Electricity



Electricity

and

Energy

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National 5

Summary Guide

1 kJ = 1 kilojoule =1000 J = 103J

1 MJ = 1 megajoule = 106J

1GJ = 1 gigajoule = 109J

1 TJ = 1 terajoule = 1012J

1 therm = 1.06 x 106J

1 kilowatt hour = 3.6 x 106J

Gravitational Potential Energy

• I can state what is meant by gravitational potential energy.

• I can carry out calculations involving the relationship between Gravitational Potential Energy, mass, gravitational field strength and height.

Gravitational Potential Energy

When an object is raised above ground level, work is done on the object and energy is transferred.

Gravitational Potential Energy is the energy an object has because of its position above the surface of the Earth.

The Gravitational Potential Energy, Ep of an object with mass, m at a height, h is defined by the equation:

Ep = mgh

Where;

• Ep is the gravitational potential energy measured in Joules, J

• m is the mass of the object measured in kilograms, kg

• g is the gravitational field strength measure in N/kg, for example the gravitational field strength on Earth is 9.8 N/kg

• h is the height measured in metres, m

Worked Example

A crane raises a load at a steady velocity from ground level through a vertical displacement of 10m. The load has a mass of 1200kg.

How much gravitational potential energy is gained by the load?

Solution

Ep= ? Ep = mgh

m = 1200 kg Ep = 1200 x 9.8 x 10

g = 9.8 N/kg Ep = 117600

h = 10m Ep = 1.18 x 105 J

Kinetic Energy

• I can state what is meant by kinetic energy.

• I can carry out calculations involving the relationship between kinetic energy, mass, and velocity.

Kinetic Energy

When you pedal a bicycle so that’s its velocity will increase you are transferring chemical energy6 from your muscles to the kinetic energy of the bicycle.

Kinetic Energy is the energy an objet has as a result of its movement. The amount of kinetic energy, Ek a moving object has depends on the mass, m and its velocity, v.

The bigger the mass and the greater the velocity of an object the greater its kinetic energy.

The kinetic energy of an object is defined by the equation:

Ek = ½ m v2

Where;

• Ek is the kinetic energy measured in Joules, J

• m is the mass measured in kilograms, kg

• v is the velocity measured in metres per second, ms-1

Worked Example

A cricket ball of mass 0.16kg and a tennis ball of mass 0.06kg are both thrown at a velocity of 25 ms-1. Which object has the greatest kinetic energy?

Solution

Cricket ball

Ek= ? Ek = ½ m v2

m = 0.16kg Ek = ½ x 0.16 x (25)2

v = 25 ms-1 Ek = 50J

Tennis ball

Ek= ? Ek = ½ m v2

m = 0.06kg Ek = ½ x 0.06 x (25)2

v = 25 ms-1 Ek = 18.75J

The cricket ball has the greatest kinetic energy.

Rearranging the kinetic energy equation

Sometimes it is necessary to rearrange the kinetic energy equation in order to calculate the mass or the velocity of a moving object

Ek = ½ m v2

[pic] What are the rearranged versions of this equation for calculating mass and velocity?

Work Done

• I can state that work done is a measure of the energy transferred

• I can carry out calculations involving the relationship between work done, force and distance

Work Done

When work is being done energy is being transferred and a force is being applied over a displacement, s.

The work done, Ew of an object is defined by the equation;

Ew = F s

Where;

• Ew is the work done measured in Joules, J

• F is the force applied measured in Newton’s, N

• s is the displacement measured in metres, m

Worked Example

A car engine moves a car with a force of 10 kN and travels 50 m . Calculate the work done.

Solution

Ew = ? Ew = F x s

F = 10kN Ew = 10000 x 50

s = 50m Ew = 500000

Ew = 5 x 105J

Conservation of Energy

• I can solve problems involving work, energy transfer and the law of conservation of energy.

The law of conservation of energy states that energy cannot be created or destroyed. Energy is transferred from one form into another.

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• A skateboarder at the top of a top of a slope has a potential energy.

• When the skateboarder rolls down the first slope they lose Ep and gain Ek.

• As they roll back up the slope they will lose Ek and gain Ep.

Worked Example Ep = Ek

A 1.3 kg ball is dropped from the top of a cliff which is 80 m high.

a) Calculate the potential energy at the top of the cliff.

b) State the kinetic energy at the bottom of the cliff, assuming that there is no friction

c) Calculate the speed of the ball at the bottom of the cliff.

d) Which piece of information given in the question is not required to find the speed?

Solution

a) Ep =? Ep = mgh

m = 1.3kg Ep = 1.3 x 9.8 x 80

g = 9.8 N/kg Ep = 1019.2 J

h = 80 m

b) Ek = Ep = 1040J

c) Ek = 1019.2J Ek = ½ m v2

m = 1.3 kg 1019.2 = ½ x 1.3 x (v)2

v = ? (2 x 1019.2) ÷1.3 = v2

√(1568) = v

v = 39.6 ms-1

d) The mass

Supply and Demand

• I can state fossil fuels are at present the main sources of energy.

• I can state that the reserves of fossil fuels are finite.

• I can classify renewable and non renewable sources of energy.

• I can explain the advantages and disadvantages associated with at least three renewable energy sources.

Industrialised countries, like Britain use large quantities of energy for heating, transport and industry. This energy large comes from the fossil fuels Coal, Oil and Gas. These are called fossil fuels because they are the remains of plants and animals which lived many millions of year ago. These fuels are also known as finite resources which mean they will eventually run out and cannot be replaced.

|Coal |Oil |Gas |

|[pic] |[pic] |[pic] |

|Used to produce electrical energy at |Used for transport |Used for heating homes and generating electricity.|

|power stations. | | |

Renewable Energy

• The Sun provides our planet with the energy required to drive the weather systems of our world and allows our plants tog grow.

• The tides are a consequence of the gravitational pull of the sun and the Moon.

• The Earth has a very hot interior which in some parts of our world come close to the surface.

All of these can offer alternative ways of obtaining the energy that we need.

Efficiency

• I can carry out calculations involving efficiency of energy transformations.

The efficiency of a power station (or any other machine) is given by

Efficiency = useful energy output x 100%

total energy input

Where;

• Efficiency is measured as a percentage, %

• Input Energy is measured in Joules, J

• Output Energy is measured in Joules, J

As the energy output from a power station or any other machine is always less than the energy input, the efficiency is always less than 100%. The useful energy we get from a machine is never as great as the energy we pout in, because some of the energy is transferred into other forms such as heat, sound or light. We say that some of the energy put into the machine is ‘lost’. However, the total amount of all the energies will remain the same i.e. the total energy is conserved.

Worked Example

A Hydroelectric power station has an efficiency of 75%. When operating, the power station produces 3MJ of energy every second.

What is the energy input to the station every second?

Solution

Efficiency = 75%

Useful Energy out = 3MJ = 3000000 J

Energy in = ?

Efficiency = useful energy output x 100%

total energy input

E input= 3000000 x 100%

75%

E input = 4000000J = 4MJ

What is electricity?

• I can carry out calculations involving charge, current and time.

• I can use correctly the term coulomb.

• I can state that the voltage of a supply is a measure of the energy given to the charges in a circuit.

All solids, liquids and gases are made up of atoms. An atom comprises of a positively charged centre or nucleus surround by a cloud of rapidly revolving negative charges called electrons. The nucleus is made of particles called protons( positively charge) and neutrons( no charge).

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Charge is measure in Coulombs, C. The charge on a proton is equal in size to that of the electron. This value is 1.6 x 10 -19 C.

Charge, Current and Time

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Consider the simple circuit shown above. The lamp will light because the negative charges (electrons) from the negative terminal of the battery move through the wires and lamp to the positive terminal of the battery. This movement of negative charges in called an electric current. A current is a flow of electrons.

The quantity of charge transferred, Q is given by: Q = I t

This defines current

I = Q

t

Where;

• Q is the charge measured in Coulombs, C

• I is the current measured in Amperes, A

• t is the time measured in seconds, s

This means that 1 Coulomb of charge is transferred when 1A of current is flowing in 1 second.

Worked Example

How many coulombs of charge are transferred in 5 minutes by a current of 0.2A.

Solution

Q = ? Q = I x t

I = 0.2A Q = 0.2 x 300

t = 5 x 60 =300s Q = 60C

Conductors and Insulators

• I can state that the electrons are free to move in a conductor.

Negative charges (i.e. electrons) can only move from the negative terminal to the positive terminal of the battery if there is an electrical path between them. ,Materials which allow negative charges to move through easily, to form an electric current, are known as conductors.

Conductors are mainly meals, such as copper, gold and silver. However, carbon is also a good conductor.

Materials which do not allow electrons to move through them easily are called insulators. Glass, plastic , wood and air are examples of insulators.

Permanent Magnets

• I can state that a magnetic field exists around a current carrying wire.

A permanent magnet has a magnetic field surrounding it which cannot be switched off. The opposite ends, or poles, of a magnet are called NORTH and SOUTH. The shape of a magnetic filed surrounding a magnet can be shown by scattering iron fillings on a piece of paper placed on top of the magnet. The direction of the magnetic field can be found using a compass. (Activity 1.2.1)

When two permanent magnets are placed close to each other , their magnetic fields produce forces which cause

• A north to repel a north poles

• A south to repel a south pole

• And a north to attract a south pole.

Remember that like poles repel and unlike poles attract.

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The Van der Graaf Generator

• I can investigate the interaction of charged objects, for example the Van der Graaf generator.

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Fact: The electrons are in an excited state because of mutual repulsion, which means that they are looking for a place to go where the electron population is not so dense. Since the electrons are excited they will jump to anything that comes close to the generator globe, such as your hand, the desk, water droplets, or dust particles in the air. When they jump to these new locations they drop to a lower energy level and when they make this jump they release this energy in the form of light, heat, and sound. This is why we hear a crack and see a spark for the generator. The electrons can even jump through a glass tube like a fluorescent light bulb.

Fact: If you put a pile of tin plates on the top of the generator’s globe, they will fly off one at a time. Since each of them will be picking up some of the excess charges the plates repel each other and they will fly off one at a time.

Fact: A piece of tinsel or Christmas tree icicles will first be attracted to the globe by induction, just like when you stick a charged balloon to the wall, and then repel away from the generator after it picks up some excess charge.

Fact: If you start with your hands on the globe and then turn the generator on you won’t get a shock, because the charge doesn’t get a chance to build up. The charge will continually leak into you and then through you to the ground. This is why you cannot have a “Bad Hair day” while standing on the ground. However if someone is standing on a crate or chair with their hands on the globe the charges will begin to build up in your body causing a “Bad Hair Day” which is an example of the charges repelling each other. You can also receive a shock from this person because he / she has built up a charge and they have electrons built up in them who are looking to jump to a less densely populated area.

The Cathode Ray Tube

• I can state that a magnetic field exists around a current-carrying wire.

• I can state that, in an electric field, an electric charge experiences a force.

• I can state that an electric field applied to a conductor causes the free electric charges in it to move.

• I can state that work W is done when a charge Q is moved in an electric field.

• I can state that the potential difference between two points is a measure of the work done in moving one coulomb of charge between the two points.

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Alternating and Direct current

• I can state that the mains supply is a.c and a battery is d.c.

• I can explain in terms of current a.c. and d.c.

• I can state that mains voltage is 230V

• I can state that mains frequency is 50Hz.

Electrical supplies can deliver either a direct current (d.c.) or an alternating current (a.c.).

Direct current (d.c.) is an electric current that always flows in one direction.

A battery is a source of direct current (d.c.).

Electronic circuits such as those in computers and stereos need direct current electricity in order to work. Direct currents cannot be transferred efficiently over large distances.

Alternating current (a.c.) is an electric current that is constantly changing direction this can happen many times per second.

Alternating current is produced by most generators and is used in mains electricity.

Motors often work using alternating current.

The voltage of alternating current is easily changed with a transformer. Alternating current can be transferred efficiently overlarge distances

The mains supply is a source of alternating current (a.c.). The frequency of the mains supply is 50 Hz.

An oscilloscope can be used to compare alternating and direct currents.

DC circuit and AC circuit are connected as shown below:

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Direct Current Trace

The voltage of a d.c. supply is steady and always in the same direction.

Alternating Current Trace

The voltage of an a.c. supply follows a repeated pattern: it rises to a peak, returns to zero changes direction and so on.

Circuits Symbols and meters

• I can draw and identify the circuit symbols for a battery, lamp, motor, switch, resistor, diode and variable resistor.

• I can state that current can be measured in a circuit using an Ammeter.

• I can state that voltage can be measured in a circuit using a Voltmeter.

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Voltage

The electrical energy given to the negative charges by a battery is a measure of the voltage of the battery. To be exact, the voltage of a battery is the electrical energy given to one coulomb of charge passing through the battery. For example, a 9V battery gives six times as much energy to each coulomb of charge passing through as a 1.5V battery.

The Volt

A potential difference of one volt exists between two points if 1 J of work is required to transfer 1 C of charge from one point to the other,

i.e. 1 V = 1 J C–1.

When a current passes through any component energy is transferred.

In a lamp the electrical energy is converted into light( and some heat).

In a motor the electrical energy is converted into kinetic energy( and some heat and sound).

The potential difference across each component in a circuit is the energy that is lost by each coulomb if charge passing through it.

The unit of potential differences is V.

Current

• I can describe an electric current using the words: electrons, positive and negative.

• I can name the device used to measure electrical current

• I can name the unit of electrical current.

• I can state that electrons are free to move in a conductor

• I can describe the electric current in terms of the movement of charges around a circuit

Can you remember the name given to the tiny invisible

particles that make up an electric current?

electrons

What type of charge does an electron have?

negative

What is the other type of charge then?

Positive

What happens when two positive charges are near each other?

repel

So what about two negative charges?

repel

So what happens when one positive and one negative are close?

attract

Ohms Law

• I can carry out calculations involving current, voltage and resistance.

• I can correctly use the term Ohm.

Ohms law defines the relationship between voltage, current and resistance. These basic electrical units apply to direct current, or alternating current. Ohm’s law is the foundation of electronics and electricity.

Ohms Law:

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Where

• V is the voltage measured in volts, V.

• I is the Current measured in amps, A

• R is the Resistance measured in ohms, Ω

It is used extensively by electricians. without a thorough understanding of ohm’s law an electrician cannot design or troubleshoot even the simplest of electronic or electrical circuits.

Ohm’s Law is named after a physics teacher called Georg Ohm.

Ohm established in the late 1820’s that if a voltage was applied to a resistance then “current would flow and then power would be consumed”.

Can you guess what nationality he was?

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Brandenburg Bratwurst Andreas

Gate sausage Hinkel

Worked Example

What is the current through a 5.6kΩ resistor when it is connected to a 230V supply?

Solution

V = 230V I = V ÷ R

I = ? I = 230 ÷ 5600

R = 5.6kΩ I = 0.041°

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Resistors in Series

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Resistors in Parallel

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If we join components in parallel we decrease the resistance of the circuit.

The current will increase.

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Combination Resistors

• I can solve problems and calculate total resistance in a combination circuit.

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Household Wiring

• I can state that household wiring connects appliances in parallel.

• I can state advantages of using the ring circuit as a preferred method of wiring in parallel.

• I can give two differences between the lighting circuit and the ring circuit

Sockets in the home are connected in a special parallel circuit called a ring circuit.

Advantages of the ring circuit

Ring circuits have the following advantages:

1. There is less current in each wire since there is more than one path to each socket

2. Thinner cable can be used since there is less current.

3. Thinner cable is cheaper than thicker cable making the system less costly

4. There is less risk of overheating with a smaller current

Differences between a ring circuit and lighting circuit

|Lighting circuit |Ring circuit |

|Simple parallel |Ring |

|5A max |30A max |

|Thin cable |Thick cable |

The Thermistor

• I can state that as the temperature across a thermistor decreases the resistance will increase.

A thermistor has the following symbol:

[pic]

The resistance of a thermistor depends on temperature. As temperature increases the resistance decreases. (Try it out for yourself set up a thermistor circuit and investigate)

Temperature

Up

Resistance

Down

Power, Energy and Time

• I can state that in a lamp electrical energy is transformed into heat and light.

• I can state the relationship between energy and power

Energy is transferred when there is an electrical current in a circuit. When you lug an electrical appliance into the mains socket and switch it on, energy is transferred as heat, light, sound or movement.

Different electrical appliances transfer energy at different rates. The power of an appliance is the rate at which it transfers energy from the main supply.

P= E

t

Where:

• P is the power measured in watts, w

• E is the energy measured in joules, j

• t is the time measured in seconds, s

Worked Example

A lamp uses 216kJ of energy in a time of 6 minutes. What is the power of the lamp?

P = ? P = E ÷t

e = 216kJ = 216000J P= 216000÷ 360

t = 6 minutes = 6x 60 = 360s P = 600 W

Power, Current and Voltage

• I can carry out calculations involving power current and voltage

The energy transferred every second by an appliance in a circuit can be calculated using the relationship:

Energy transferred per second = Voltage x Current

Power = Voltage x Current

P = I x V

Where:

• P is the power measured in watts, w

• I is the current measure in Amperes, A

• V is the voltage measured in Volts, V

Power equivalence equations

Since we know that P=IV and from earlier we know Ohms Law stated that V=IR we can combine these two equations.

Lets start with

P= I V and we know that V = I R

Substitute for V

Therefore P = I (I R)

So;

P = I2R

Lets try it again

P= I V and we know that I = V / R

Substitute fo I

Therefore P = (V/R) x V

So;

P = V2

R

Electronics

• I can draw and identify the symbol for the following gates: AND; OR; NOT.

• I can state what a truth table is.

• I Can draw the truth table for the following gates: AND; OR; NOT.

These are examples of electronic systems:

• radio

• computer

• stopwatch

• digital thermometer

• calculator

• clock

• burglar alarm

• fire alarm

All electronic systems have 3 parts:

• input

• process (or processor)

• output

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So, an electronic fire alarm might have a heat detector as the input, and an electric bell as the output.

Inputs

An input device converts an

Input into an electrical signal.

• microphone

• switch

• keyboard

• solar cell

• light dependent resistor (LDR)

• thermocouple

• thermistor

Outputs

An output device converts an

electrical signal into an output.

• light emitting diode (LED)

• loudspeaker

• 7-segment display

• printer

• electric motor

• electric bell

• monitor or screen

Processors

A processor makes changes to electrical

signals between input and output.

• transistor

• logic gate

• clock signals

• amplifiers

Logic Gates

There are three types of logic gates. They are

1. NOT

2. AND

3. OR

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Transmission lines and Transformers.

• I can state that transformers are used to change the magnitude od an a.c. suply.

• I can describe the structure of a transformer.

• I cna carry out caluclatiions invlolving the realtionship between curren, number of turns and voltage.

• I can describe qualitivitively the transmissin of electrical energy by the national grid.

• I can state that high voltages are used in the transmission of electicity to reduce power losses.

Most of our electrical energy is generated in Power Stations. For social and technical reasons, they are usually located away from highly populated areas like cities or towns.

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Our electricity is transmitted throughout the UK via a network of transmission lines. Pylons are used to support the cables of the transmission lines. This network of pylons and transmission lines is called the National Grid.

Devices called step up transformers link the power stations with the National Grid. The high voltage from the transformer is applied to the electrical transmission cables. High voltages are used in order to reduce power losses in transmission cables. Heat is produced in the cables during the transmission of electricity. Much more heat would be produced if lower voltages were used for transmission.

Step down transformers are used at the consumer end of the transmission lines. These transformers step down the high voltages to a level that is suitable for factory machinery and the electrical appliances in our home.

Remember: Power losses because of resistance in the transmission lines make it difficult to pass electricity over long distances.

High Voltages are used in the transmission of electricity to reduce power loss. The amount of energy changed to heat every second (the power loss) is given by:

P = I2R.

Transformers overcome this problem.

Transformers

The voltage of an alternating current can be changed using a device called a transformer. A transformer contains two coils that are wound around a soft iron core.

The alternating current in the primary (input) coil produces an alternating magnetic field.

This alternating magnetic field induces an alternating current in the secondary (output) coil.

The voltage induced in the secondary (output) coil depends on the number of turns on the primary and secondary coils.

A step-up transformer has more turns on the secondary coil and so increases voltage.

A step-down transformer has fewer turns on the secondary coil and so decreases voltage.

The transformer circuit symbol is shown below. The straight line in between the coils represents the iron core

[pic]

A television needs a very high voltage to operate. It contains a step-up transformer, which increases the voltage of the electricity supplied to the television.

This outdoor transformer decreases the voltage of the electricity carried by the national grid. It is an example of a step-down transformer.

Transformer Equation:

NP = VP

NS VS

Where:

• NP = no. of turns in primary coil

• NS = no. of turns in secondary coil

• VP = voltage in primary coil (in volts)

• VS = voltage in secondary coil (in volts)

Worked Example:

The transformer shown below allows a computer to operate properly

from the 230 V mains.

Calculate the computer's operating voltage.

Solution

NP = 4600

NS = 100

VP = 230V

VS = ?

For an ideal transformer (100% efficient), all the power produced at the primary coil is transferred to the secondary coil.

IPVP = ISVS (P = IV)

Rearranging, this gives:

VP = IS so VP = IS = NP

VS IP VS IP NS

I is the current in the coils, measured in amperes (A).

What is heat?

• I can use the following terms correctly in context temperature; heat and Celsius.

• I can state the same mass of different materials require different quantities of energy to raise their temperature of the unit mass by one degree.

Heat is the transfer of energy, between one body or object and another, that occurs because of a difference in temperature between them.

Sometimes, it is also used to describe the amount of energy an object has before or after energy has been transferred.

It is measured in joules (J).

Heat is transferred in three ways:

• conduction

• convection

• radiation

Heat always flows:

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What is temperature?

Temperature is a measure of how hot or cold an object becomes when heat flows to or from it. It is not the amount of energy an object has.

If a cup of tea is left on a table, its temperature decreases as heat flows from the tea to the surroundings.

Temperature can be measured using the Celsius scale (units = °C).

This scale uses the freezing and boiling point of water as two fixed points on a scale labelling them 0 and 100. Objects are measured on the scale, relative to these points.

How does energy affect materials?

Different materials require different amounts of energy to raise their temperatures by the same amount.

To increase the temperature of 1 kg of water by 1°C, requires 4200 J.

To increase the temperature of 1 kg of copper by 1°C, requires 390 J.

Water and copper require different amounts of energy to raise the temperature of 1 kg because they have different values for a property called specific heat capacity.

Specific heat capacity is the amount of energy required to increase the temperature of 1 kg of a material by 1°C.

The specific heat capacity for water is 4200 J/kg°C or 4.2 kJ/kg°C and for copper is 390 J/kg°C or 3.9 kJ/kg°C.

Specific heat capacity

• I can carry out calculations based on practical applications involving heat, mass, specific heat capacity and temperature change.

The specific heat capacity of a material is the amount of energy required to raise the temperature of 1 kg by 1 °C.

It can be used to work out how much energy is needed to raise the temperature of a material by a certain amount:

[pic]

• Energy is measured in joules (J).

• Mass is measured in kilograms (kg).

• Temperature change is measured in °C.

• Specific heat capacity is measured in J/kg°C.

Worked Example

Using the specific heat capacity

of water (4200 J/kg°C), how much energy is needed to increase the temperature of 600 g of water by 80°C in a kettle?

Note: mass = 600 g = 0.6 kg

[pic]

energy = 0.6 x 4200 x 80 = 201 600 J

Pressure

• I can state the relationship between pressure, force and area.

Examples to Consider and discuss:

1. Pushing a drawing pin.

2. Stiletto heels.

3. Using a ladder to cross a frozen pond.

Pressure is defined as the force per unit area. The unit of pressure is the Pascal (Pa). Alternative units for pressure are: N m-2.

1 Pa = 1 N m -2

P=F/A

Where;

• P is the pressure measured in Pascal’s, Pa

• F is the force measured in Newton’s, N

• A is the area measure in meters squared m2

Worked Example

A 3 kg block has dimensions as shown.

[pic]

Calculate the pressure the box exerts on the surface of a desk.

Solution

Weight of Box

W = m g

W = 3 ×9.8

W = 29.4 N

Area of Base

A = l×b

A = 0.3 ×0.2

A = 0.06 m2

Pressure on Desk

P=F/A

P = 29.4 / 0.06

P = 490 Pa

The kinetic Model

• I can discuss the relationship between the volume, pressure and temperature of a fixed mass of gas using the kinetic theory.

Kinetic Theory of Gases

The kinetic model of matter explains the behaviour of gases using a model.

The model considers gases to be made up from a large number of very small particles which are far apart, move randomly at high speeds and collide (elastically) with everything they meet.

Volume

The volume of a gas is taken as the volume of the container. The volume occupied by the particles themselves is so small it is neglected.

Temperature

The temperature of a gas depends on the kinetic energy of the gas particles.

The faster the particles move the greater their kinetic energy and the higher the temperature.

Pressure

Pressure of a gas is due to the particles colliding with each other and the walls of the container. The more frequent and harder the collisions, the greater the pressure.

Absolute Zero

Absolute zero is the point where no more heat can be removed from a system, according to the absolute or thermodynamic temperature scale.

This corresponds to 0 K or -273.15°C. In the kinetic theory, there should be no movement of individual molecules at absolute zero, but experimental evidences shows this isn't the case.

We know that temperature is used to describe how hot or cold an object it. The temperature of an object depends on how fast its atoms and molecules oscillate. At absolute zero, these oscillations are the slowest they can possibly be. Even at absolute zero, the motion doesn't completely stop.

It's not possible to reach absolute zero, though scientists have approached it.

**Notes to remember:

The lowest possible temperature is -273°C. This temperature is called absolute zero. The absolute temperature scale calls -273°C, zero kelvin (K).

Converting °C to K

add 273

Converting K to °C

subtract 273

An increase of 5°C = an increase of 5 K.

No negative temperatures on the Kelvin scale.

Boyles Law:

Boyle's Law, is a principle that describes the relationship between the pressure and volume of a gas. According to this law, the pressure exerted by a gas held at a constant temperature varies inversely with the volume of the gas.

For example, if the volume is halved, the pressure is doubled; and if the volume is doubled, the pressure is halved. The reason for this effect is that a gas is made up of loosely spaced molecules moving at random. If a gas is compressed in a container, these molecules are pushed together; thus, the gas occupies less volume. The molecules, having less space in which to move, hit the walls of the container more frequently and thus exert an increased pressure.

Boyle's Law actually applies only to an ideal, theoretical gas. When real gases are compressed at a constant temperature, changes in the relationship between pressure and volume occur. However, the law is accurate enough to be useful in a number of practical applications. It is used, for example, in calculating the volume and pressure of internal-combustion engines and steam engines.

The law was first stated in 1662 by Robert Boyle. In 1676, Edme Mariotte of France independently stated the same law, and it is sometimes called Mariotte's Law.

Boyle's Law states that:

P1 V1 = P2 V2 (at constant temperature)

Where;

• P1 is the initial pressure, measured in Pascal’s, Pa

• P2 in the new pressure, measured in Pascal’s, Pa

• V1 is the initial volume, measured in meters cubed, m3

• V2 is the new volume, measure in meters cubed, m3

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Worked Example and Solution

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Pressure Law

Pressure and Temperature

A fixed mass of gas at a constant volume is considered. If the temperature is increased, the particles move with greater speed and kinetic energy. Collisions are more frequent and with greater force, causing pressure to increase.

When the temperature is decreased, the particles move with less speed and kinetic energy. Collisions are less frequent and with smaller force, causing pressure to decrease.

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The Pressure law states that ;

P1 = P2

T1 T2

Where;

• P1 is the initial pressure, measured in Pascal’s, Pa

• P2 in the new pressure, measured in Pascal’s, Pa

• T1 is the initial temperature, measured in Kelvin, K

• T2 is the new volume, measured in Kelvin, K

Worked Example

A quantity of gas has a pressure of 2.5x105 Pa at a temperature of 20 °C.

(a) Calculate the new pressure when the temperature

reaches 37 °C.

(b) State two important assumptions made in part (a).

Solution

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Charles Law

Charles' Law, is a principle that deals with the effect of heat on the expansion of gases. The law states:

If the pressure of a gas remains constant, the volume of the gas will increase as the temperature increases.

Thus if the temperature increases, the gas takes up more space. If the temperature decreases, the gas takes up less space. The principle was first formulated by the French physicist Jacques Alexandre Cesar Charles in 1787.

Charles' law is stated this way in formula form:

V1 = V2

T1 T2

Where;

• T1 is the initial temperature, measured in Kelvin, K

• T2 is the new volume, measured in Kelvin, K

• V1 is the initial volume, measured in meters cubed, m3

• V2 is the new volume, measure in meters cubed, m3

In using Charles' law, temperatures must be converted to the Kelvin scale, in which the zero point is absolute zero (-273.15° C.).

Volume and Temperature

A fixed mass of gas at a constant pressure is considered. If the temperature increases, the particles move with a greater speed and kinetic energy. The volume increases. If the temperature decreases, the particles move with less speed and kinetic energy. The volume decreases.

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Worked Example

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General Gas Law

Pressure, Volume and Temperature

In combining Boyle’s Law, the Pressure Law and Charles’ Law we can get one relationship that relates pressure, volume and temperature of a fixed mass of gas.

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This equation is used in the form:

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When using this equation – temperature must be in Kelvin (K).

Gas Laws and the Kinetic Theory

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What will happen to the pressure?

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What will happen to the pressure?

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What will happen to the volume?

Ideal Gases

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An ideal gas is a theoretical gas composed of a set of randomly moving, non-interacting point particles. The ideal gas concept is useful because it obeys the ideal gas law, a simplified equation of state, and is amenable to analysis under statistical mechanics.

At normal conditions such as standard temperature and pressure, most real gases behave qualitatively like an ideal gas. Many gases such as nitrogen, oxygen, hydrogen, noble gases, and some heavier gases like carbon dioxide can be treated like ideal gases within reasonable tolerances. Generally, a gas behaves more like an ideal gas at higher temperature and lower pressure, as the work which is against intermolecular forces becomes less significant compared with the particles' kinetic energy, and the size of the molecules becomes less significant compared to the empty space between them.

The ideal gas model tends to fail at lower temperatures or higher pressures, when intermolecular forces and molecular size become important.

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Resistors

When we put a resistor into a circuit, we increase the resistance of a circuit.

An increase in resistance of a circuit leads to a decrease in the current in that circuit.

Resistance is measured in ohms (©).

If we join components in series, we increase tstor into a circuit, we increase the resistance of a circuit.

An increase in resistance of a circuit leads to a decrease in the current in that circuit.

Resistance is measured in ohms (Ω).

If we join components in series, we increase the resistance of the circuit.

The current will decrease.

The total resistance in series is equal to the sum of the individual resistances.

RS = R1 + R2 + R3

R1

R2

R3

R3

R2

R1

input

process

output

The And Gate

An AND gate only gives a high output when both input A and input B are high.

A truth table for a logic gate shows the output for all possible combinations of inputs.

A

B

Circuit symbol

for an AND gate

An AND

logic circuit

0

0

1

A

0

1

0

1

input

B

0

0

1

1

0

output

The OR Gate

An OR gate has two inputs, A and B, and one output.

0

1

1

A

0

1

0

1

input

B

0

0

1

1

1

output

It gives a high output when input A or input B is high.

A

B

An OR

logic circuit

Circuit symbol

for an OR gate

The NOT Gate

A NOT gate has only one input and one output. The output is always the opposite of its input.

0

0

1

input

output

1

A NOT gate is also called an inverter. NOT gates are often used in combination with other logic gates.

Circuit symbol

for a NOT gate

primary

coil

iron core

secondary

coil

230 V ac

4600

turns

100

turns

Computer

NP = VP 4600 = 230

NS VS 100 Vs

46 Vs = 230 therefore : Vs = 230 / 46 = 5V

HOT COLD

energy

specific heat

capacity

temperature

change

=

mass

x

x

energy

specific heat

capacity

temperature

change

=

mass

x

x

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