which a large radioactive nucleus splits into two small nuclei and releases energy. Figure 1, p. 456 shows the decay of uranium-235.

Matter can be changed into energy, from Einstein’s famous equation E = mc2. What Einstein showed was that matter is a form of energy. In Figure 1, if you compare the total mass of the products (233) with the total mass of the reactants (235), you’d see that the total mass of the products is slightly less than the total mass of the reactants. Why are the masses different? Do we have a violation of the principle of the conservation of mass? Not at all – some of the matter was converted into energy, in this case three neutrons.

During nuclear fission, such as we have shown in Figure 1 with a uranium-235 nucleus, the neutrons produced can split other uranium-235 nuclei, so that energy and more neutrons are given off. This process can keep continuing, leading to a nuclear chain reaction – a continuous series of nuclear fission reactions (see Fig. 3, p. 457). When this process is uncontrolled, huge amounts of energy are given off very quickly – such as in an atomic bomb. However, nuclear power plants (Fig. 4, p. 458) use controlled chain reactions, and in this case large amounts of electricity can be generated that can provide power to millions.

There are advantages and disadvantages to using fission as a form of energy. On the down-side, there is the possibility of accidents occurring, such as the one that occurred in Chernobyl, Ukraine, in 1986. The nuclear waste produced (this includes fuel rods, chemicals used to process uranium, etc.) is also a serious problem – the waste will give off high levels of radiation for thousands of years. On the up-side, while nuclear power plants cost more to build than those that use fossil fuels like coal, nuclear power plants often cost less to run. They also don’t release gases like carbon dioxide into the atmosphere. However, the supply of uranium, like coal and other fossil fuels, is limited.

Fusion is another type of nuclear reaction in which matter is converted into energy. In nuclear fusion, two or more nuclei that have small masses combine, or fuse, to form a larger nucleus. This is the process that generates energy in the sun and other stars. In the case of the sun and stars of similar mass, four hydrogen nuclei - the lightest element - fuse together to make a helium-4 nucleus - the next lightest element (Fig. 6, p. 460).

In order for fusion to happen, the repulsion between positively charged nuclei must be overcome. Recall that protons have positive charges, which will cause them to repel, or push away from each other. Very high temperatures are needed to make the protons come very close to each other, and fuse – more than 100,000,0000C! These high temperatures cause matter to be in a state called plasma, when electrons have been removed from atoms. So plasma is made up of ions and electrons.

Like fission, fusion has its disadvantages and advantages. On the down-side, very high temperatures are needed. Additionally, more energy is needed to make and hold the plasma than is generated by fusion. Scientists feel that these problems can be resolved, perhaps within a few decades. On the up-side, fusion reactors are less accident prone than fission reactors. Fusion reactors also release more energy per gram than fission reactors do, and they are cleaner burning, producing less radioactive waste than fission reactors.

Information Posted on 4/2/06

Chapter 15 – Chemical Compounds

Section 1 – Ionic and Covalent Compounds

One way of grouping compounds is by the kind of bonds that they have. A chemical bond is the combining of atoms to form molecules or compounds. This bonding can occur between the valence electrons (those in the outermost energy level) of different atoms. This section focuses on two different compounds that can be formed, ionic and covalent.

Compounds that contain ionic bonds are called ionic compounds. An ionic bond is an attraction between oppositely charged ions. Ionic compounds can be formed by the reaction of a metal with a nonmetal. Therefore, when ionic compounds form, there is a transfer of electrons from the metal to the nonmetal. This transfer also causes atoms to become ions. A good example of an ionic compound is sodium chloride (table salt). When sodium (a metal) gives up its one valence electron to chlorine, sodium becomes a positively charged ion, and chlorine, since it receives an electron, becomes a negatively charged ion.

Properties of ionic compounds include brittleness – the tendency to break apart when hit. This is due to the arrangement of ions in a crystal lattice - a repeating three-dimensional pattern (see Fig. 1, p. 418). Ionic compounds, because of the strong ionic bonds that hold them together, have high melting points. Sodium chloride, for example, melts at 801oC. Fig. 2, p. 419 shows the melting points for other ionic compounds. Many ionic compounds are highly soluble – that is, they are easily dissolved in water. Water molecules attract each of the ions in an ionic compound and pull them away from each other. The resulting solution can conduct an electric current (Fig. 3, p. 419) because the ions are charged. An undissolved crystal of an ionic compound does not conduct an electric current.

Most compounds are covalent compounds – compounds that form when a group of atoms share electrons. This sharing of electrons forms a covalent bond, and the atoms that join together to form a covalent bond are nonmetals. The group of atoms that make up a covalent compound is called a molecule – the smallest particle into which a covalently bonded compound can be divided and still be the same compound.

The properties of covalent compounds are very different from those of ionic compounds. Because the bonds that hold covalent compounds together are weaker, less heat is needed to break them apart – therefore covalent compounds have a low melting point. Also, many covalent compounds are not soluble in water – meaning that they don’t dissolve easily in water (oil in salad dressing is a good example). Some covalent compounds do dissolve in water (such as sugar). However, when sugar dissolves in water, ions are not formed, therefore a solution of sugar and water does not conduct electricity, as shown in Fig. 5, p. 421. Some covalent compounds, such as acids, do form ions when dissolved in water, and will conduct an electric current.

Section 2 – Acids and Bases

An acid is any compound that increases the number of hydronium ions, H3O+, when dissolved in water. Hydronium ions form when a hydrogen ion, H+, separates from the acid and bonds with a water molecule, H2O, to form a hydronium ion, H3O+. Some of the properties of acids include a sour flavor (such as in a lemon or lime). The taste of such foods is a result of citric acid. Many acids are corrosive, meaning that they destroy tissue and many other things. Acids change color in indicators. An indicator is a substance that changes color in the presence of an acid. Some indicators include bromthymol blue (Fig. 2, p. 423) and litmus, which contains strips of red and blue paper. When an acid is added to blue litmus paper, the color of litmus changes to red. In our acids-bases lab, citric acid turned the indicator paper a very deep shade of red.

Acids also react with some metals, producing hydrogen gas. For examples, hydrochloric acid reacts with zinc metal to produce hydrogen gas (Fig. 3, p. 423). Acids also conduct electric current. When acids dissolve in water, they break apart and form ions in the solution, which make it possible for the solution to conduct an electric current. An example of this is a car battery. Sulfuric acid in the battery conducts electricity to help start the car’s engine. Acids, such as sulfuric acid, nitric acid, and hydrochloric acid, are used in many areas of industry and in homes. They are used to make products including paper, paint, detergents, fertilizers, rubber, and plastics. They can even be used to in swimming pools to help keep them algae-free.

A base is any compound that increases the number of hydroxide ions, OH-, when dissolved in water (Fig. 5, p. 425). These hydroxide ions give bases their properties, which include a bitter flavor (i.e., as tasting soap will show!), a slippery feel (as soap feels). Many bases, like acids, are corrosive. Also like acids, bases change color in indicators. Bases change the color of red litmus paper to blue. Or, if using the indicator bromthymol blue, it turns a deep shade of blue when a base is added to it, as shown in Fig. 6, p. 426. In our acids-bases lab, ammonia turned the indicator paper a deep blue. Because bases, when dissolved in water, increase the number of hydroxide ions, OH-, an electric current can be conducted in solutions of bases.

Like acids, bases have many uses. For example, sodium hydroxide is used to make soap and paper. Calcium hydroxide is used to make cement and plaster, and ammonia is found in many household cleaners. Magnesium hydroxide and aluminum hydroxide are used in antacids to treat heartburn.

Section 3 – Solutions of Acids and Bases

Acids and bases can be strong or weak. This strength or weakness is not the same as the concentration of an acid or base. The concentration is defined as the amount of acid or base dissolved in water. The strength of the acid or base depends on the number of molecules that break apart when the acid or base is dissolved in water.

As an acid dissolves in water, its molecules break apart and form hydrogen ions, H+. If all of the acid’s molecules break apart, the acid is a strong acid. Examples of strong acids include hydrochloric acid and sulfuric acid. If only a few molecules of the acid break apart, then the acid is a weak acid. Weak acids include citric acid and carbonic acid. In a similar manner, if all of the molecules of a base break apart in water to produce hydroxide ions, OH-, then the base is a strong base. Strong bases include sodium hydroxide and potassium hydroxide. When only a few of the base’s molecules break apart, then it is a weak base, such as ammonium hydroxide and aluminum hydroxide.

When acids and bases meet and undergo a reaction, it is called a neutralization reaction. Acids and bases neutralize each other – take away any ions – because the hydrogen ions (H+), present in an acid, and the hydroxide ions (OH-), which are present in a base, react to form water, H2O, which is neutral. The chemical equation is: H+ + OH- ( H2O

Notice that the positive and negative charges cancel each other out, giving us neutral water. In some cases, the water evaporates, and the ions join together to form a compound called a salt.

As we have seen, an indicator, such as litmus, can be used to identify an acid or a base. The pH scale is used to describe how acidic or basic a solution is. The pH of a solution is a measure of the hydronium ion concentration in the solution. A pH of 7 is neutral – neither acidic nor basic. Basic solutions have a pH greater than 7, and acidic solutions have a pH less than 7 (see Fig. 2, p. 429). Besides litmus strips or hydrion paper, pH meters can be used to detect and measure hydronium ion concentration electronically. Living things depend on having a steady pH in their environment. For example, some plants prefer acidic soil. Flowers of different colors can be produced by growing them in soil that is acidic or basic (Fig. 4, p. 430).

When an acid neutralizes a base, a salt and water are produced. A salt is an ionic compound that forms when a metal atom replaces the hydrogen of an acid. Salts have many uses in industry and in homes. For example, sodium chloride is used to season foods. Sodium nitrate is a salt used to preserve food, and calcium sulfate is used to make wallboard, used in construction. Salt is also used to help keep roads free of ice by decreasing the freezing point of water.

Section 4 – Organic Compounds

Organic compounds are covalent compounds composed of carbon-based molecules. Fuel, rubbing alcohol, cotton, paper, plastic, and sugar are all examples of organic compounds. All organic compounds contain carbon. Each carbon atom contains four valence electrons. So each carbon atom can make four bonds with four other atoms. Figure 1, p. 432 shows the three structural formulas for organic compounds. They are used to show how atoms in a molecule are connected. They are also called carbon backbones because carbon forms the basis for the three types of chains – straight, branched, and ring.

Although many organic compounds contain several kinds of atoms, some contain only two – hydrogen and carbon. These organic compounds containing only these elements are called hydrocarbons, of which there are three groups (see Figs. 2 & 3, p. 433). Saturated hydrocarbons, or alkanes, have each carbon atom in the molecule sharing a bond with each of four other atoms. A single bond is a covalent bond made up of one pair of shared electrons. An unsaturated hydrocarbon, such as ethane or ethyne, has at least one pair of carbon atoms sharing a double or triple bond. They are called unsaturated because the double or triple bonds can be broken and more atoms can be added to the molecules. Hydrocarbons that contain two carbon atoms connected by a double bond are called alkenes, and hydrocarbons that contain two carbon atoms connected by a triple bond care called alkynes. The third type of hydrocarbon is the aromatic hydrocarbons, most of which are based on benzene. As shown in Fig. 3, benzene has a ring of six carbons that have alternating double and single bonds.

Organic compounds that are made by living things are called biochemicals, and are divided into four categories: carbohydrates, lipids, proteins, and nucleic acids. Carbohydrates are biochemicals such as cellulose and glycogen, and are composed of one or more simple sugar molecules bonded together, and are used as a source of energy. There are two kinds of simple carbohydrates: simple and complex. Lipids are biochemicals that do not dissolve in water, and include fats, oils, and waxes. Proteins are biochemicals that are composed of ‘building blocks’ called amino acids. Nucleic acids are biochemicals made up of nucleotides, molecules made of carbon, hydrogen, oxygen, nitrogen, and phosphorus atoms. Nucleic acids are sometimes called the blueprints of life, because they contain all the information needed for a cell to make all of its proteins. The two kinds of nucleic acids are DNA and RNA.

Notes for Quarter 3

Information Posted on 3/20/06

Chapter 14 – Chemical Reactions

Section 1 – Forming New Substances

A chemical reaction is a process in which one or more substances changes to make one or more new substances. You know that a chemical reaction has occurred when the chemical and physical properties of the new substances differ from those of the original substances. A good example is photosynthesis, the process whereby plants make food for themselves. Three substances chemically react with each other – sunlight, carbon dioxide, and water, and produce new substances – glucose (sugar) and oxygen:

sunlight + H2O + CO2 glucose + O2

The above equation is properly read “sunlight reacting with water and carbon dioxide produces glucose and oxygen.” The substances on the left side of the arrow are called the reactants (since they react chemically with one another), and the substances on the right side are called the products (since they are the substances produced as a result of the chemical reaction).

There are several signs that you can observe that tell you that a chemical reaction has taken place. In some chemical reactions, gas bubbles form. Others form solid precipitates – solid substances that are formed in a solution. During other chemical reactions, energy is given off, in the form of light, thermal energy (heat), or electricity. Reactions often have more than one of these signs present (see Fig. 2, p. 389 for a diagram showing these four common signs.

As already mentioned, the properties of substances that are formed in a chemical reaction (the products) are very different from the substances that react (the reactants). Sodium is a soft metal that reacts violently with water. Chlorine is a poisonous yellow-green gas. But when they react together, a new substance – sodium chloride (table salt) – is produced. This new substance has very different properties than sodium and chlorine alone – table salt is a white, granular solid safe to put on our food.

In order for new substances to form in a chemical reaction, the chemical bonds in the starting substances (the reactants) must break. This can be done when molecules collide with enough energy. The atoms then rearrange, and new bonds form to make the new substances, as Fig. 4, p. 390 shows in the example of hydrogen (H2) and chlorine (Cl2).

Section 2 – Chemical Formulas and Equations

All substances are formed from about 100 elements. Each element has a chemical symbol. Chemical formulas are a shorthand way to use chemical symbols and numbers to represent a substance. The chemical formula F2 means that this fluorine molecule is made up of two fluorine atoms. The chemical formula for NH4 tells us that 1 atom of nitrogen and 4 atoms of hydrogen bonded together to make the compound ammonia. Look at Fig. 1, p. 392 for some other examples.

We can write formulas for both covalent compounds and ionic compounds. You can write formulas for covalent compounds by using the prefixes in the names of the compounds. For example, take carbon dioxide (CO2). Like the examples above, when there is an absence of a prefix (like we have here for carbon), it indicates one atom. So there is one carbon atom, and two oxygen atoms – but notice the prefix di- in front of oxide – this prefix tells us that there are two atoms of oxygen. With dinitrogen monoxide (N2O), the prefix di- indicates two nitrogen atoms, and the prefix mono- indicates one oxygen atom (Table 1 on p. 393 gives you a list of commonly used prefixes).

When we write formulas for ionic compounds, you have to be sure that the compound’s charge is 0. Recall that ionic compounds are formed from a metal atom and a nonmetal atom. So, the compound magnesium chloride (MgCl2) is formed from Mg and Cl2 bonded together. The magnesium ion has a 2+ charge (it’s a metal and gives up its 2 valence electrons when it reacts with chlorine). The chloride ions have a 1- charge (it’s a nonmetal, and the two chlorine atoms will receive the two electrons from magnesium). Add up the charges: one magnesium ion has a charge of 2+, and two chloride ions will have a charge of 2- together – so we have a total charge of 0. And that’s why we write the formula for magnesium chloride as MgCl2. Fig. 3, p. 393 gives you another example using NaCl.

Chemical equations use chemical symbols and formulas as a shortcut to describe a chemical reaction. As explained in the notes for section 1 using the example of photosynthesis, the substances that start a chemical reaction are called the reactants, and are listed to the left of the arrow. The substances that are formed from the reaction are the products, and are listed to the right of the arrow. So in a chemical equation like the one listed in Fig. 5, p. 394 (C + O2 CO2), what the equation is telling us is that one atom of carbon is reacting with two atoms of oxygen to form the compound carbon dioxide. Notice the plus sign (+) on the reactant side – this always separates the formulas of two or more reactants or products from one another. The number 2 that you see is called a subscript, and is always written below the element or formula symbol. The arrow ( ), also called the yields sign, separates the formulas of the reactants fro the formulas of the products.

Notice the above equation: on the reactant side, there is one atom of carbon, and two atoms of oxygen. On the product side, there is one atom of carbon, and two of oxygen. There’s a balance; an equality of atoms on both the reactant and product side. So this equation is said to be balanced. But why must chemical equations be balanced? IN the 1700’s, French chemist Lavoisier found that the total mass of the reactants was always the same as the total mass of the products. His work led to the law of conservation of mass, which states that mass is neither created nor destroyed in ordinary chemical and physical changes. So the numbers and kinds of atoms on both sides of the arrow must be equal – just like our above example. To balance equations, you use coefficients, which are numbers placed in front of a chemical symbol or formula. For example, 2CO represents two carbon monoxide molecules. The number 2 is the coefficient. For an equation to be balanced, all atoms must be counted. So you must multiply the subscript of each element in a formula by the formula’s coefficient. For example, 2H2O contains a total of 4 hydrogen atoms and two oxygen atoms. Only coefficients – NOT SUBSCRIPTS – are changed when balancing equations. Look at Figure 7, p. 396 for a step-by-step example of how to balance the equation H2 + O2 H2O.

Section 3 – Types of Chemical Reactions

Most chemical reactions can be placed into one of four categories: synthesis, decomposition, single-displacement, and double-replacement. Each type of reaction has a pattern that shows how reactants become products.

In a synthesis reaction, two or more substances combine to form one new compound. You start out with simpler substances (reactants), and you end up with a more complex substance (product). So something more complex is being synthesized, or built up, from something less complex. Look carefully at the following examples of synthesis reactions:

2Na + Cl2 ( 2NaCl

P4 + 3O2 ( 2P2O3

Just the opposite occurs in a decomposition reaction. The word ‘decompose’ means to break down, and that is exactly what happens in this type of reaction: a single substance breaks down to form two or more simpler substances; something complex (reactant) decomposes into its simpler components (products). Here are some examples of decomposition reactions:

H2CO3 ( H2O + CO2

2NO2 ( 2O2 + N2

In some reactions, an element replaces another element that is part of a compound. When this occurs, a single-displacement reaction has occurred. As the following examples will show, the products of a single-displacement reaction are a new compound and a different element:

Zn + 2HCl ( ZnCl2 + H2

SeCl6 + O2 ( SeO2 + 3Cl2

In the first equation, on the reactant side zinc is the lone element. However, after the reaction occurs (the product side), zinc has bonded with chlorine (forming the new compound ZnCl2), and hydrogen is now the lone element. In the second equation, after the reaction occurs, oxygen is no longer by itself – it has bonded with selenium and formed a new compound (SeO2), and chlorine is now the different element.

In a single-displacement reaction, a more reactive element can displace a less reactive element in a compound, as Figure 4, p. 400 shows in the case of copper and silver. The only nonmetals that participate in single-displacement reactions are the halogens (group 17).

In a double-displacement reaction, ions from two compounds exchange places. So this tells us that metals bonded to nonmetals will typically form this type of reaction. One of the products of a double-displacement reaction is often a gas or a precipitate. Look at the following examples of a double-displacement reaction:

NaCl + AgF ( NaF + AgCl

Na3PO4 + 3KOH ( 3NaOH + K3PO4

Notice in both equations that the compounds involved in the reactions have switched bonding partners. For example, in the first equation on the reactant side sodium was bonded with chlorine, and silver with fluorine. But after these compounds reacted, notice what we have on the product side: sodium now has bonded with fluorine to form NaF, and silver has bonded with chlorine to form AgCl.

Section 4 – Energy and Rates of Chemical Reactions

Chemical energy is part of all chemical reactions. Energy is needed to break the chemical bonds in reactants. As new bonds form in the products, energy is released. Upon comparing the chemical energy of the reactants and products, it is possible to tell if energy is released or absorbed in the overall reaction.

An exothermic reaction is a chemical reaction in which energy is released. The prefix exo means “go out” or “exit.” Energy can be given off in exothermic reactions in several forms: as light, as electrical energy, or as light and thermal energy (Fig. 1, p. 402). Energy released in an exothermic reaction is often written as a product:

2Na + Cl2 ( 2NaCl + energy

An endothermic reaction is a chemical reaction in which energy is taken in or absorbed. The prefix endo means “go in.” Energy taken in during an endothermic reaction is often written as a reactant:

2H2O + energy ( 2H2 + O2

The law of conservation of energy states that energy can be neither created nor destroyed – it can only change forms. Energy can also be transferred from one object to another, much the same way that a baton is transferred from one runner to another. What this law means for chemical reactions is this: if you could measure all the energy in a reaction, you would find that the total amount of energy (of all types) is the same before and after the reaction.

Just as a bowler must first put in some energy to start the ball rolling down the lane, a chemical reaction must also get a boost of energy before a reaction can start. This boost of energy is called the activation energy – the smallest amount of energy that molecules need to react. Sources of activation energy include friction, as when a match is lit – the heat produced by the friction of moving the match against the match box strip provides the activation energy needed to start the reaction. Other possible sources of activation energy include electricity (as in the spark in a car’s engine that starts it), and light.

A reaction takes place only if the particles of reactants collide. But there must be enough energy to break the bonds that hold particles together in a molecule, and form new bonds in new substances. The speed at which new particles form is called the rate of a reaction.

There are four factors that affect the rate of a reaction: temperature, concentration, surface area, and the presence of an inhibitor or catalyst.

Temperature: At high temperatures, particles of reactants move quickly, and collide often and with a lot of energy – so, many particles have the activation energy to react. And many reactants can change into products in a short time.

Concentration: A high concentration of reactants causes a fast rate of a reaction. Concentration is a measure of the amount of one substance dissolved in another substance, as shown in Fig. 6, p. 406.

Surface Area: Increasing the surface area of solid reactants increases the rate of a reaction. For example, grinding a solid into a powder makes a larger surface area – greater surface area exposes more particles of the reactant to other reactant particles. This leads to reactant particles colliding with each other more often, and so the reaction rate is increased.

The Presence of an Inhibitor or Catalyst: An inhibitor is a substance that slows down or stops a chemical reaction. This can be useful at times, as when preservatives are added to food to slow the growth of bacteria or fungi. Some antibiotics also serve as inhibitors, preventing certain kinds of bacteria from multiplying. A catalyst is a substance that speeds up a chemical reaction without being permanently changed. Because it isn’t changed, the catalyst is not a reactant. A catalyst lowers the activation energy of a reaction, which allows the reaction to happen more quickly. Some examples of catalysts include certain enzymes in your body that speed up chemical reactions, as well as the catalytic converter in cars.

Information Posted on 3/6/06

Chapter 13 – Chemical Bonding

Section 1 – Electrons and Chemical Bonding

Chemical bonding is the joining of atoms to form new substances. The properties of these new substances are different from the properties of the original elements. For example, sodium is a white solid metal, and chlorine is a green, poisonous gas. But when an atom of sodium reacts with an atom of chlorine, a chemical bond is formed, and a new substance – sodium chloride, is made. This new substance is white and granular (and edible!) So sodium chloride has very different properties than either sodium or chlorine.

Electrons in an atom are arranged in energy levels. The first energy level, closest to the nucleus, can hold up to 2 electrons. The second energy level can hold up to 8 electrons. The third energy level can hold up to 18 electrons (Fig. 2, p. 365 shows the electron arrangement for a chlorine atom.

When atoms combine to make chemical bonds, it is almost always the valence electrons – those electrons in the outermost energy level of an atom – that are responsible for forming the bond. Determining the number of valence electrons in main group elements (Groups 1-2, and 13-18) is easy – the element’s group number tells us the number of valence electrons. For example, all of the elements in group 1 (Alkali Metals) have 1 valence electron. All elements in group 16 (Oxygen Group) have 6 valence electrons (see Fig. 3, p. 365).

All atoms do not have the same ability to form bonds – some rarely form bonds at all. An example of atoms that rarely form bonds would be the Noble Gases (Group 18). Elements in this group have 8 valence electrons (with the exception of helium, which has 2). Atoms like the Noble Gases are in a very stable state, as their outermost energy levels are full.

Atoms with fewer than 8 valence electrons are more likely to form bonds. Atoms bond by gaining, losing, or sharing electrons to have a filled outermost energy level – so a filled outermost level contains 8 valence electrons. As Fig. 5, p. 367 shows, an atom of sulfur, which has 6 valence electrons, can either gain or share 2 more electrons, thereby giving it 8 valence electrons. Magnesium has 2 valence electrons. It can have a full outer level by losing the 2 valence electrons in the third energy level. Now the second energy level becomes the outermost energy level and has 8 electrons. Light elements like helium, hydrogen, lithium and beryllium will have full outermost energy levels with only 2 valence electrons when reacting with other atoms. For example, lithium, upon reacting with another atom, will lose its one valence electron in the second energy level, and the first energy level now becomes its outermost energy level, holding 2 valence electrons. As we shall see in section 2, when reacting with other atoms, metals (like magnesium) will give up electrons, and non-metals (like sulfur) will receive electrons.

Section 2 – Ionic Bonds

An ionic bond forms when electrons are transferred from one atom to another. During ionic bonding, one or more valence electrons are transferred from one atom to another. Like all chemical bonds, ionic bonds form so that the outermost energy levels of the atoms in the bonds are filled.

An atom is neutral because the number of protons equals the number of electrons – the positive and negative charges cancel each other out, giving the atom an overall charge of 0. However, when ionic bonds form, atoms are transferring electrons – either giving them away, or receiving them. The number of protons, of course, stays the same, but now the number of protons and electrons is different – the charges don’t cancel out, and the atoms become ions – charged particles that form when atoms gain or lose electrons.

Since metals have few valence electrons, it’s easier for them to lose those valence electrons when reacting to form bonds. Only a small amount of energy is needed to take electrons from metal atoms. Non-metals, on the other hand, have more valence electrons, and so it would be easier for them to gain what little electrons they need to have a full outermost energy level. Look at Figure 2, p. 369. Sodium, a metal, has 11 protons and 11 electrons. But during chemical changes, sodium will lose its 1 valence electron – it now has 11 protons and 10 electrons – 11 minus 10 is positive 1. Now we no longer have a neutral sodium atom, but rather a positively charged sodium ion (Na+). For aluminum, since this metal has 3 valence electrons, when it loses them upon reacting, the aluminum ion is written as Al3+, since the original aluminum atom lost 3 electrons (13 protons minus 10 electrons equals positive 3).

As we’ve seen, metals like sodium and aluminum for positive ions when reacting, since they give away electrons. Non-metals, however, form negative ions, since they receive electrons upon reacting with another atom. Look at Figure 3, p. 370. Oxygen, with 8 protons and 8 electrons, becomes an oxide ion (O2-) when reacting, since it receives two electrons from the metal that its forming a bond with (8 protons minus 10 electrons equals -2). Chlorine, with 7 valence electrons, gains the one electron it needs to have a full outermost energy level when reacting with a metal such as sodium. The chlorine atom, originally with 17 protons and 17 electrons, now becomes a chloride ion (Cl-) with 17 protons and 18 electrons.

Energy is given off by most nonmetal atoms when they gain electrons – the more easily an atom gains an electron, the more energy the atom releases. So atoms of Group 17 (the halogens) give off the most energy when they gain an electron. An ionic bond will form between a metal and a nonmetal if the nonmetal releases more energy than it takes to take electrons from the metal. And since it takes only a small amount of energy to pull the few valence electrons from metal atoms, forming ionic bonds isn’t hard.

When ionic bonds form, the number of electrons lost by the metal atoms equals the number gained by the nonmetal atoms. The ions that bond are charged, but the compound formed is neutral because the charges of the ions cancel each other out. For example, when sodium and chlorine react, the compound sodium chloride is formed. The ionic configuration is Na+Cl-. But since the charges are equal in number (1) but opposite, they cancel, so the compound NaCl is neutral. When ions bond, they form a repeating three-dimensional pattern called a crystal lattice, like the sodium chloride (table salt) shown in Figure 4, p. 371.

Section 3 – Covalent and Metallic Bonds

A covalent bond forms when atoms share one or more pairs of electrons. When two atoms of nonmetals (such as hydrogen) bond, they do so by sharing electrons with each other – see Fig. 1, p. 372). So unlike ionic bonds, covalent bonds DO NOT form by the losing and gaining of electrons – rather the atoms that make up a covalent bond SHARE the valence electrons that are used to form the bond. For example, look at the diagram below, which shows how two atoms of hydrogen bond to form molecular hydrogen (H2).

[pic]

Notice how each electron from the hydrogen atoms are shared equally by the two molecules – this is referred to as a nonpolar covalent bond. In the other type of covalent bond – polar covalent bonds - there is an unequal sharing of electrons among the atoms that form the bond, such as in the water molecule (H2O). Because oxygen is a more massive atom than hydrogen, it tends to pull electrons towards it, leaving the oxygen atom with a slight negative charge. This results in the two hydrogen atoms having a slight positive charge, as shown below:

[pic]

Notice in both cases that all atoms in the bonds have full valence levels – hydrogen with two, and oxygen with eight.

There are a couple of ways to represent atoms and molecules that can form covalent bonds. One way is the ball and stick model, in which atoms are joined together with one stick (representing a single bond), two sticks (representing a double bond), or three sticks (representing a triple bond). The number of valence electrons needed by the atoms to have a complete valence level will determine what kinds of bonds are formed. The other method is known as the electron dot diagram, which represents atoms and molecules with dots drawn around them, representing the number of valence electrons. To write an electron dot diagram for a single atom, first write the element’s symbol. Then, starting at the top of the element, write single dots (each dot representing a valence electron). If the element has more than 4 valence electrons, such as oxygen with 6, then any remaining dots are paired up. For molecules, paired dots are drawn between the atoms making up either polar or nonpolar covalent bonds (see Fig. 2 & 3, p. 373).

Substances containing covalent bonds consist of individual particles called molecules – two or more atoms joined in a definite ratio. Molecules are composed of at least two covalently bonded atoms. Diatomic molecules are made up of two atoms of the same element (examples would be H2, O2, N2 and the halogens I2, F2, Br2 and Cl2). Look at the fluorine molecule in Fig. 5, p. 374 – note that the shared electrons are counted as valence electrons for each atom. So, both atoms of the molecule have filled outermost (valence) energy levels. There are more complex molecules as well, such as the proteins in your body. Carbon atoms are the basis of many of these more complex molecules. Each carbon atom needs to make 4 covalent bonds to have 8 valence electrons – these bonds can be with other atoms of other elements or with other carbon atoms, as shown in the model of the sugar sucrose (Fig. 6, p. 375).

A metallic bond is formed by the attraction between positively charged metal ions and the electrons in the metal. Positively charged metal ions form when metal atoms lose electrons. Bonding in metals is the result of metal atoms being so close to one another that their outermost energy levels overlap. This overlapping allows valence electrons to move throughout the metal (Fig. 8, 376). So a metal can be thought of as being made up of positive metal ions that have enough valence electrons ‘swimming’ around to keep the ions together. The electrons also cancel the positive charge of the ions.

Metallic bonding is what gives metals their properties of electrical conductivity (ability to conduct electricity, such as copper in wiring), malleability (ability to be hammered into sheets, as in aluminum), and ductility (ability to be drawn into wires (such as copper). The bonds formed in metals prevent them breaking. When a piece of metal is bent, some of the metal ions are forced closer together. One might expect the metal to break because all of the metal ions are positively charged. But since these ions are surrounded by a sea of electrons, no breaking occurs. It’s theses moving electrons around and between the metal ions that maintain the metallic bonds, no matter how the shape of the metal changes (Fig. 9, p. 377).

Information Posted on 2/13/06

Chapter 12 – The Periodic Table

Section 1 – Arranging the Elements

In the early 1860’s, scientists knew of the properties of more than 60 elements – but no one had organized the elements according to those properties. When this was eventually done, scientists would be greatly aided in their understanding of how elements interact with each other. It was Russian chemist Dmitri Mendeleev who is credited with first discovering a pattern to the elements in 1869. He arranged the elements in order of increasing atomic mass. When he did so, he noticed a pattern emerging. When the elements were arranged in order of increasing atomic mass, those with similar properties occurred in a repeating pattern – in other words, the pattern was periodic. Periodic means “happening at regular intervals.” Mendeleev found that the elements’ properties followed a pattern that repeated every seven elements, and his table became known as the periodic table of the elements. Figure 2, p. 337 shows his first try at arranging the elements.

In 1914, Henry Moseley, a British scientist, determined the number of protons-the atomic number—in an atom. All elements fit the pattern in Mendeleev’s periodic table when they were arranged by atomic number. Looking at the periodic table in your textbook on p. 338-339, all of the more than 30 elements discovered since 1914 follow the periodic law, which states that the repeating chemical and physical properties of elements change periodically with the elements’ atomic numbers.

Elements are classified as metals, nonmetals, and metalloids, according to their properties. The zigzag line on the periodic table helps one recognize which elements are metals, which are nonmetals, and which are metalloids. Looking at your periodic table’s key, it’s clear that most of the elements are metals – these are found on the left side of the periodic table. Atoms of most metals have few electrons in their outer energy level, and are solid at room temperature (mercury is a notable exception, being a liquid at room temperature). As Figure 3, p. 340 illustrates, some other properties of metals are that they are shiny, malleable (can be flattened with a hammer), ductile (can be drawn through a wire), and are good conductors of thermal energy. Nonmetals are found on the right of the zigzag line on the periodic table. Atoms of most nonmetals have an almost complete set of electrons in their outer energy level. Group 18 elements, the noble gases, have a complete set of electrons. More than half of the nonmetals are gases at room temperature, and they have properties that are the opposite of metals, as shown in Figure 4, p. 341. Metalloids (semiconductors) border the zigzag line on the periodic table. Atoms of metalloids have about half of a complete set of electrons in their outer energy level, and they have properties of metals and some properties of nonmetals, as shown in Figure 5, p. 341.

Each square on the periodic table includes the element’s name, chemical symbol, atomic number, and atomic mass. Each element is identified by a chemical symbol. Names of elements come from a variety of sources, such as scientists (i.e., mendelevium) and places (i.e., californium). Some names have their roots in other languages. For example, the element sodium’s symbol is Na. This is from the Latin word ‘Natrium.’ The first letter of a chemical symbol is always capitalized, and any other letter is always lowercase.

Each horizontal row of elements (left to right) on the periodic table is called a period. There are seven periods on the periodic table. Look at the period 4 elements in Figure 6 on p. 342. The physical and chemical properties of elements in a row follow a repeating, or periodic, pattern as you move across the period. Properties such as conductivity and reactivity change gradually from left to right in each period. Additionally, all of the elements in a row have the same number of energy levels. Each vertical column of elements (top to bottom) on the periodic table is called a group. There are 18 groups on the periodic table. Elements in the same group have the same physical and chemical properties, and for this reason a group is also called a family. Elements in a group also have the same number of valence electrons – the number of electrons in their outermost energy level. So all elements in group 2 have 2 electrons in their outermost energy level. For groups 13-18, you subtract 10 from the group number to get the number of valence electrons. So elements in group 13 would have 3 valence electrons, group 17 elements would have 7 valence electrons, etc.

Section 2 – Grouping the Elements

In this section we will look at the particular groups of elements that make up the periodic table. Starting at the far left, we have Group 1, the Alkali Metals. The elements in group 1 (Fig. 1, p. 344) are the most reactive metals because their atoms can easily give away the one electron in their outer energy level. For example, when sodium and chlorine react together to yield the compound sodium chloride, sodium gives up its one electron in its outer energy level to chlorine, which, being a nonmetal, receives electrons. Group 2 elements (Fig. 2, p. 345) are the Alkaline-Earth Metals. Atoms of alkaline-earth metals have 2 electrons in their outer energy level. Because it is more difficult for atoms to give two electrons than to give one when joining with other atoms, the alkaline-earth metals are less reactive than the alkali metals. Group 2 elements and compounds formed from them have many uses; for example, in making low-density materials used in airplanes.

Groups 3-12 do not have individual names, but are collectively referred to as the transition metals. The atoms of transition metals do not give away their electrons as easily as atoms of the Group 1 and Group 2 metals do. Therefore, transition metals are less reactive than both alkali metals and alkaline-earth metals are. Properties of transition metals vary widely, as shown in Figure 3, p. 346. But being metals, they share the properties of metals (i.e., shiny, good conductors, etc.).

The Lanthanides and Actinides (p. 346), which are actually transition metals from periods 6 and 7, appear in two rows at the bottom of the periodic table to keep the table from being too wide. Lanthanides are elements in the first (top) row. They are shiny, reactive metals, some of which are used to make steel. Second (bottom) row elements are called actinides. All atoms of actinides are radioactive, or unstable. The atoms of a radioactive element can change into another element. Many of the actinides are not found in nature, but rather are made in laboratories. Americium (element 95), for example, is used in some smoke detectors.

Group 13 is the Boron Group (p. 347). Aluminum, besides being the most common element in this group, is also the most abundant metal in Earth’s crust. Elements in the boron group are reactive. Note in your text that boron is a metalloid, and the rest of group 13 elements (Al, Ga, In, Tl) are metals. Group 14 is the Carbon Group (p. 347), and it contains a nonmetal (C), two metalloids (Si, Ge), and three metals (Sn, Pb, Uuq). Carbon forms many important compounds found in living things. Group 15 is the Nitrogen Group (p. 348). There are two nonmetals (N, P), two metalloids (As, Sb), and one metal (Bi) in this group. Nitrogen, a gas at room temperature, makes up about 78% of the Earth’s atmosphere. Group 16 is the Oxygen Group (p. 348). It comprises three nonmetals (O, S, Se), one metalloid (Te), and a metal (Po). Oxygen makes up about 20% of the atmosphere. Group 17 is the Halogens (p. 349). These are very reactive nonmetals because their atoms need to gain only one electron to have a complete outer level (when an atom has 8 electrons in its outermost level, it is complete). All elements in this group are nonmetals (F, Cl, Br, I, At). Group 18 are the Noble Gases (p. 350). This group contains only nonmetals (He, Ne, Ar, Kr, Xe, Rn). Noble gases are non-reactive metals – they do no need to lose or gain electrons, as they have a full set of electrons in their outer level. The properties of hydrogen (p. 350) do not match the properties of any single group, so it is set apart from the other elements in the periodic table. Hydrogen is above Group 1 because atoms of the alkali metals also have one electron in their outer level. Hydrogen is reactive.

Information Posted on 2/1/06

Chapter 11 –Introduction to Atoms

Section 1 – Development of an Atomic Theory

Since ancient times, humankind has wondered how the natural world was put together. Some Greek philosophers, like Aristotle, thought that you could keep cutting up matter forever, never ending up with a particle that couldn’t be cut further. Another philosopher, however, named Democritus, thought that you would eventually end up with a particle that could not be cut. He called this particle atomos, meaning “not able to be divided.” Democritus turned out to be right: matter is made of particles called atoms, which are the smallest particles into which an element can be divided and still be the same substance.

Since the time of Democritus, the development of atomic theory experienced a gradual progression of ideas that would eventually lead to the modern atomic theory. One advantage that later scientists had over philosophers like Democritus is that they could test their ideas. In the late 1700’s, scientists learned that elements combine in certain proportions based on mass to make compounds. In 1803, John Dalton, a British chemist and teacher, wanted to know why, and so began experimenting with different substances. Following results that he obtained, Dalton published his atomic theory, which stated that: (1) all substances are made of atoms. Atoms are small particles that cannot be created, divided, or destroyed. (2) Atoms of the same element are exactly alike, and atoms of different elements are different. (3) Atoms join with other atoms to make new substances.

While Dalton’s ideas were largely correct, new data would be obtained toward the end of the 1800’s that didn’t fit with some of Dalton’s ideas. The stage was set for atomic theory to evolve further. In 1897, British scientist J.J. Thomson pointed out a mistake in Dalton’s theory, showing by experiment that there were particles inside the atom. This meant that atoms can be divided into even smaller parts. From Thomson’s experiment (see Fig. 3, p. 314), he discovered electrons – particles with a negative charge. Thomson proposed a new model of the atom, sometimes called the plum-pudding model, because Thomson thought that electrons were mixed throughout an atom like plums in a pudding. Here is a diagram of Thomson’s model:

[pic]

In 1909, Ernest Rutherford tested Thomson’s theory. His famous ‘gold-foil experiment’ is shown on p. 315, Figure 5. Starting out with Thomson’s idea that atoms are soft ‘blobs’ of matter, he expected the particles to pass right through the gold. Most of the particles did just that. But to his surprise, some of the particles were deflected, or even bounced straight back. He realized that in order to explain this, atoms must be considered mostly empty space, with a tiny part made of highly dense matter. Figure 7, p. 316 gives you an idea for the dimensions of an atom. In 1911, Rutherford revised the atomic theory, proposing that in the center of the atom was a tiny, extremely dense positively charged part called the nucleus. Here is a diagram of Rutherford’s model:

[pic]

In 1913, Niels Bohr, a Danish scientist who worked with Rutherford, studied the way that atoms react to light. His results led him to propose that electrons move around the nucleus in certain paths, or energy levels. In Bohr’s model there are no paths between levels. But electrons can jump from a path in one level to a path in another level. His model was a valuable tool in predicting some atomic behavior, but there was still room for some improvement. Here is a diagram of Bohr’s model:

[pic]

Many 20th century scientists added to our current understanding of the atom. Physicists Erwin Schrodinger and Werner Heisenberg did very important work, further explaining the nature of electrons in an atom. Their work showed that electrons do not travel in definite paths as Bohr suggested. According to the current theory, often called the Electron Cloud Model (see diagram below), there are regions inside the atom where electrons are likely to be found. These regions are called electron clouds.

Section 2 – The Atom

The dimensions of atoms are difficult to comprehend. To give you some idea of just how small they are, a piece of aluminum foil is about 50,000 aluminum atoms thick! As tiny as an atom is, it is made of even smaller particles. Those particles are protons, neutrons, and electrons. Here is a diagram of the atom and its parts:

[pic]

The nucleus is the small dense, positively charged center of the atom. It contains most of the atom’s mass. Inside the nucleus are found two type of particles; protons and neutrons. Protons are positively charged particles. Neutrons are particles that do not have a charge; they are neutral; hence the name ‘neutron.’ Outside of the nucleus are found electrons – particles that have a negative charge. Electrons are found in electron clouds outside the nucleus. It is the size of the electron clouds that determine the size of the atom. In the diagram above, the nucleus is enlarged to show you the protons and neutrons clearly. However, in reality the diameter of the nucleus is 1/100,000 the diameter of the atom. Remember that an atom is mostly empty space, with the nucleus occupying only a tiny part of it.

In terms of mass, scientists use a unit called the atomic mass unit to express masses of particles. Each proton has a mass of about 1.7 x 10-24 grams, or 1 amu. Neutrons are a little more massive than protons, although the difference is so small that the mass of a neutron is also give as 1 amu. Electrons, compared with protons and neutrons, are very small in mass. It takes more than 1,800 electrons to equal the mass of one proton.

The charges of protons and electrons are opposite but equal, so their charges cancel out. Because an atom has no overall charge, it is neutral. Atoms can be stripped of electrons, or they can gain electrons. In this case the balance between protons and electrons is lost, and an ion is formed. An atom that loses one or more electrons becomes a positively charged ion, and an atom that gains one or more electrons becomes a negatively charged ion.

Atoms of different elements differ because they have different numbers of protons in the nucleus – the number of protons in nucleus of an atom is called the atomic number of that atom. All atoms of an element have the same atomic number. For example, every carbon atom has only 6 protons in its nucleus. However, it is possible for atoms of the same element to differ in the number of neutrons that they have. Isotopes are atoms that have the same number of protons but have different numbers of neutrons. So atoms that are isotopes of each other are always the same element, because isotopes always have the same number of protons. The different numbers of neutrons, however, gives them different masses. The isotopes of hydrogen are written as hydrogen-1, hydrogen-2, and hydrogen-3. See Figure 4, p. 321 for a diagram of two isotopes of hydrogen.

You can identify each isotope of an element by its mass number. The mass number is the sum of the protons and neutrons in an atom. For example, as Figure 5, p. 322 shows, one isotope of boron has 5 protons and 6 neutrons, and 5 electrons. So the mass number of this boron isotope is 5 + 6 = 11. Electrons are not included in the mass number because their mass is so small, and therefore have very little effect on the atom’s total mass. The carbon isotope with a mass number of 12 is called carbon-12. Knowing that the atomic number for carbon is 6, you can find the number of neutrons in carbon-12 by subtracting the atomic number from the mass number. For carbon-12, the number of neutrons is 12 – 6, or 6.

The atomic mass of an element is the weighted average of the masses of all the naturally occurring isotopes of that element. A weighted average accounts for the percentages of each isotope that are present. For example, Chlorine-35 makes up 76% of all the chlorine in nature, and chlorine-37 makes up the other 24%. To find the atomic mass of chlorine, just multiply the mass number of each isotope by its percentage abundance in decimal form, then add those amounts together:

(35 x 0.76) = 26.60

(37 x 0.24) = +8.88

_______

35.48 amu

There are forces acting within the atom (Figure 7, p. 324). These forces are the gravitational force, electromagnetic force, strong force, and weak force. Gravity is the weakest force, and so the gravitational force plays the virtually no role on scales the size of atoms, because the masses of particles in atoms are so small. The electromagnetic force holds the electrons around the nucleus. The strong force is responsible for holding the nucleus together – otherwise the protons in the nucleus would fly apart, since they are positively charged, and like charges repel each other. The weak force is an important force in radioactive atoms. In certain unstable atoms, a neutron can change into a proton and an electron – the weak force makes this possible.

Notes for Quarter 2

Information Posted on 1/18/06

(Temperature and Heat Notes for Chapter 10)

Section 1 - Temperature

Temperature is a measure of the average kinetic energy of the particles in an object. All matter is made of atoms or molecules in motion – the faster the particles are moving, the more kinetic energy they have, and the more kinetic energy the particles of the object have, the higher the temperature of the object is (see Fig. 1 p. 274).

When you measure the temperature of an object, you are measuring the average kinetic energy of all the particles in the object – the particles are moving around randomly at different speeds, and so have different amounts of kinetic energy. So the average kinetic energy of the particles is measured, and that is what is registered as a certain temperature.

A thermometer is used to measure temperature. Most thermometers are thin glass tubes filled with a liquid, usually mercury or alcohol, since they remain liquid over a wide range of temperatures (although mercury is not used as widely today because of safety concerns). Thermometers can measure temperature because of a property called thermal expansion – the increase in volume of a substance because of an increase in temperature. As a substance’s temperature increases, its particles move faster and spread out – so there is more space between them, and the substance expands. Mercury and alcohol expand by constant amounts for a given change in temperature.

There are three principle scales used to measure temperature; Fahrenheit, Celsius, and Kelvin (see Fig. 3, p. 276 – you should be familiar with the boiling and freezing points for the three scales). Absolute zero is the lowest temperature on the Kelvin scale (0 K). Absolute zero (about -459oF), is the temperature at which all molecular motion stops.

Section 2 – Heat

Heat is the energy transferred between objects that are at different temperatures. When two objects at different temperatures come into contact, energy is always transferred from the object that has the higher temperature to the object that has the lower temperature. The type of energy that is transferred is called thermal energy - the total kinetic energy of the particles that make up a substance. Measured in joules (J), thermal energy depends partly on temperature (something at a high temperature has more thermal energy), and partly on how much of a substance there is (the more particles there are in a substance at a given temperature, the greater the thermal energy of the substance is).

There are three ways of energy is transferred. Thermal conduction is the transfer of thermal energy from one substance to another through direct contact (a metal spoon in a hot bowl of soup displays conduction). Substances that conduct thermal energy well are called thermal conductors, and those that do not are called thermal insulators (see Table 1, p. 283). Convection involves the transfer of thermal energy by the circulation or movement of a liquid or gas (the rising and sinking of water during boiling is an example of convection). The third way that energy is transferred is radiation – the transfer of energy by electromagnetic waves, such as visible light, infrared, or other types of radiation in the electromagnetic spectrum). Energy from the sun is a form of radiation.

Information Posted on 1/17/06

Notes for Chapter 9 – Energy and Energy Resources

Section 1 – What Is Energy?

Energy is the ability to do work. Work is done when a force causes an object to move in the direction of the force. As Figure 1, p. 240 shows, the tennis player does work on her racket by exerting a force on it. The racket, in turn, does work on the ball, and the ball does work on the net. When one object does work on another, energy is transferred from the first object to the second object – this allows the second object to do work. So, work is a transfer of energy.

In our tennis example, energy is transferred from the racket to the ball. As the ball flies over the net, the ball has kinetic energy – energy of motion. An object’s kinetic energy can be found by the following equation:

Mv2

Kinetic energy = ----------

2

The m stands for mass in kilograms. The v represents an object’s speed – the faster something is moving, the more kinetic energy it has. Also, the greater the mass of a moving object, the greater its kinetic energy is. But notice that the speed (v) in the equation is squared. So speed has a greater effect on kinetic energy than mass does. Car crashes are more dangerous at higher speeds than at lower speeds. A moving car has 4 times the kinetic energy of the same car going half the speed! Look at the example in the Math Focus section on p. 241 to see this equation used in a real problem.

Not all energy has to do with motion. Potential energy is the energy an object has because of its position. As Figure 3 p. 242 shows, a stretched bow has potential energy – the bow has energy because work has been done to change its shape. The energy of that work is turned into potential energy. Gravitational potential energy is a special case that occurs when you lift an object. As the object is lifted, work is being done on it – you use a force that is against the force of gravity. When you do this, you transfer energy to the object and give the object gravitational potential energy. You can find gravitational potential energy by using the following equation:

Gravitational potential energy = weight x height

Mechanical energy is the total energy of motion and position of an object. Both potential energy and kinetic energy are kinds of mechanical energy. Mechanical energy can be all potential energy, all kinetic energy, or some of each (see Fig. 4, p. 243). You can find mechanical energy by using the equation:

Mechanical energy = potential energy + kinetic energy

There are other forms of energy as well (see p. 244-245 for detailed descriptions). Thermal energy is all of the kinetic energy due to random motions of the particles that make up an object. Chemical energy is the energy of a compound that changes as its atoms are rearranged. Electrical energy is the energy of moving electrons. Sound energy is caused by an object’s vibrations. Light energy is produced by the vibrations of electrically charged particles. Nuclear energy comes from changes in the nucleus of an atom.

Section 2 – Energy Conversions

Energy conversion is a change from one form of energy to another. Any form of energy can change into any other form of energy. Often, one form of energy changes into more than one form. Figure 1, p. 248 shows how the skateboarder changes from potential to kinetic energy, depending on his position in the skate arena. When he’s at the very top, his potential energy is at maximum. As he speeds through the bottom, his kinetic energy is at maximum. A rubber band provides another example of energy conversion. Stretching a rubber band takes a little effort. The energy that you put into it becomes elastic potential energy. When its let go, the rubber band snaps back to its original shape – it releases its stored up potential energy as it does so.

Many other examples of energy conversations can be put forth. Chemical energy of food is converted into kinetic energy when you are active, plus thermal energy to help maintain body temperature. In photosynthesis in plants, light energy is converted into chemical energy (Fig. 4, p. 250). The chemical energy from a tree can be changed into thermal energy if you burn it.

Energy conversions are needed for everything that we do. For example, a hairdryer (Fig. 5, p. 251) converts electrical energy into thermal energy to dry your hair. Sound energy is also produced, which you can hear. Look through Table 1, p. 251 for other examples of electrical energy conversions.

Machines of course use energy – a machine makes work easier by changing the size or direction (or both) of the force needed to do the work. As Figure 6, p. 252 shows, when using a nutcracker, some of the energy you transfer to the nutcracker is converted to sounds energy as the nutcracker transfers energy to the nut. Figure 7 shows energy conversions that occur in the act of riding a bicycle. Machines help us use energy by converting it into the form that you need.

Section 3 – Conservation of Energy

Look at Figure 1, p. 254. As the cars go up and down the hills on the track, potential energy get converted into kinetic energy, and back again. But the cars never return to the same height that they started at. But energy is not lost along the way, just converted into other forms of energy. The original potential energy of the roller coaster is lost in part to friction – a force that opposes motion between two surfaces that are touching. For the roller coaster to move, energy must be used to overcome friction. There is friction between the cars’ wheels and the track, as well as between the cars and the air around them. So because of this, not all the potential energy gets converted into kinetic energy as the cars go down the hill. The roller coaster is an example of a closed system – a group of objects that transfer energy to each other.

Energy is conserved in all cases – no exception to this has ever been found, and so this rule is described as a law. According to the Law of Conservation of Energy, energy cannot be created or destroyed – it can only be changed from one form to another. The total amount of energy in a system is always the same. See Figure 2 p. 255 for a diagram detailing energy conservation in a light bulb.

Any time one form of energy is converted into another form, some of the original energy always gets converted into thermal energy – the thermal energy due to friction that results from energy conversions is not useful energy – not used to do work. In a car, not all of the gasoline’s chemical energy makes the car move. Some wasted thermal energy will always result from the energy conversions, leaving through the radiator and tail pipe. Because of this, it is impossible to build a perpetual motion machine (Fig. 3, p. 256). Such a machine would put out exactly as much energy as it takes in.

While we can’t build machines that would run forever without any additional energy, we can strive to make machines (such as cars) more energy efficient. In terms of energy conversions, energy efficiency is a comparison of the amount of energy before a conversion with the amount of useful energy after a conversion. So a car with high energy efficiency can go farther than other cars with the same amount of gas.

Section 4 – Energy Resources

An energy resource is a natural resource that can be converted into other forms of energy in order to do more useful work. Some energy resources, called nonrenewable resources, cannot be replaced or are replaced much more slowly than they are used. Fossil fuels are the most important nonrenewable resources. Fossil fuels are energy resources that formed from the buried remains of plants and animals that lived millions of years ago. These plants stored energy from the sun by photosynthesis. Animals used and stored that energy by eating the plants. So, fossil fuels are concentrated forms of the sun’s energy (see Fig. 1, p. 258 to see how fossil fuels are formed). Today, millions of years later, energy from the sun is released when these fossil fuels are burned.

As Figure 2, p. 259 shows, fossil fuels can be used in different ways in society. Energy can be generated by burning the three main types of fossil fuels – coal, petroleum, and natural gas. Electrical energy is one of the main types of power derived, or obtained, from fossil fuels. For example, electric generators convert the chemical energy in fossil fuels into electrical energy by process shown in Figure 3, p. 260. Nuclear energy can also be used to generate electrical energy. Like fossil-fuel power plants, a nuclear power plant generates thermal energy that boils water to make steam. The steam then turns a turbine, which runs a generator. The spinning generator converts kinetic energy into electrical energy. In nuclear power plants, nuclear energy is generated from radioactive elements such as uranium. In a process called nuclear fission, the nucleus of the uranium atom is split into two smaller nuclei, which releases nuclear energy.

Renewable resources are naturally replaced more quickly than they are used. Examples of renewable resources include solar energy, energy from water, wind energy, biomass, and geothermal energy (see p. 261-262). Sunlight can be changed into electrical energy through solar cells. The potential energy of water in a reservoir can be changed into kinetic energy as the water moves through a dam. This falling water turns turbines, which are connected to a generator that changes kinetic energy into electrical energy. The kinetic energy of wind can turn the blades of a windmill. A wind turbine changes the kinetic energy of the air into electrical energy by turning a generator. Geothermal energy is thermal energy caused by the heating of the Earth’s crust. Some geothermal power plants pump water underground next to hot rock. The water turns to steam, which can then turn the turbine of a generator. Finally, biomass - organic matter such as plants, wood, and waste can be burned to release energy.

As Table 1, p. 262 shows, there are both advantages and disadvantages to using these various sources of renewable and nonrenewable energy resources to generate other forms of energy.

Information Posted on 1/6/06

Notes for Chapter 8 – Work and Machines

Section 1 – Work and Power

Work is done when a force causes an object to move in the direction of the force. Applying a force doesn’t always result in work being done – pushing a stalled car that doesn’t budge – although a tiring task – doesn’t result in any work being done because the car hasn’t moved. If the car moves, however, then it could be said that work had been done on the car – because the car would move in the same direction as the force being applied by you on it (see Fig. 2, p. 211 for both examples and non-examples of work).

Work depends on distance as well as force. Look at the climbers in Figure 4 on p. 212. The climbers on the left are walking up a slope, and those on the right are going straight up the cliff. In which case is more work being done? Well, the same amount of work is being done in both cases. Why? Because of the relationship between force and distance. The climbers who walk up the slope don’t need to use as much force as the climbers who go straight up the cliff. But the climbers walking up the slope have to go farther than the climbers going straight up the cliff. In the first case, less force being applied over a greater distance, and in the second case you have more force applied over a shorter distance. But in both cases the amount of work is the same.

The amount of work (W) done in moving an object can be calculated by multiplying the force (F) applied to the object by the distance (d) through which the force is applied:

W = F x d

Force is expressed in newtons, the meter is the basic metric unit for length or distance. So the unit used to express work is the newton-meter (N x m), or simply the joule.

Power is the rate at which energy is transferred. You calculate power (P) by dividing the amount of work done (W) by the time (t) that it takes to do that work:

W

P = ----

t

The unit used to express power is joules per second (J/s), or the watt. One watt is equal to 1 J/s. How can power be increased? If you sand a shelf by hand (Fig. 6, p. 214), then the power output is lower. An electric sander can do the same amount of work faster – therefore, the electric sander has more power.

Section 2 – What is a Machine?

A machine is a device that makes work easier by changing the size or direction of a force. While we often think of complex things such as cars and computers as machines, we use many much more simpler devices in our everyday lives that qualify as machines as well. We can understand how a machine functions in terms of two important concepts; work input and work output. The work that you do on a machine is called work input. The force that you apply to the machine through a distance is called the input force. The work done by the machine on an object is called work output. The force that the machine applies through a distance is called the output force.

It is important to understand that work output can never be greater than work input. Looking at Figure 2, p. 217, if you multiplied the forces by the distances through which the forces are applied (recall that W = F x d), you’d find that the screwdriver does not do more work on the lid (output force) than you do on the screwdriver (input force). So machines allow force to be applied over a greater distance, which means that less force will be needed for the same amount of work. Look at Figure 3, p. 218. Lifting the box straight up from the ground will require a greater input force applied over a shorter distance. Pushing it up the ramp, however, will involve less input force applied over a greater distance. But notice something significant – the same amount of work – 450J – is done.

Mechanical advantage is the number of times the machine multiplies force. Put another way, the mechanical advantage of a machine compares the input force with the output force:

Output force

Mechanical advantage (MA) = ---------------------

Input force

Using this equation, if you had to push a 500 N weight up a ramp and only needed to push 50 N of force the entire time, than the mechanical advantage of the ramp would be 10 (500 N / 50 N = 10). A machine that has a mechanical advantage greater than 1 will help move or lift heavy objects because the output force (the work that the machine does on the object) is greater than the input force (the work that you do on the machine).

Mechanical efficiency is a comparison of a machine’s work output with the work input. A machine’s mechanical efficiency is calculated using the equation:

Work output

Mechanical efficiency = ---------------- x 100

Work input

The 100 in the equation means that mechanical efficiency is expressed as a percentage. So mechanical efficiency tells you what percentage of the work input gets converted into work output. While an ideal machine would have 100% mechanical efficiency, this is unfortunately impossible. Every machine has moving parts, and these moving parts always use some of the work input to overcome friction. But new technologies continuously increase machine efficiency so that more energy is available to do useful work (see Fig. 6, p. 221).

Section 3 – Types of Machines

In this section, we looked at six simple machines, as well as machines composed of two or more simple machines called complex machines. A lever is a simple machine that has a bar that pivots at a fixed point called a fulcrum. Levers are used to apply a force to a load. There are three classes of levers, which are based on the locations of the fulcrum, the load, and the input force (see the figures on p 222-223 for examples of the three classes of levers). A pulley is a simple machine that has a grooved wheel that holds a rope or cable. A load is attached to one end of the rope, and input force is applied to the other end. There are three types of pulleys - fixed, movable, and block and tackle. Study Figure 4 on p. 224 to see how these three types of pulleys operate, the relationship between input force and output force, and the mechanical advantage is each.

The wheel and axle is a simple machine that has two circular objects of different sizes (see Fig. 5 p. 225). Doorknobs, steering wheels, and wrenches all use a wheel and axle. The mechanical advantage of a wheel and axle is the radius of the wheel divided by the radius of the axle (see Fig. 6 p. 225). An inclined plane is a simple machine that is a straight, slanted surface. A ramp would be a good example of an inclined plane. The mechanical advantage of an inclined plane is found by dividing the length of the inclined plane by the height to which the load is lifted (see Fig. 7 p. 226 – the inclined plane in the picture has a mechanical advantage of 5, making it relatively easy to push the piano into the truck).

A wedge is a pair of inclined planes that move. A knife, plows, axes and chisels are all examples of wedges. Since wedges are useful for cutting, the longer and thinner the wedge is, the greater its mechanical advantage – this is why axes and knives cut better when you sharpen them – you are making the wedge thinner. The mechanical advantage of a wedge is found by dividing the length of the wedge by its greatest thickness (see Fig. 8 p. 227). A screw is an inclined plane that is wrapped in a spiral around a cylinder, as shown in Fig. 9, p. 227). When a screw is turned, a small force is applied over the long distance along the inclined plane of the screw. Meanwhile, the screw applies a large force through the short distance it is pushed. The longer the spiral on a screw is and the closer together the threads are, the greater the screw’s mechanical advantage is. A jar lid is a screw that has a large mechanical advantage.

Compound machines consist of two or more simple machines. A block and tackle is actually a complex machine, since it consists of two or more pulleys. A can opener (Fig. 10, p. 228) is also a compound machine consisting of three simple machines: the handle is a second-class lever, the knob is a wheel and axle, and a wedge is used to open the can. The mechanical efficiency of most compound machines is low, since they have more moving parts than simple machines do, and so there is more friction to overcome.

Information Posted on 12/10/05

Notes for Chapter 7 – Forces in Fluids

Section 1 – Fluids and Pressure –

A fluid is any material that can flow and that takes the shape of a container. Because of these properties, both liquids and gases and considered to be fluids. Fluids can flow because the particles that make them up are spread farther apart than in a solid, so they can move, or flow around one another. This also makes them take the shape of whatever container they are put in.

Because of this, fluids exert pressure. A tire stays inflated because the millions of gas particles are both striking each other and the inner walls of the tire. Together, these collisions create a force on the tire. The amount of force exerted on a given area (in this case, the inner tire), is called pressure.

The SI unit for pressure is the pascal. One pascal (1 Pa) is the force of one Newton exerted over an area of one square meter (1 N/m2). Pressure can be calculated by using the equation:

force

pressure = -----------

area

Atmospheric pressure is the pressure caused by the weight of the atmosphere. It is exerted on everything on Earth. Every square centimeter of your body feels about 10 N (2 lbs) of weight caused by this atmospheric pressure. However, like air inside a balloon (see Fig. 2, p. 181), fluids in your body also exert a pressure to counteract atmospheric pressure – this is why you don’t feel the crushing weight of the air above. Atmospheric pressure varies with altitude (how high or low you are with respect to Earth’s surface. Pressure is greatest at sea level, because the full pressure of the atmosphere is being exerted on you. High atop a mountain, however, the atmospheric pressure is about 1/3 of that at sea level (see Fig. 3, p. 182). If you travel to higher or lower points in the atmosphere, the fluids in your body have to adjust to maintain equal pressure (this is what happens anytime your ears ‘pop’ – because of pressure changes in pockets of air behind your eardrums). Atmospheric density also decreases as you increase in altitude. This is why airplanes have to be pressurized so that there is enough oxygen for people to breath comfortably. High above sea level, oxygen molecules are farther apart, therefore the need to provide a means for additional oxygen, whether they be on an airplane, or climbing a mountain.

Water is a fluid, and so exerts pressure just like the atmosphere does. A diver, then, experiences not only the force of water pressure, but also atmospheric pressure, since atmospheric pressure presses down on the water that they are swimming in. Water pressure does not depend on the amount of fluid present – a swimmer would feel the same pressure swimming 3 m below a small pond and at 3 m below the ocean surface. Because water is about 1,000 times denser than air, water exerts more pressure than air does (see Fig. 4, p. 183).

Fluids flow from areas of high pressure to areas of low pressure. We see this principle at work, for example, in the action of drinking from a straw – pressure is reduced in the straw when air is removed when you drink from it. The outside pressure forces the liquid up the straw and into your mouth. Also, when breathing, the pressure in your lungs becomes lower than the pressure outside your lungs. Air then flows into your lungs (see Fig. 5, p. 184). A tornado’s air pressure is actually very low. They tend to suck up material around them, however – the pressure outside the tornado is higher than inside, and so objects are pushed into the tornado.

Section 2 – Buoyant Force –

Buoyant force is the upward force that fluids exert on all matter. When an object is immersed in water, the water exerts fluid pressure on all sides of the object. The pressure exerted horizontally on either side of the object is equal – therefore the pressures cancel out one another. The only fluid pressures affecting the net force on the object are at the top and bottom (see Fig. 1, p. 186). Since pressure increases as depth of water increases, the pressure at the bottom of the object is greater than the pressure at the top. The water, therefore, exerts a net upward force – buoyant force – on the object.

The ancient Greek mathematician Archimedes discovered how one could determine buoyant force. Archimedes’ principle states that the buoyant force on an object in a fluid is an upward force equal to the weight of the fluid that the object takes the place of, or displaces.

An object in a fluid will sink if its weight is greater than the buoyant force (the weight of the fluid it displaces). An object floats only when the buoyant force on the object is equal to the object’s weight (see Fig. 2, p. 187). An object sinks when its density is greater than that of water, which has a density of 1 g / cm3, and an object floats if its density is less than one. We use the following equation to compute the density of an object or substance:

mass

density = --------------

volume

Most substances are denser than air, so few substances float in the air. One exception is the element helium, which has a density one-seventh that of air. A given volume of helium displaces an equal volume of air that is much heavier than itself – so helium floats in air, which is why it’s used in parade balloons (Fig. 3, p. 188).

We can change the overall density of an object by changing its shape. For example, ships are built with a hollow shape. So although the amount of steel in the shape is the same as there would be if the steel was in the shape of a block, the hollow shape increases the volume the ship, so following the above equation, increasing the ship’s volume leads to a decrease in its density. Thus, ships made of steel float because their overall density is less than that of water (see Fig. 4, p. 189).

We can also change an object’s mass to control its overall density. As Figure 5, p. 190 shows, a submarine can stay afloat when its ballast tanks are filled mostly with air (which is about 1000 times less dense than water). When the crew want the submarine to descend to a certain depth, the ballast tank vent holes are opened, and water fills them. The submarine’s mass is increased, although its volume stays the same. From the above equation, the submarine’s density in this situation is increased, and so it sinks down into the water. When the submarine rises toward the water’s surface, compressed air pumped into the tanks forces the water out, thereby decreasing the mass, and decreasing the density – which allows the vessel to rise.

Similar to a submarine, certain fish adjust their overall density to stay at a certain depth in the water. Fish that have a swim bladder (Fig. 6, p. 191) can fill it with gases, thereby inflating the bladder. This increases the fish’s volume and thereby decreases the fish’s overall density, which keeps the fish from sinking in the water.

Section 3 – Fluids and Motion –

Bernoulli’s principle states that as the speed of a moving fluid increases, the fluid’s pressure decreases. In the class demonstration of this principle with the two pieces of paper, air speed between the two sheets increased when air was blown between them. Because the air speed was increased, the pressure between the two sheets decreased. Thus, the higher pressure on the outside of the sheets pushed them together.

There are many other examples that demonstrate Bernoulli’s principle. Look at Figure 1, p. 192, which shows a tennis ball attached to a string and swung into a stream of water. You might think that the ball would be pushed out of the water, but instead it is held there. The reason for this is that the water is moving faster than the air around it, so the water has a lower pressure than the surrounding air. The higher air pressure pushes the ball into the area of lower pressure – the water stream.

Airplanes and birds in flight also demonstrate Bernoulli’s principle. As Figure 2,p. 193 shows, since air is moving slower beneath the plane’s wing, the greater pressure below the wing exerts an upward force. This upward force is called lift, and it pushes the wings (and the rest of the aircraft or bird) upward against the downward pull of gravity. While the amount of lift created by a plane’s wing is determined partly by the speed at which air travels around the wing, the speed of a plan is determined mostly by its thrust – the forward force produced by the plane’s engine.

Lift also depends partly on the size of a plane’s wings. As Figure 3, p. 194 shows, the large thrust of the jet pushes it through the air at great speeds. But the glider doesn’t have an engine. It does, however, have a large wing area. So its large wings create the lift it needs to stay in the air. A similar situation exists with bird wings. A bird with small wings has to flap them quickly to stay in the air. But a hawk only flaps them occasionally because it has large wings. When fully extended, the hawk can fly with little effort, gliding on wind currents and generating the lift needed to stay in the air.

Bernoulli’s principle even extends into the world of sports, as Figure 4, p. 195 shows in the case of the infamous pitch known as the screwball. Because the ball is spinning clockwise, the direction of air flow on the left is moving in the opposite direction, and so moves slower than the air on the right of the ball, which is moving in the same direction of the ball’s spin. So the lower air speed on the left side of the ball translates into greater pressure on the left side of the ball, causing it to be pushed to the right side.

If you’ve ever walked against the force of a strong wind, you probably felt like you were being pushed backward. Fluids (in this case air) exert a force that opposes the motion of objects moving through the fluids. Drag is the force that opposes or restricts motion in a fluid. Drag can also work against the forward motion of a plane or bird in flight. Drag is usually caused by an irregular flow or air called turbulence. To help deal with turbulence, plans are equipped with flaps like those shown in Figure 5, p. 196.

Pascal’s principle states that a change in pressure at any point in an enclosed fluid will be equally transmitted to all parts of that fluid. So if a local water pumping station increased the water pressure by 20 Pa, the water pressure will be increased the same at a home 2 km away as that of a store a few blocks away from the station. Hydraulic devices use Pascal’s principle to move or lift objects. Since liquids cannot be easily squeezed or compressed into a small space, they are often used in hydraulic devices such as those found in cranes and forklifts. Hydraulic devices can also multiply forces, as in the case of car brakes (see Figure 6, p. 197).

Information Posted on 11/29/05

Notes for Chapter 6 – Forces and Motion

Section 1 – Gravity and Motion –

Challenging the Greek philosopher Aristotle, who taught that the rate at which an object falls was dependent on its mass, the Italian scientist Galileo dropped two cannonballs of different masses from the Leaning Tower of Pisa in Italy. The result? Disproving Aristotle, the cannonballs hit the ground at the same time. So objects fall to the ground at the same rate because the acceleration due to gravity is the same for all objects.

Acceleration is the rate at which velocity changes over time. The acceleration of an object is the object’s change in velocity divided by the amount of time during which the change occurs. All objects accelerate toward Earth at a rate of 9.8 meters per second per second. This rate is written as 9.8 m/s/s, or 9.8 m/s2.

To find the velocity of falling objects, the following equation is used:

Δv = g x t

In the equation, (Δv) is the change in velocity, (g) is the acceleration due to gravity, and (t) is the time the object takes to fall in seconds. See p. 151 for a sample problem.

An object stops accelerating only when the upward force of air resistance is equal to the downward force of gravity (so the net force is 0 N). The object then falls at a constant velocity called the terminal velocity.

The force of air resistance pushes up on a falling object, such as apple (Fig. 3, p. 152). The amount of air resistance acting on an object depends on the size, shape, and speed of the object. For example, air resistance would affect a flat sheet of paper more than a crumpled one.

Only objects that are only acted on by the force of gravity can be said to be in a state of free fall. So this would apply to objects orbiting the Earth, such as the space shuttle (see Fig. 7, p. 154). As this diagram shows, two motions combine to cause orbiting – the forward movement, and the object’s free fall toward Earth.

Besides satellites and spacecraft, many natural objects are in orbit in the universe, such as moons around planets and planets around stars. Any object in circular motion is constantly changing direction, and to do this an unbalanced force is needed. The unbalanced force that causes objects to move in a circular (or near circular) path is called centripetal force. Centripetal means “toward the center.”

Projectile motion is the curved path that an object follows when thrown, launched, or otherwise projected near the surface of the Earth. Projectile motion has two components – horizontal motion and vertical motion. The two components have no effect on each other. When the two motions are combined, they form a curved path, as shown in Fig. 9, p. 155.

Section 2 – Newton’s Laws of Motion –

Newton’s First Law of Motion:

An object at rest remains at rest, and an object in motion remains in motion at constant speed and in a straight line unless acted on by an unbalanced force. Any object that is not moving is said to be at rest. A golf ball sitting on a tee will remain there unless acted on by an unbalanced force (the golf club). The second part of the law states that a moving object will continue to move forever unless acted on by an unbalanced force that will stop its motion. That unbalanced force is friction. Friction will, for example, make a car slow down, and will cause a rolling ball to slow down and stop.

The tendency of objects to remain in motion or stay at rest is call inertia. The more massive an object the more inertia it has. Thus the inertia of an object is related to its mass (see Fig. 3, p. 160).

Newton’s Second Law of Motion:

The acceleration of an object depends on the mass of the object and the amount of force applied. In the first part, acceleration depends on mass. For example, you only have to exert a small force on an empty cart to accelerate it. But the same amount of force will not accelerate a full cart as much as the empty cart – you’d have to apply more force to accelerate the full cart (see Fig. 4, p. 161). So Newton’s second law of motion shows how force, mass and acceleration are related. The equation that is used to express this relationship is:

F

F = m x a or a = --------

m

Newton’s Third Law of Motion:

Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.

So the third law of motion states that for every action, there is an equal and opposite reaction - every force must have an equal and opposite reaction. So forces act in pairs – action and reaction force pairs are present even when no motion is taking place, as in the force you exert on a chair when you sit on it. Your weight pushing down on the chair is the action force. The reaction force is the force exerted by the chair that pushes up on your body. The force is equal to your weight. For more examples of this principle, see Figures 6 & 7, p. 163-164.

Section 3: Momentum – The momentum of an object depends on the object’s mass and velocity. The more momentum an object has, the harder it is to stop the object or change its direction. For example, imagine a compact car and a large truck traveling with the same velocity. The drivers of both vehicles put on the brakes at the same time. Which vehicle will stop first? Because the truck has more mass and more momentum than the car has, a larger force is needed to stop the truck. So the compact car will stop first.

Momentum (p) can be calculated with the equation:

p = m x v

In the equation, m is the mass of an object in kilograms, and v is the object’s velocity in meters per second (see the example at the top of p. 167).

The Law of Conservation of Momentum states that any time objects collide, the total amount of momentum stays the same. Look at Figure 2 on p. 167. The white cue ball had a certain amount of momentum before the collision. During the collision, the cue ball’s momentum was transferred to the red billiard ball. After the collision, the billiard ball moved away with the same amount of momentum the cue ball had. So this example shows how momentum is conserved, or stays the same.

Sometimes objects stick together after a collision – like the football players on p. 168. After two objects stick together, they move as one object. The mass of the combined object is equal to masses of the two objects added together. In a head on collision, the combined objects move in the direction of the object that had the greater momentum before the collision. But together, the objects have a different velocity than the velocity of either object before the collision. The objects have a different velocity because momentum is conserved and depends on mass and velocity. When mass changes, velocity must change too.

Sometimes objects bounce off of each other in collisions - for example, the bowling ball and pins shown in Figure 3, p. 168. During these types of collisions, momentum is usually transferred from one object to another object. This transfer of momentum causes the objects to move in different directions at different speeds. The total momentum, however, of all the objects will remain the same before and after the collision.

Conservation of momentum can be explained by Newton’s third law of motion, as Figure 4, p. 169 shows. The action force (the cue ball hitting the billiard ball with a certain amount of force) makes the billiard ball begin moving, and the reaction force (the equal but opposite force exerted by the billiard ball on the cue ball) stops the ball’s motion.

Information Posted on 11/14/05

Notes for Chapter 5 – Matter in Motion

Section 1 – Measuring Motion –

We often think of the motion of an object as something easy to detect – you just watch the object. But you are actually watching the object in relation to another object that stays in place. When an object changes position over time relative to a reference point, that object is in motion.

Speed is the distance traveled by an object divided by the time taken to travel that distance. To determine average speed, the following equation is used:

Total distance

Average speed = ---------------------

Total time

The SI (Metric) unit for speed is meters per second (m/s). However, kilometers per hour (km/h), feet per second (ft/s), and miles per hour (mi/h) are other units commonly used to express speed.

Velocity is the speed of an object in a particular direction. It is important to remember that speed and velocity are not the same. For example, two birds might leave a tree at the same speed – 10 km/h for 5 minutes, 12 km/h for 8 minutes, and 5km/h for 10 minutes, but not end up in the same place. Why? They went in different directions. So the speeds were the same, but they had different velocities.

Velocity must include a reference direction. To say that an airplane’s velocity is 600 km/h would not be correct. You would have to include a reference direction, such as 600 km/h south. NOTE: See Fig. 3 & 4 on p. 121.

Acceleration is the rate at which velocity changes. Velocity changes if speed changes, if direction changes, or if both change. So, an object accelerates if its speed, its direction, or both change.

• If an object is speeding up, we say that the object has a positive acceleration.

• If an object is slowing down, we say that the object has a negative acceleration.

• You can find the average acceleration by using the equation:

Final velocity – starting velocity

Average acceleration = ----------------------------------------------

Time it takes to change velocity

*The SI (Metric) units for Acceleration:

• Velocity is measured in meters per second (m/s)

• Time is measured in seconds (s)

Final velocity – starting velocity m/s

Average acceleration = ---------------------------------------------- -----

Time it takes to change velocity s

• The units are meters per second ÷ second which is meters per second per second or meters per second squared.

• The SI unit meters per second per second mean that an object speeds up by a certain velocity every second.

• Example: If a cyclist speeds up to 5m/s2 from 1 m/s2 in 4 s, they have increased their speed by 1 m/s every second, or have an acceleration of 1 m/s2.

NOTE: See Fig. 5 on p. 122.

Section 2 – What is a Force?

• In science, a force is simply a push or a pull. Put in a more detailed way, a force is a push or pull exerted on an object in order to change the motion of the object.

• Forces cause changes in three things:

– Shape

– Direction

– Speed (causing acceleration or deceleration)

The SI unit for force is the Newton (N). This unit is names after the famous scientist Sir Issac Newton.

Usually, more than one force is acting on an object. The net force is the combination of all forces acting on an object. To determine the net force on an object if all forces act in the same direction, you add the two forces together. If one person pushes with a force of 25N and the other pulls in the same direction with a force of 20N, then you simply add these numbers to find the net force (25N + 20N = 45N). See Fig. 3, p. 125. If the two forces are opposing one another, as the two dogs pulling on a rope in Fig. 4 p. 126, then you subtract the two opposing forces: 12N – 10N = 2N.

Forces act on all objects at all times. They can be balanced or unbalanced. Balanced forces will not cause a change in the motion of a moving object. Why? Because when the forces on an object produce a net force of 0N, then the forces are said to be balanced (See Fig. 5, p. 126). On the other hand, when the net force on an object is not 0N, then the forces on an object are unbalanced (See Fig. 127, p. 127).

Section 3 – Friction- A Force That Opposes Motion

Friction is a force that opposes motion between two surfaces that are in contact. Basically, when the microscopic hills and valleys of one surface stick to the hills and valleys of another surface, friction is created.

Rougher surfaces have more microscopic hills and valleys than smooth surfaces do. So the rougher the surface is, the greater the friction.

There are two types of friction. If you slide a stack of books across a table, you are witnessing kinetic friction. Kinetic means “moving,” so the amount of kinetic friction between two surfaces depends in part on how the surfaces move. Surfaces can slide past each other, or a surface can roll over another surface. Usually, the force of sliding kinetic friction is greater than the force of rolling kinetic friction – which is why it’s easier to move a piece of heavy furniture on wheels rather than just sliding it across the floor (See Fig. 3, p. 130). When a force is applied to an object but does not cause the object to move, static friction occurs. Static means “not moving.” Suppose you try to push a stack of heavy books across a table with your finger. The books don’t move because the force of static friction balances the force applied by your finger (See Fig. 4, p. 131).

Friction can be both helpful and harmful. Without friction, the tires on your car could not push against the ground to move the car forward, and the brakes could not stop the car – so this is a good example of friction being helpful. On the other hand, friction between moving engine parts causes the parts to eventually wear down, so this is an example of friction being harmful.

Friction can be reduced. You can reduce the amount of friction by using lubricants such as oil, wax, or grease. Friction can also be reduced by switching from sliding kinetic friction to rolling kinetic friction (as in the example of moving heavy furniture). Still another way to reduce friction is to make surfaces that rub against each other more smoothly. Sanding a park bench will make the bench smoother. Therefore, the bench will be more comfortable now to sit on because the friction between your leg and the bench is reduced.

Friction can be increased. One way to make friction increase is to make surfaces rougher. For example, sand scattered on icy roads keeps cars from skidding. You often see baseball players using textured batting gloves to increase friction between the bat and their hands so that the bat won’t fly out of their hands when they swing.

Section 4 – Gravity: A Force of Attraction

Gravity is a force of attraction between objects that is due to their masses. All matter has mass, and gravity is a result of mass. Therefore, all matter is affected by gravity. Put another way, all objects experience an attraction toward all other objects. You usually don’t notice objects moving towards each other because the mass of most objects is too small to cause a force large enough to move objects toward each other. Of course, we are all familiar with one object massive enough to cause a noticeable attraction on other objects – Earth. Earth has a huge gravitational force, and pulls everything on its surface toward the center. Because of this force, books, tables and chairs stay in one place, and dropped objects fall to Earth rather than moving together or toward you.

Sir Isaac Newton realized that the force of gravity could answer two basic questions that people had asked for thousands of years: Why do objects fall toward Earth, and what keeps the planets moving in the sky? Newton summarized his ideas in a law now known as the law of universal gravitation, which states that all objects in the universe attract each other through gravitational force.

The Law of Universal Gravitation can be broken into two parts:

Part 1: Gravitational force increases as mass increases – Astronauts bounce when they walk on the moon? Why? Because the moon’s mass is about 1/6 that of the Earth. So the moon has less mass, which causes less of a gravitational pull on the astronaut’s body. Gravitational force is small between objects that have small masses, and gravitational force is large when the mass of one of both objects is large (see Fig. 3, p. 136).

Part 2: Gravitational force decreases as distance increases – If you jump up, you are immediately pulled down to the surface of the Earth by the Earth’s gravitational force. On the other hand, the Sun is more than 300,000 times more massive than Earth. However, the Sun’s gravitational force doesn’t affect you more than Earth’s because the Sun is so far away (93 million miles)! If, however, you could stand on the Sun, you would find it impossible to move, because the gravitational force acting on your body would be so great that you could not move any part of your body. So, in summary gravitational force is strong when the distance between two objects is small. If, however, the distance between two objects increases, then the gravitational force pulling them together decreases rapidly.

Weight and mass, while related to one another, is not the same thing! Mass is the amount of matter in an object. It does not change if you change your location, even if it’s on another planet or moon. Weight, however, is a measure of the gravitational force on an object. So an astronaut on the moon will weigh about 1/6th his or her weight on Earth, but his or her mass remains constant – it doesn’t change (see Fig. 6, p. 138). We learned in an earlier section that the SI unit of force is a newton (N). Since gravity is a force, it too is measured in newtons. The SI unit for mass is kilograms (kg). Mass is also measured in grams (g) and milligrams (mg) as well.

Notes for Quarter I

Information Posted on 10/19/05

Notes for Chapter 4 – Elements, Compounds, and Mixtures

Section 1 – Elements –

• An element is a pure substance that cannot be separated into simpler substances by physical or chemical means.

• A pure substance is a substance in which there is only one type of particle.

ex. – a sample of gold is a pure substance, since every atom in the sample is like every other gold atom.

Properties of Elements -

• Each element has its own characteristic properties

• Physical properties would include such things as melting point, density, and boiling point.

• Chemical properties would include reactivity with another substance, flammability, etc.

• Elements may share properties with other elements (ex.-helium & krypton are both unreactive gases but have different densities)

Identifying Elements by Their Properties –

• Each element can be identified by its own unique properties

• Examples of element identification by chemical properties – zinc is reactive with acid; hydrogen and carbon are flammable

• Examples of element identification by physical properties – sulfur is yellow (color), aluminum is malleable

Classifying Elements by Their Properties –

• Elements are grouped into categories by the properties they share

• Elements are classified as metals, nonmetals, or metalloids

Metals –

• Metals are shiny, and conduct heat and electric current

• Metals are also malleable (can be hammered into thin sheets), and ductile (can be drawn through a wire)

• Examples of metals are lead, copper, and tin.

Nonmetals -

• Nonmetals do not conduct heat or electric current, and solid nonmetals are dull (not shiny) in appearance, and may be brittle and unmalleable.

• Examples of nonmetals are iodine, sulfur, neon, and xenon.

Metalloids –

• Metalloids have properties of both metals and nonmetals – some are shiny, some are dull.

• Metalloids are somewhat malleable and ductile.

• Some metalloids conduct heat and electric current well, but some do not.

• Examples of metalloids include boron, antimony, and silicon

Section 2 – Compounds – A compound is a pure substance composed of two or more elements that are chemically combined.

• There are many examples of compounds that we encounter every day. The elements sodium and chlorine, for example, combine to form table salt – sodium chloride. Iron and oxygen combine to form rust – iron oxide.

• Compounds can be identified by their physical properties (melting point, density, color, etc.), as well as chemical properties (reactivity with acid, reactivity to light, etc.).

• It is important to understand that a compound has properties that differ from those of the elements that form it. For example, take sodium chloride. When we look at the elements that form it, we have sodium – a soft, silvery-white metal that reacts explosively if brought into contact with water. Chlorine is a poisonous, green gas. But when we chemically combine these two elements, a harmless compound is formed that has unique properties – properties that differ from the elements that formed it.

• Some compounds can be broken down into their elements by chemical changes by applying heat or electric current. For example, you can heat the compound mercury oxide, and a chemical change occurs, causing it to separate into its component elements mercury and oxygen. Other compounds break down to form simpler compounds instead of elements. For example, carbonic acid (found in soda) breaks down into the compounds carbon dioxide and water.

• Compounds are found in nature, but often the compounds found in nature are not the raw materials needed by industry. Ammonia is a common compound used in industry, used to make fertilizers. Combining the elements nitrogen and hydrogen makes ammonia. Many examples of compounds can be found in the natural world. Proteins, for example, are compounds found in all living things. Carbon dioxide is another important compound in nature, and is of central importance in the process of photosynthesis – the means by which plants make food and release oxygen into the atmosphere.

Section 3 – Mixtures – A mixture is a combination of two or more substances that are not chemically combined. So when two or more materials are put together they form a mixture if they do not react chemically to form a compound. For example, cheese and tomato sauce do not react when they are used to make a pizza – so a pizza is a mixture. A salad (with lettuce, tomatoes, onions, etc.) would be another common example of a mixture.

• You can sometimes easily separate mixtures through physical methods – for example, taking mushrooms off of a pizza. Other mixtures are not so easily separated. For example, you can’t pick the salt out of a saltwater mixture. Although you could heat the saltwater mixture, evaporating the water and leaving the salt behind (see p. 99 for other ways to separate mixtures).

• Whereas a compound is made of elements in a specific mass ratio, the components of a mixture do not need to be mixed in a definite ratio. For example, granite is a mixture made of three minerals, feldspar, mica, and quartz. Even though the proportions of these minerals may change, this combination of minerals is always a mixture called granite.

A solution is a mixture that appears to be a single substance. It is composed of particles of two or more substances that are distributed evenly among each other. Solutions have the same appearance and properties throughout the mixture.

• The process in which particles of substances separate and spread evenly throughout a mixture is called dissolving. The solute is the substance that is being dissolved, and the solvent is the substance in which the solute is dissolved. For example, salt water is a solution. Salt is soluble in water, meaning salt dissolves in water. So salt would be the solute, and water the solvent.

• Solutions can be liquids (ex. Gasoline, soft drinks, many cleaning supplies, etc.). However, solutions may also be gases, such as air, or even solids, such as steel. Alloys are solid solutions of metals or nonmetals dissolved in metals (brass is an alloy of the metal zinc dissolved in copper).

• Particles in solutions are so small that they never settle out. They also cannot be removed by filtering (see, for example, figure 4 on p. 101).

• A measure of the amount of solute dissolved in a solvent is concentration. Concentration is expressed in grams of solute per milliliter of solvent (g/mL). Solutions can be described as being concentrated or dilute. These terms, however, do not tell you the amount of solute that is being dissolved. In Figure 5 on p. 102, the two solutions have the same amount of solvent, but one has less solute than the other. Accordingly, one is dilute, and the other concentrated. So a dilute solution will contain less solute, and a concentrated solution will contain more solute.

• The solubility of a solute is the ability of the solute to dissolve in a solvent at a certain temperature. Most solids are more soluble in liquids at higher temperatures, but gases become less soluble in liquids as the temperature is raised.

• A suspension is a mixture in which particles of a material are dispersed throughout a liquid or gas but are large enough that they settle out.

• Some mixtures have properties between those of solutions and suspensions – these mixtures are known as colloids. A colloid is a mixture in which the particles are dispersed throughout but are not heavy enough to settle out. So the particles in a colloid are small and well mixed. Milk, mayonnaise, and whipped cream are all examples of colloids.

Information Posted on 10/10/05

Notes for Chapter 3 – States of Matter

Section 1 – Three States of Matter – The three states of matter that we will focus on are solid, liquid, and gas (a fourth state, plasma, occurs at very high temperatures, such as those found within stars like the Sun. Electrons are stripped away from their parent atoms in a plasma). States of matter are the physical forms in which a substance can exist. Water, for example, commonly exists in three states: solid (ice), liquid (water), and gas (steam). Solids have a definite shape and volume. There are two kinds of solids – crystalline and amorphous. Crystalline solids have a very orderly, 3-D arrangement of particles (ex. Diamonds, iron and ice). Amorphous solids are made of particles that do not have a special arrangement (ex. Glass, rubber and wax). Liquids have a definite volume, but they take the shape of the container that they are in. A special property of liquids is surface tension – a force that acts on the particles at the surface of a liquid. This is what causes some liquids to form spherical drops like dew on grass. Another important property of liquids is viscosity – a liquid’s resistance to flow. Generally, the stronger the attraction between molecules that make up a liquid, the more viscous it is (honey has a higher viscosity than water). Gas is the state of matter that has no definite shape or volume. The particles of a gas move quickly, so they can break away completely from one another. The particles of a gas move quickly, so they can break away completely from one another. So the particles of a gas have less attraction between them than do particles of the same substance in the solid or liquid state.

Section 2 – Behavior of Gases – Gases behave differently from solids or liquids, in that the particles that make up gases are spaced widely apart. To understand gas behavior, we have to understand the relationship between temperature, volume, and pressure. Temperature is a measure of how fast the particles in an object are moving. If you take a balloon and put it outside on a hot day, you will notice that the balloon will expand. This is because the heat will causes the particles in the balloon to move faster – they collide with the inner walls of the balloon with greater force, causing the balloon to grow in size. But on a cool day, the particles of gas in the balloon have less energy, and so do not push as hard on the walls of the balloon. Volume is the amount of space that an object takes up. Because the particles of a gas are spread out, the volume of any gas depends on the container that the gas is in. Gas particles can be compressed much more easily than particles of a liquid, which is why you can bend and twist a balloon filled with air, but you cannot do so with one filled with water – the balloon would break! Pressure is the amount of force exerted on a given area of surface. You can also think of it as the number of times the particles of a gas hit the inside of their container. A basketball has a higher pressure than a beach ball because inside the basketball, there are more particles of gas in it, and they are closer. The particles collide with the inside of the ball at a faster rate. The beach ball has a lower pressure, on the other hand, because there are fewer particles of gas, and they are farther apart. The particles in the beach ball collide with inside of the ball at a slower rate, thereby leading to lower pressure.

Robert Boyle, a 17th century Irish chemist, described the relationship between the volume and pressure of a gas. Boyle’ Law states that the volume of a gas is inversely proportional to the pressure of a gas when temperature is constant (see Fig. 3 on p. 72). So as pressure increases, the volume of a gas decreases, and as pressure is decreased, the volume of a gas increases. Charles’ Law states that the volume of a gas is directly proportional to the temperature of a gas when pressure is constant (see Fig. 4 on p. 73). So decreasing the temperature of a gas causes the particles to move more slowly, thereby leading to a decrease in volume. Increasing the temperature of the gas causes particles to move more quickly, thereby leading to a greater volume.

Section 3 – Changes of State – A change of state is the change of a substance from one physical form to another (ice to liquid to gas, for example). All changes of state are physical changes – the identity, or chemical composition never changes. The particles of a substance move differently, and have different amounts of energy, depending on what state it is in. Particles in liquid water have more energy than particles in ice, and so move faster. Particles in steam have more energy still, and so move even faster. In order to change a substance from one state to another, energy must be added or removed. Melting is the change of state from solid to liquid. By adding energy to an ice cube, for example, the temperature of the ice cube is increased, and the ice particles move faster. When a certain temperature is reached, the ice will melt. A substance’s melting point is that temperature at which the substance changes from a solid to a liquid. For a solid such as ice to melt, particles must overcome some of their attractions to each other. When a solid is at its melting point, and energy added to it is used to overcome the attractions that hold the particles in place. So melting is an endothermic reaction – because energy is gained by the substance as it changes state.

Freezing is the change of state from a liquid to a solid. The temperature at which a liquid changes into a solid is the liquid’s freezing point. Freezing is the reverse process of melting; thus freezing and melting occur at the same temperature (see Fig. 3, p. 75). So if energy is added at 0 degrees Celsius, the ice will melt, because any energy added goes into breaking the bonds of attraction of the particles. If energy is removed at 0 degrees Celsius, the liquid will freeze, because removing energy will cause the particles to begin locking into place. So freezing is an exothermic reaction, because energy is removed from the substance as it changes state.

Evaporation is the change of a liquid to a gas. It is possible for evaporation to occur at the surface of a liquid that is below its boiling point (leave a glass of water out and you will notice that the water eventually evaporates). So in the process of evaporation, some particles at the surface of the liquid move fast enough to break away from the particles around them and become a gas. Boiling is the change of a liquid to a vapor, or gas, throughout the liquid (see Fig. 4, p. 76). This is an important point – evaporation occurs at the surface of a liquid, and boiling occurs throughout the liquid. Boiling occurs when the pressure inside the bubbles (called vapor pressure) equals the outside pressure on the bubbles (called atmospheric pressure). The temperature at which a liquid boils is its boiling point. Water boils at 100 degrees Celsius at sea level. However, in Denver, about 1.6 km above sea level, the atmospheric pressure is lower, since there is less air above you. Therefore, since there is less atmospheric pressure, the boiling point of water will be slightly less – about 95 degrees Celsius.

Condensation is the change of state from a gas to a liquid. Condensation and evaporation are the reverse of each other. The condensation point of a substance is the temperature at which the gas becomes a liquid. And the condensation point is the same temperature as the boiling point at a given pressure. Sublimation is the change of state in which a solid changes directly into a gas. Dry ice (solid carbon dioxide) is much colder than ice made from water.

When most substances gain or lose energy, one of two things happen to the substance: its temperature changes or its state changes. The temperature of a substance, remember, is a measure of the speed of the particles that make up the substance. So when the temperature of a substance changes, the speed of the particles also changes. But the temperature of a substance does not change until the change of state is complete. For example, the temperature of boiling water stays at 100 degrees Celsius until it has all evaporated (see Figure 7, p. 79).

Information Posted on 10/02/05

Notes for Chapter 2 – The Properties of Matter

Section 1 – What is Matter? – Matter is anything that has mass and takes up space. The amount of space take up, or occupied, by an object is known as the object’s volume. The volume of liquids is most often expressed in the units liters (L) and milliliters (mL). A graduated cylinder is used to measure the volume of a liquid. Because the surface of a liquid in any container is curved, to properly read the volume of a liquid in a graduated cylinder one must look at the bottom of the curve of the meniscus (see p. 39 in text). To measure the volume of a solid object, the units must be expressed in cubic units (‘cubic’ means having three dimensions). A block of wood, therefore, has three dimensions – height, width, and length. To find the volume of such an object, one simply multiplies these three dimensions:

V = l x w x h

So the block of wood might have a length of 14 cm, a width of 5 cm, and a height of 3 cm. Multiplying each of these values gives one the volume of the block of wood. Or, a graduated cylinder can be used to measure the volume of a solid object by water displacement – subtracting the water level after an object has been dropped into a graduated cylinder from the original water level. So, if a graduated cylinder is filled to the 80 mL line, and a small lead weight is dropped in, the water line might rise to 120 mL. So to find the volume of the lead, one simply subtracts new from original:

V = 120 mL – 80 mL = 40 cubic centimeters

Mass is the amount of matter in an object. Weight is the measure of the gravitational force exerted on an object. Going to the moon won’t change your mass, but your weight will be 1/6 of what it is on the Earth, because the moon is 1/6 the size of Earth, and therefore it will exert a much weaker gravitational pull on an object at its surface. Inertia is the tendency of an object to resist a change in its motion. So, an object at rest will stay at rest unless acted on by an outside force, and an object moving will keep moving at the same speed and direction unless something acts on it to change its speed or direction. Inertia is the tendency of an object to resist a change in its motion – an object will remain at rest until something causes it to move. Likewise, a moving object will keep moving at the same speed and in the same direction unless something acts on it to change its speed or direction. A more massive object will therefore have greater inertia – pushing a loaded grocery cart is harder than pushing an empty one.

Section 2 – Physical Properties – A physical property of matter can be observed or measured without changing the matter’s identity. So, you don’t have to change an orange’s identity to see its color or measure its volume. Density is one physical property that describes the relationship between mass and volume, and is found by the formula:

M

D = ______

V

Other physical properties include conductivity, state, solubility, ductility, and malleability. Physical changes do not produce new substances – crushing a can, tearing a piece of paper, or melting an ice cream bar are all physical changes – no change to the original substance (in terms of its chemical makeup) was made – it’s still a can, a piece of paper, and an ice cream bar – only its size or appearance has changed, or state.

Section 3 – Chemical Properties – Chemical properties describe matter based on its ability to change into new matter that has different properties. Some chemical properties include flammability – the ability of a substance to burn - and reactivity – the ability of two or more substances to combine and form one or more new substances. Burning paper produces heat and smoke – new substances. An iron nail can react with oxygen in the air to form iron oxide, or rust – these are examples of chemical changes – when one or more substances are changed into new substances that have new and different properties. These two terms are not the same – chemical properties of a substance describe which chemical changes will and will not occur. Chemical changes are the process by which substances actually change into new substances. The key question to ask when trying to determine whether a physical or chemical change has occurred is: Has a new substance been formed? Did the original composition change? Many physical changes can be reversed – an ice cube that has melted can be turned back into solid ice by freezing the liquid water again. However, most chemical changes are not easily reversed.

Information Posted on 9/22/05

Notes for Chapter 1 – The World of Physical Science

Section 1 – Exploring Physical Science – Focuses on the nature of science, which is a process of gathering knowledge about the natural world. When we make observations of phenomena, we are prompted to ask questions-the beginning of scientific inquiry.

Physical science is defined as the study of matter and energy. It is divided into the study of physics and chemistry. Physics looks at energy and the way that energy affects matter. Chemistry studies the structure and properties of matter and how matter changes.

**Knowledge of physical science is important for many areas of science, such as geology, meteorology, and biology.

Section 2 – Scientific Methods – Scientific methods are the ways in which scientists answer questions and solve problems. There are certain steps that scientists use whenever they are engaged in scientific inquiry. First, an observation is made – this is any use of the senses to gather information (for example, noting that the sky is blue, or that a cotton ball feels soft). Scientists are then led to ask questions about their observations. After gathering preliminary information, scientists are then ready to form a hypothesis – a possible explanation or answer to a question. A good hypothesis is always testable. In other words, information can be gathered or an experiment can be designed to test the hypothesis. Scientists then make a prediction of what they think will happen before testing the hypothesis. One way to test a hypothesis is to do a controlled experiment, which compares the results from a control group with the results from experimental groups. Pieces of information obtained through experimentation are called data. After testing a hypothesis, it is important to analyze your results by using calculations, tables, and graphs. Then, after analyzing your results, you should draw conclusions about whether your hypothesis is supported. Finally, communicating your results allows others to check or continue your work.

Section 3 – Scientific Models – A model is a representation of an object or system. There are three kinds of scientific models: Physical, mathematical, and conceptual. A physical model might be a model space shuttle, or the human eye. Mathematical models are made up of equations and data, and sometimes use computers. Conceptual models are often ideas; for example, the big bang theory is a conceptual model. Some models are smaller than the objects they represent (i.e., globes, solar system models), while other models are larger than the objects they represent (i.e., molecules, DNA). A scientific theory is an explanation for many hypothesis and observations. A scientific law summarizes experimental results and observations. The different between the two is that a theory is an explanation of why something happened the way it did, and a law is a statement that tells how things work.

Section 4 – Tools, Measurement, and Safety – A tool is anything that helps you do a task. Scientists use many tools to help them in their experiments. One way to collect data is to take measurements. But to do this, you need the proper tools. Stopwatches, metersticks, and balances are some tools that can be used to make measurements. (See metric information posted on 9/19 for detailed information on the International System of Units (SI), or metric system).

Length, volume, mass, and temperature are types of measurement used in science. The meter is the basic SI unit of length. Mass is the amount of matter in an object, and the kilogram (kg) is the basic unit for mass. The kilogram is used to describe the mass of large objects, and the gram is used to measure the mass of smaller objects. Volume is the amount of space that something occupies. Liquid volume is expressed in liters (L). Liters are based on the meter. A cubic meter is equal to 1,000 L. Volumes of solid objects are expressed in cubic meters. If you measure the mass and volume of an object, you have enough information to measure its density – the amount of matter in a given volume. Density is called a derived quantity because it is found by combining the two basic quantities of mass and volume. The equation that relates density to mass and volume is:

m

D = --------

V

The temperature of a substance is a measurement of hot or cold the substance is. Degrees Fahrenheit and degrees Celsius are often used to describe temperature. The SI unit for temperature is the Kelvin (K).

You will frequently encounter different safety symbols and rules when engaging in scientific investigations. Always pay attention to any safety labels on the sides of chemicals or other equipment – these alert you to what precautions you need to take, such as wearing goggles or gloves.

Information Posted on 9/19/05

Metric units of measurement

The French Academy of Sciences in the late 1700’s set out to make a simple and reliable measurement system. Over the next 200 years, the metric system was formed. This system is now the International System of Units (SI). We will be working with mass, volume, length, and temperature in our metric studies and conversions.

When working with mass, volume, and length, the “metric staircase” is a helpful visual tool, especially when doing conversions between units:

km

hm

dkm

m

dm

cm

mm

You can remember how the order of the units goes by remembering a simple mnemonic, or memory aide: King Henry Died Monday Drinking Chocolate Milk.

You can use the metric staircase for meters (length), as we did above, or grams (mass), or liters (volume). Just be sure to substitute the ‘m’ on the right of the unit with ‘L’ for liters and ‘g’ for grams (km – kL – kg).

Common SI (Metric) units:

km (kilometer) = 1000 meters

hm (hectometer) = 100 meters

dkm (dekameter) = 10 meters

m (meter) = 1 meter (base unit)

dm (decimeter) = 1/10 of meter

cm (centimeter) = 1/100 of meter

mm (millimeter) = 1/1000 of meter

Information Posted 9/11/05

The Scientific Method is a useful tool for engaging in scientific inquiry. The traditional steps are:

• Observe

• Ask a question

• Research

• Form a hypothesis

• Test the hypothesis

• Record and analyze data

• Form a conclusion

It is also important to repeat your experiment, to exclude the possibility of error in your experimental setup. You also must communicate your results to the rest of the scientific community. In this way, you contribute to building up of knowledge and experience in a particular scientific discipline, and others benefit from your work.

Let’s put these steps into a practical example. You might observe that leaves are starting to change from green to shades of red, brown, and orange during the fall season. You then ask a question: “What is causing the leaves to change color during this time every year?” You then do some background research. You are now in a position to form a hypothesis: The leaves are changing color as a result of chemical reactions occurring. You must test this hypothesis, and then record and analyze data that you obtain. Only now can you form a reasonable conclusion: The leaves are changing color because of a breakdown in chlorophyll, and secondary chemical reactions that cause the green color to disappear and hues of red, brown, and orange to appear in its place.

Experiments are made up of variables, which are factors in an experiment that change. The Independent Variable (IV) is the factor that the experimenter changes on purpose. In an experiment that seeks to determine the effect of differing amounts of water on plant growth, the differing amounts of water would be the IV – the experimenter might give Plant A 10mL of water, Plant B 20mL, Plant C 30mL, and Plant D 40mL. The factor that changes as a result of the purposely-changed factor is called the Dependent Variable (DV). In other words, the experiment changes it. In our plant growth experiment example, plant growth would be the DV. The independent variable will have different levels, or ways that the experimenter changes it – this is referred to as Level of Independent Variable (LIV). In our example, the different ways that he/she changes, or manipulates the IV are applying 10, 20, 30, and 40mL of water to the different plants. Variables that do not change in an experiment are called Constants. In our sample experiment, some constants might be same type of water (distilled), same type of soil, same type of pot, etc.). An experiment will usually have a Control. A control group is used for comparison with the experimental groups. So in our example, Plant E might be the control – it is not given any water. It is a good idea to repeat our experiment, to reduce the possibility of error. So the Number of Repeated Trials (NRT) refers to the number of times that each level of independent variable is tested. In our example, Plants A-D will be given the specified amounts of water (10, 20, 30, and 40mL) of water a total of three times – so the NRT for this experiment would be 3.

υ We want to be able to formulate good title and hypothesis statements for our experiment. When you write an experimental title, you are basically stating what the effect of the independent variable is on the dependent variable, and you write it in this format:

The Effect of IV on DV.

So a good title for our plant experiment would be:

The Effect of Amount of Water on Plant Height.

| |

IV DV

We then can write a hypothesis statement for our experiment. Hypothesis statements follow an “If, then” format. Basically, in a hypothesis statement you are making a prediction about how the dependent variable will change if you make a certain change to the independent variable: If how you are changing the IV, then how you predict the DV will change. So a good hypothesis statement for our experiment would be:

If the amount of water given a plant is increased, then the height the plant will grow will be increased.

There are of course several different hypothesis statements that could be written for an experiment such as this. Some are written in such a way that a change in magnitude in the IV reflects a similar magnitude change in the DV. These are in direct proportion:

If IV increases, then DV increases.

If the amount of water given a plant is increased, then the plant height will be increased.

If IV decreases, then DV decreases.

If the amount of water given a plant is decreased, then the plant height will be decreased.

Others are written in such a way that a change in magnitude in the IV reflects the opposite change in magnitude in the DV. These are in inverse proportion:

If IV increases, then DV decreases.

If the amount of water given to a plant is increased, then the plant height will be decreased.

If IV decreases, then DV increases.

If the amount of water given a plant is decreased, then the plant height will be increased.

Let’s take all of this information and place it on an Experimental Design Diagram:

Experimental Design Frame/Diagram

Title: The Effect of Amount of Water on Plant Height

Hypothesis: If the amount of water given a plant is increased, then the plant height will be increased.

IV: Amount of water

Levels of IV

(LIV) 10mL 20mL 30mL 40mL No

water

(control)

# of Repeated 3 3 3 3 3

Trials (NRT)

DV: Plant height

Constants – C: Type of water, type of soil, type of pot

-----------------------

If going from a big to a smaller unit, move decimal point to RIGHT the number of times you jump down the staircase.

Ex.- 1 meter = _______millimeters

Meters to millimeters is 3 jumps. So there’s a decimal point after 1. Move decimal point 3 places to right.

1. _ _ _ = 1000.

So 1 meter = 1000 millimeters

If going from a smaller to a bigger unit, move decimal point to LEFT the number of times you jump up the staircase.

Ex. – 1 millimeter = _______meters

Millimeters to meters is 3 jumps.

There’s a decimal point after 1.

Move decimal point 3 places to left.

_ _ _ 1.

So 1 millimeter = 0.001 meters

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