Unit 9: Waves and Sound



Physics I Honors Semester 2 Exam Review Unit 9: Waves and Sound Essential questions: x What is simple harmonic motion? Motion with a restoring force to bring something back through equilibrium. It occurs over and over in regular time intervals. x What factors affect simple harmonic motion of a pendulum and a mass-spring system? For a pendulum: length, gravity…NOT the mass of the bob For a mass-spring system: k (the spring constant…how stiff the spring is), mass…NOT gravity x What is a mechanical wave? Any wave requiring a medium for travel x What distinguishes one mechanical wave from another? How the particles vibrate. For a transverse wave: particles vibrate perpendicular to wave motion(think doing ‘the wave’ in the football stadium…you stand up and down…wave moves left and right). For a longitudinal wave: particles vibrate parallel to wave motion (sound). x How do the different characteristics used to measure waves relate to each other? Characteristics are speed (v), frequency (f), wavelength (λ), amplitude (A), and period (T). Relationships: v = λf and f = 1/T and T = 1/f x Why does the speed of a wave change passing from one medium to another? Because waves are a transmission of energy. That energy will travel differently in different materials. FREQUENCY NEVER CHANGES from medium to medium…so speed and wavelength change to compensate. x What determines whether an incident wave pulse striking a boundary reflects inverted or not? It depends on the boundary. At FIXED boundaries, reflections are INVERTED. At FREE boundaries, reflections are UPRIGHT. x How can the principle of superposition be used to describe wave interference in a variety of situations? If the waves are both on the same side of equilibrium will have constructive interference; if on opposite sides of equilibrium will have destructive interference. x What are characteristics of a sound wave? Sound is longitudinal (like a slinky); sound needs a medium to travel, loudness is related to amplitude…or how much energy the sound wave has. x Why does the speed of a sound wave change as it travels through different media or changes in the properties of the same media? Because waves are a transmission of energy. That energy will travel differently in different materials. FREQUENCY NEVER CHANGES from medium to medium…so speed and wavelength change to compensate. x What is the Doppler Effect and how does it occur? A perceived shift in frequency. It occurs when there is movement between the sound source and the observer. For example, a train moves toward you blowing its horn. While moving towards you, you will perceive a higher frequency that is really being emitted because the motion of the train causes the wave crests to be closer together when they arrive at your ear. The opposite is true when its moving away from you. Key vocabulary: (I am giving you the shortest possible definitions here…easy to remember…mostly to jog your memory. If you need more than this…LOOK THEM UP online or in the textbook.) Simple harmonic motion – motion repeated over and over in the same time interval due to a restoring force Spring constant – stiffness of a spring (k) Restoring force – for SHM brings back through equilibrium Period – seconds per cycle/wave (T) Frequency – waves per second (f) Mechanical wave - requires a medium for transport (all waves are mechanical EXCEPT electromagnetic waves) Transverse wave – particles vibrate perpendicular to wave motion (like a wave on a rope) Longitudinal wave – particles vibrate parallel to wave motion (like a slinky) constructive interference - waves meet and add up destructive interference – waves meet and subtract Doppler effect - shift in frequency because either the sound is moving or the observer is moving or both are moving Practice: List 4 examples of systems which exhibit simple harmonic motion. A child on a swing (is like a pendulum, right?) A metronome (used to keep time for musicians) A mass-spring system (like the shocks on your car) The pendulum on a grandfather clock A spring is stretched to a displacement of 30 cm where it experiences a restoring force F. If the spring is then stretched to a displacement of 60 cm, what is the restoring force, in terms of F, exerted by the spring? ? = ??? If initially F = -k(30), and then we stretch the same spring to 60 cm, it must become 2F = -k(60). The new force is 2F. If a 43 N force stretches a spring 0.35 m, what is the spring constant (k)? Given: F = 43N Find: k x = - 0.35m (make either F or x negative because they work in opposite directions) ? = ??? 43 = ??(?0.35) ? = = 122.86 k = 123 N/m A 2 kg mass is attached to a vertically hanging spring and then released. The system stops oscillating, and eventually comes to rest with the mass hanging 12 cm from its equilibrium position. Calculate the spring constant (k). Given: m = 2kg Find: k x = - 12 cm = - 0.12m 5797775-73986? = ??? Problem is…we don’t have F. DRAW a free body diagram! The upward arrow is the spring force supporting the mass, -kx. The downward arrow is the weight of the mass, mg. SO… ? = ??? = ?? ??? = ?? ??(?0.12) = (2)(9.8) a) Write the equation for the period of a mass-spring system. ?3992978-91612? = 2? ? What two variables affect the period of a mass-spring system? mass and the spring constant a) Write the equation for the period of a pendulum undergoing simple harmonic motion. ? = 2? What two variables affect the period of a pendulum? length and gravity Frequency and period have a(n) inverse relationship for simple harmonic motion. 2030320272752If a pendulum is adjusted so that its frequency changes from 25 Hz to 75 Hz, its period will change from t seconds to t/3 seconds . Here’s why: when period is t seconds, the frequency is 25Hz. Let’s say 25Hz = f. When we change the frequency to 75Hz, we’ve really changed it to 3f. Because frequency and period are INVERSELY related, if f went to 3f, then t must go to t/3. Identify the type of wave and all of the labeled parts below: 4594323126436698395-28123 The wave type is transverse . A: crest B: trough C: wavelength D: amplitude Describe the motion of the particles in a transverse wave. Particle motion is perpendicular to wave motion. Think about a wave on a string…the string itself actually moves up and down…but the wave…the energy moves left to right. Describe the motion of the particles in a longitudinal wave. Particle motion is parallel to wave motion. Think about a slinky that you have stretched out on a table. Pinch 10-15 coils together and you will see a pulse move down the wave. The motion of the wave is in the same direction as the motion of the individual coils. Describe the motion of the particles in a surface wave. This particle motion is both parallel and perpendicular to wave motion. See your textbook…pg. 382…for more information. A periodic wave has a wavelength of 0.85 m and a speed of 64 m/s. a) What is its frequency? Given: λ = 0.85m Find: f v = 64 m/s ? = ?? 64 = 0.85? ? = = 75.29 f = 75.3 Hz What is its period? Given: λ = 0.85m Find: T v = 64 m/s f = 75.3Hz 1? = ?? = = 0.01328 T = 1.33 x 10-2seconds a) Label the nodes and antinodes shown in the standing wave below using the letter “N” for node and “A” for antinode. There are 5 nodes and 4 antinodes in the standing wave below. 69809026981 Draw two waves that will meet and constructively interfere. Include wave direction arrows! Draw two waves that will meet and destructively interfere. Include wave direction arrows! N N N N A N A A A Draw two waves that will meet and constructively interfere. Include wave direction arrows! Draw two waves that will meet and destructively interfere. Include wave direction arrows! N N N N A N A A A 15. 16. 17. Draw the reflected pulse for the following two scenarios: a) Draw the appropriate Reflected Pulse on the b) Draw the appropriate Reflected Pulse on the diagram diagram above for Fixed End Reflection. above for Free End Reflection. 18. Sound waves are longitudinal waves. 18. Sound waves are longitudinal waves. Draw a picture of a sound wave. a) Calculate the wavelength a 392-Hz sound wave in air (v=343 m/s). Given: f = 392Hz Find: λ v = 343m/s ? = ?? 343 = ?(392) ? = = 0.875 λ = 0.875m b) Calculate the wavelength of the speed of sound in copper (v=3560 m/s) of a 392-Hz wave. Given: f = 392Hz Find: λ v = 3560m/s ? = ?? 3560 = ?(392) ? = = 9.0816 λ = 9.08m You are standing on a sidewalk. An ambulance with its siren on approaches then passes by you. How does the sound change as the ambulance approaches then moves away from you? On approach the sound you hear is a higher frequency (pitch) that the siren is actually making. When it is directly in front of you, you hear the actual frequency (pitch), as it moves away you hear a lower frequency (pitch). The pitch never actually changes…it is just that what you hear changes. Unit 10: Light Essential questions: x Does an object have to produce its own light in order to be seen? Nope…we see the moon…it does not produce its own light! Luminous bodies (like the sun) produce their own light. Illuminated objects can be seen because they reflect light. This is basically everything that is not a light source (the moon, you, me, book, trees, cars, etc.). x How is comparing properties of light waves to mechanical waves helpful in learning about light? Because light waves are indeed waves. They reflect, refract, diffract, disperse, etc…just like all other waves. x Why do some light waves pass through a polarized filter and some do not? It depends on how the filter is turned. See the pictures in the polarization questions that are in the practice section. x How can polarization be used to improve environmental conditions? There are a variety of different ways we can used polarized light to improve conditions…scattering aerosols, useful for weather radar, etc. Feel free to research different applications to expand and enhance your knowledge here. x What is the speed of light in a vacuum, and why does this speed get a special equation symbol c, instead of using v, for velocity? vlight = c = 3.00 x 108m/s It gets its own symbol because this value is the only true known constant in the universe. In every test, under every different condition, the speed of light always measures precisely the same amount. It is special, so it gets special status. x What are some similarities and differences between mechanical and light waves? Difference: mechanical waves require a medium for transmission…they have to have something to travel through; electromagnetic waves DO NOT require a medium…they can travel through a vacuum (and it’s a good thing…because space is a vacuum…and that’s how the Earth gets all her energy!). Similarities: everything else; all other wave behaviors that occur in mechanical waves also occur in electromagnetic waves. x How can we apply the mathematical equations used for mechanical waves to electromagnetic waves? We apply them in exactly the same way: v = λf (except really here c = λf) and f = 1/T. x Why is it important to make observations and record data to help understand and explain the universe around us? We can learn new things about our planet, our solar system, and our universe. Specifically, this is how Hubble determined that the universe is expanding…a pretty big discovery. x How does the roughness of a surface affect the light reflecting off the surface into a person’s eyes? This is diffuse reflection. Diffuse reflection is when the rays do not reflect of a surface parallel to one another. This allows us to see the thing…but it is not blinding or glaring to us. For example, mirrors, shiny metals, etc…those have specular reflection. You can reflect all the incoming rays in one path. Diffuse reflection reflects the incoming light rays in different paths so that the object is visible. x How can the ray model of light be used to explain the law of reflection? One ray in…hits the surface and changes direction…bounces away. The incoming angle is equal to the reflected angle. This is the law of reflection. x How are image formation, position, height, magnification, and orientation determined for plane mirrors? Images in plane mirrors are ALWAYS: upright, same sized (same height and magnification = 1), same distance into the mirror as the object, and virtual. x How are image formation, position, height, magnification, and orientation determined for spherical mirrors? See ray diagrams. x How does refraction of light affect what we see? Refraction causes light to bend. Which way it bends depends on whether it’s moving from a medium with a high speed of light to a lower speed of light or vice versa. The refraction will cause things to appear in a different position than they really are. For example…when you stand on a boat or dock or bank and look into the water and see a fish. If you try to grab it the fish is always in a different position under the water than it looked to you above the water due to light’s refraction. x How is construction of a lens related to how the lens refracts light? Concave lenses cause light to diverge; convex lenses cause light to converge. This is important because their image formation is different based on their shape. x How are image formation, position, height, magnification, and orientation determined for thin lenses? Using the lens and magnification equations OR using ray diagrams. Key vocabulary: Luminous sources – emit their own light Illuminated source – reflect light rather than emitting their own Polarization – property of electromagnetic waves that describes the orientation of their oscillations Diffuse reflection – reflection off a surface such that the waves are reflected in many different directions Specular reflection – reflection off a surface such that the waves are reflected in the same direction (think mirrors) Reflection – change in direction of a wave due to bouncing off of surface Refraction – change in direction of a wave due to a change in medium Index of refraction – (n) ratio of the speed of light in the material to the speed of light in a vacuum; optical density.Practice: List 2 examples each of a luminous source and an illuminate source. Luminous source: the sun, a lightbulb Illuminated source: a full moon, a mirror A concave mirror forms a real image 28 cm away from the mirrored surface. If the object is 13 cm away, what is the focal length of the mirror? Given: di = 28 cm Find: f (concave mirrors are converging, so f will be positive) do = 13cm 111=+ ???1113173066-261556= + = 0.1126 ? 28 131? == 0.1126= 8.88 ?f = 8.88cm Carson Busses is driving down the road on a sunny day. Reflection of light off the road surface results in a large amount of polarization and a subsequent glare. Annoyed by the glare, Carson pulls out his Polaroid sunglasses. How must the axes of polarization be oriented in order to block the glare? (Note: the lines on the filters below represent the axis of polarization.) Use the picket-fence analogy to assist you with your answer! Use the picket-fence analogy to assist you with your answer! The glare from the road is light that has been partially polarized already by reflecting off the road. It is reflecting parallel to the road’s surface and making its way through our NON-polarized glasses, like the drawing on the left above. In order to block that glare, we want a transmission axis that is NOT parallel to what is already coming through…if it is leaving the road in one direction…we want something like what is on the right. Leaves the road in one direction, we block it with our glasses in the other direction. The reason we know the answer is A, is because think when it leaves the road it will reflect parallel to the road’s surface…so it will reflect like B…meaning B WON’T block the glare…but A will block the glare. C and D just don’t make sense…in either case you’d get glare in one eye but not the other…which is silly. An object placed 16 cm from a thin converging lens along the axis of the lens produces a real image behind the lens at 7 cm from the lens. What is the focal length? Given: di = 7 cm Find: f (converging lens means f will be positive) do = 16cm 111=+ ???111=+= 0.2054 ?71613208626-600084? == 0.2054= 4.87 ?f = 4.87cm 698395417783Think of a material that reflects almost all the light that shines on it, but in which you cannot see your reflection. Is this diffuse or specular reflection? Diffuse! Most surfaces reflect all that light shined on it…it just isn’t reflected in the same direction…so it’s diffuse reflection. Describe the images produced by a flat (plane) mirror using the following choices: Real or Virtual? Virtual 1809340-18319Enlarged or Reduced? Neither, SAME SIZE Upright or Inverted? Upright Its location along the principal axis. Same distance behind the mirror as the object is in front (di = do) A light ray strikes a flat (plane) mirror at 42 degrees. What is the angle of reflection? 42 degrees The law of reflection states that Θi = Θr In the spaces provided below, describe the image that will be formed when an object is placed in the locations listed. The description should include whether the image is: a. Real or Virtual Enlarged or Reduced Upright or Inverted Its location along the principal axis. Convex Lens Concave Mirror Object Location Image Description Object Location Image Description beyond 2F real reduced inverted between f and 2f beyond C real reduced inverted between f and C @ 2F real same size inverted at 2f @ C real same size inverted at C between F and 2F real enlarged inverted beyond 2f between F and C real enlarged inverted beyond C @ F NO IMAGE @ F NO IMAGE in front of F virtual enlarged upright same side of lens as the object in front of F virtual enlarged upright opposite side of mirror as the object An object is placed 22 cm from a diverging lens. If a virtual image appears 6 cm from the lens on the same side as the object, what is the focal length of the lens? Given: di = -6 cm (virtual images have negative distances) Find: f (diverging lens means f will be negative) do = 22 cm 111=+ ???111=+= ?0.1212 ??62213110074-600157? == ?0.1212= ?8.25 ?f = -8.25cm If a virtual image is formed 13 cm along the principal axis from a convex mirror with a focal length of -25 cm, what is the object’s distance from the mirror? Given: di = -13 cm (virtual images have negative distances) Find: do f = -25 cm (diverging lens means f is negative) 111 2830674-721316?do = 27.08 cm A light ray goes from water (n = 1.33) to crown glass (n = 1.52) at an angle of 37 degrees. Draw and label a picture including the boundary, normal, incident ray, and refracted ray. water glass water glass Describe, using WORDS, what is occurring in the picture you drew in part a) above. The light ray in the water, hits the surface of the glass and because glass has a higher index of refraction, the ray travels through it, but bends closer to the normal. (The normal is an imaginary line we use as a reference point to measure angles of incidence and angles of reflection.) Use Snell’s Law to calculate the angle of refraction. Given: ni = 1.33 Find: Θr nr = 1.52 Θi = 37 ? sin ? = ? sin ? 1.33 sin 37 = 1.52 sin ? 347545301.sin ? = = 0.5265 ? = sin0.5265 = 31.775 Θ = 31.8° Describe the conditions necessary for total internal reflection to occur. Must be going from a higher index of refraction to a lower index of refraction (from high n to low n) Must be at an angle of incidence higher than the critical angle Define index of refraction (as a ratio) in equation form. ?? = ? Unit 11: Electricity Essential questions: x What are current, potential difference, resistance, and power? Current: flow of charge carriers (I) Potential difference: a difference in electric potential between two points (ΔV) Resistance: opposition to current that is present in the material it is trying to flow through (R) Power: rate of energy dissipationv(P) x What do the circuit schematics look like? They are drawings of all the elements in a circuit. x How do you calculate the equivalent resistance and total current in a series circuit? o Equivalent resistance: ? ?= ?+ ?+ ? (resistance adds up) How do you calculate the current in each resistor in a series circuit? ? ? = ?= ?= ? (current is the same in each part as the total) How do you calculate the potential difference across each resistor in a series circuit? ? ? = ??, but ? = ? + ? + ? (voltage drops in each part add up to the total voltage) x How do you calculate the equivalent resistance and total current in a parallel circuit? o Equivalent resistance: 138414477123? = + + ? (the inverses of the resistances add up) How do you calculate the current in each resistor in a parallel circuit? ? = ? + ?+ ? (current in each part adds up to the total) How do you calculate the potential difference across each resistor in a parallel circuit? ? = ? = ?= ?(voltage is the same in each part as the total) x How do you calculate the equivalent resistance, total current, current in each resistor, and potential difference across each resistor in a complex circuit? First reduce down to either all parallel or all series. Then apply the rules for whichever applies and carry back your results to each previous step until you are back at the original complex circuit. x How do you calculate the power dissipated by a given electrical device? ? = ?? ?? ? = ? ? Key vocabulary: Electric field – a region where electric force on a test charge can be detected (E = F/q) Current – electric charge per unit of time (I = Q/t) Resistance - opposition to current that is present in the material it is trying to flow through (R) Conductor – materials where resistance is low (think: metals) Insulator – materials where resistance is high (think: rubber)4899885-38414Practice: In the figure shown at right, calculate the… [Note: The 40.0Ω should be 40.0V] a. Equivalent resistance Step 1: 8+2 = 10Ω 131556466973Step 2: = + = .2 ?= (. 2)= 5? Step 3: 5 + 5 = 10Ω = Req Total current Vt = ItReq (rearranged…) It = Vt/Req It= 40/10 = 4A Current through each resistor In series, total current is the same as the current through each piece, so the 5Ω resistor has 4A. Also, the entire reduced green box gets 4A…but not each resistor there…the whole thing. So now…consider that inside that green box you have two branches, each with 10Ω of resistance. Because the two parallel branches are equal, each gets half the current. So…the 10Ω resistor gets 2A of current. That leaves the red box. There, the two resistors are in series, so they get the same as the total through the branch, which is 2A. Voltage (V) Current (I) Resistance (R) 16 2 8 4 2 2 20 2 10 20 4 5 Total 40 4 10 Voltage through each resistor Now, just apply V = IR to each resistor since you know the I and the R in each one to calculate V. Check your work in the chart below. Four resistors with values of 1.0 Ω, 2.0Ω, 3Ω, and 4Ω are connected …in series. What is the equivalent resistance of this combination? 1+2+3+4 = 10Ω … in parallel. What is their equivalent resistance? 46979565520 = + ++ = 2.083 ?= (2.083) = 0.48Ω In general, as you connect more resistors …in series what happens to the equivalent resistance of the comibination? increases …in parallel what happens to the equivalent resistance of the comibination? decreases Unlike charges attract ; like charges repel . An electric hair dryer requires 1500 W at 110 V. What is the resistance of the heating coil in the hair dryer? Given: P = 1500W Find: R V = 110V ? = ?? and ? = ?? First, use ? = ?? to find current. 1500 = ?(110) ? = = 13.636 Next, use ? = ??to find R. 110 = (13636)? ? = = 8.067 R = 8.07Ω A small handheld flashlight has 2 – 1.5 V (AA size) batteries. This gives a potential difference of 3 V across it. The bulb has a resistance of 5.0Ω. How much current is in the bulb filament? Given: V = 3V Find: I R = 5Ω ? = ?? 3 = ?(5) ? = = 0.6 I = 0.6A An electric field of 4000 N/C is produced by a charge of 7 x 10-9 C. How far away is the charge? Given: E = 4000N/C Find: r q = 7 x 10-9C k = 8.99 x 109 N?m2/C2 (remember that this is a constant) ??? = ?3087722-176358(8.99 x 10 )(7?10)62.934000 = = ???= = 0.0157 r = 0.125m What is the relationship between current, charge and time? Current equals charge over time ? = Current and time are inversely related; current and charged are directly related. If you hold charge constant and increase the time, what happens to the current? The bottom of the fraction gets bigger…current decreases. If you hold charge constant and decrease the time, what happens to the current? The bottom of the fraction gets smaller…current increases. When a positive charge moves in the direction of the electric field, what happens to the electrical potential energy associated with the charge? The PEe decreases (the charge is moving in the direction it wants to move naturally…so it ‘loses’ PE) When a positive charge moves in the direction away from the electric field, what happens to the electrical potential energy associated with the charge? The PEe increases When a negative charge moves in the direction of the electric field, what happens to the electrical potential energy associated with the charge? The PEe increases (the charge is being forced to move in a direction it does not naturally want to move…so it’s ‘gaining’ PE) When a negative charge moves in the direction away from the electric field, what happens to the electrical potential energy associated with the charge? The PEe decreases Two point charges, initially 5 cm apart, experience a force, F. What is the electric force between the charges when they are moved to each of the distances below? ?? ?? = ?…10 cm? if r was originally 5cm, and is now 10cm we can replace r with 2r. ?? ??? ?1 ?? ?? === (2?)4?4? So, the new force is ? …15 cm? if r was originally 5cm, and is now 15cm we can replace r with 3r. ?? ??? ?1 ?? ?? === (3?)9?9? So, the new force is ? …2.5 cm? if r was originally 5cm, and is now 2.5cm we can replace r with ?. ?? ??? ??? ?309445359689? === 4 ( ?) ?? So, the new force is 4? Two point charges having charge values of 5.0μC and -7.0 μC, respectively, are separated by 2.0 cm. What is the electrostatic force between them? Given: r = 2cm = 0.02m Find: F q1 = 5x10-6C q2 = 7x10-6C k = 8.99x109 N?m2/C2 (constant) ?? ?? = ?? = ? = 786.6?; ?????????? (??????? ??? ?? ???????? ??? ??? ?? ????????) How is the power affected by… Use this power equation: ? = ? ? …doubling the current in a resistor? ? = (2?) ? = 4? ? So…Power goes from P to 4P …tripling the current in a resistor? ? = (3?) ? = 9? ? So…Power goes from P to 9P …decreasing the current in a resistor to half its original value? ?? = ( ?) ? =? ? So…Power goes from P to Unit 12: Magnetism Essential questions: x What are the names of the poles of a magnet? North and south. Which magnetic poles attract? North attracts to south. Which magnetic poles repel? Like poles repel (north-north or south-south) x What does the magnetic field look like: o Around a bar magnet? Between two like poles? Between two unlike poles? Around a long, straight, current-carrying wire? Around a loop of current-carrying wire? Around the Earth? x What is a magnetic domain made of? a region composed of a group of atoms whose magnetic fields are aligned in the same direction x What is the direction of the force on a wire carrying a current in a magnetic field? Right hand rule #1: thumb points in direction of current, fingers wrap in direction of magnetic field. o What is the size of that force? ? = ??? x What is the direction of the force on a charged particle moving in a magnetic field? Right hand rule #2: 1. Fingers in direction of magnetic field 2. Thumb in direction of velocity of the proton 3. Palm points in direction of force. [NOTE: if it’s an electron either use left hand rule OR reverse the direction of the force] o What is the size of that force? ? = ??? x How can a current be produced in a loop of wire and what is the direction of an induced current? Three ways to produce a current: Direction of the induced current: 1. Vary the strength of the magnetic field The direction of an induced current is always such 2. Move the circuit into/out of the field that its magnetic field opposes the change causing it.3. Spin/rotate the circuit in the field x What is the mathematical relationship between the frequency and wavelength of an electromagnetic wave? ? = ?? x What are the seven regions of the electromagnetic spectrum in order of decreasing wavelength? Radio, microwave, infrared, visible, ultraviolet, x-ray, gamma ray x How is an electromagnetic wave produced? Oscillating electric and magnetic fields are perpendicular to each other and induce one another. They create the EM wave that is perpendicular to each of them. Key vocabulary: Magnetic domain – a region composed of a group of atoms whose magnetic fields are aligned in the same direction Electromagnetic induction – the process of creating a current in a circuit by changing the magnetic field strength Magnetic field – a region where a magnetic force can be detected Practice: Explain Lenz’s Law. Draw a picture or describe an example. 46949044012 The direction of an induced current is always such that its magnetic field opposes the change causing it. Example: In the picture below, the blue magnetic field lines are from the magnet. As it is lowered into the loop of wire, there are MORE downward field lines. The current induced in the wire, will create a magnetic field that opposes the increase in those downward field lines. Meaning that inside the loop of wire, the induced current will have a magnetic field that points upward. Using right hand rule for wires (thumb in direction of current, fingers wrap in direction of magnetic field) you will find that the current must point as indicated in order for its magnetic field to oppose the change causing it. 5265010285830What is the instantaneous direction of the force on a proton that enters the magnetic field shown at right? The proton enters from the left and moves toward the right. UP (toward the top of the page) Remember right hand rule: 1) Fingers in direction of magnetic field 2) Thumb in direction of velocity of the proton 3) Palm points in direction of force. [NOTE: if it’s an electron either use left hand rule OR reverse the direction of the force] What is the instantaneous direction of the force on an electron that enters the magnetic field shown at right? The electron enters from the left and moves toward the right. DOWN (toward the bottom of the page) What is the instantaneous direction of the force on a proton that enters the magnetic field shown at right? The proton enters from the top and moves toward the bottom. to the RIGHT What is the instantaneous direction of the force on an electron that enters the magnetic field shown at right? The electron enters from the top and moves toward the bottom. to the LEFT Describe all the methods of inducing an emf in a loop of wire. Vary the strength of the magnetic field Move the circuit into/out of the field Spin/rotate the circuit in the field Physics I Honors Semester 2 Exam Review Describe the transmission/propagation of electromagnetic waves. Oscillating electric and magnetic fields are perpendicular to each other and induce one another. They create the EM wave that is perpendicular to each of them. An electron moves north at a velocity of 5 x 105m/s and experiences a magnetic force of 6 x 10-12N west. If the magnetic field points upward, what is the magnitude of the magnetic field? Given: q = 1.6x10-19C Find: B (magnetic field) v = 5x105m/s F = 6x10-12N west ? = ??? 6?10= (1.6?10)(5?10 )? ? = = 75 B = 75 Teslas If a wire carries a current of 6A and a magnetic field of 0.004T produces a 5 x 10-3N force on it, how long is the wire? Given: I = 6A Find: l (length) B = 0.004T F = 5x10-3N ? = ??? 5?10= (6)?(0.004) ? = = 0.208 l = 0.208m (which is 20.8cm) A coil with 300 turns of wire has an area of 0.4m2. In 0.4 s it turns through 90°. If the average induced emf is 50V, what is the strength of the magnetic field? Given: N = 300 turns Find: B (magnetic field) A = 0.4m2 Δt = 0.4s emf = 50V ?????? = ? ??? ? = ?? ??In order to find B, we need to first find Φ (300)?50 = ? 0.4? = ? = 0.0667 ? = ?? 0.0667 = (0.4)? 308683300.? = = 0.16675 B = .167T Physics I Honors Semester 2 Exam Review Unit 13: Modern Essential questions: x What phenomena’s could not be explained by the wave model of light and how do they demonstrate the particle-like nature of light? Photoelectric Effect and Compton Effect x What evidence is there for the wave-like nature of matter? de Broglie’s evidence x How did the early models of the atom lead to the Quantum Model? x What led to the current nuclear model and explain how energy produced in nuclear reaction? x What are the three families of elementary particles and how are the electron, proton, and neutron classified? x What are the four fundamental interactions/forces in nature? Strong nuclear – strongest, holds nuclei together Weak nuclear – responsible for beta decay Electromagnetic Gravitational – weakest, holds the universe together Key vocabulary: de Broglie waves –matter waves; evidence of the wave like nature of matter Photoelectric effect – when electrons are ejected from a material when light of a certain frequency is shined on the surface of the metal Quantum Model – model dealing with the physical phenomena where action in the atom is on the order of Planck’s constant (really small stuff!) Fusion – small nuclei are fused/combined together and there is a lot of energy expelled. Fission – larger nuclei are split and there is a lot of energy expelled, but also there is nuclear waste. Practice: The de Broglie wavelength of an object has what sort of relationship with speed? Inverse! ?= According to Einstein, what it the relationship between a particle’s mass and the energy that it could potentially release? ? = ?? Photoelectric electric effect is most easily observable for light under what conditions? The light waves have short wavelengths (but high energy, high frequency), will deliver packets of to the electrons and cause them to eject from the material’s surface. The Quantum Model of light is useful for what sort of predictions? The photons wavelength and the characteristic light emitted help identify the element. This allows us to analyze the composition of materials. Astronomers and astrophysicists use this heavily to determine the composition of stars. Describe each of the four fundamental forces of nature. Strong nuclear – strongest, holds nuclei together Electromagnetic – responsible for attraction and repulsion of charges Weak nuclear – responsible for beta decay Gravitational – weakest, holds the universe together Describe each of the following types of nuclear decay: Alpha decay – an alpha particle is emitted from a helium nucleus [least powerful penetration] Beta decay – an electron is emitted from a neutron Gamma decay – photons are released (high energy, but no mass) [most powerful penetration] Compare and contrast fission and fusion. Fission Similarities Fusion Large nuclei split (usually uranium) Example: the type of power at a nuclear power plant Both involve rearranging/changing the nucleus of an element Both release LOTS of energy Combines smaller nuclei – usually hydrogen Example: all the energy the sun has is released from fusion reactions ................
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