Chapter 24 Thermal Physics Questions & Solutions 1. What is the meaning of the Greek words from which we get the word thermodynamics?

Heat and movement

2. Is the study of thermodynamics concerned primarily with microscopic or microscopic processes?

Macroscopic

3. What is the lowest possible temperature on the Celsius scale? On the Kelvin scale? -273.15 ºC, 0 K

4. What is the temperature of melting ice in Kelvin? Or boiling water? 273 K; 373 K

5. How does the law of the conservation of energy relate to the first law of thermodynamics?

The first law of thermodynamics is the law of conservation of energy applied to thermal systems

6. What happens to the internal energy of a system when work is done on it? What happens to its temperature?

Increases; increased

7. What is the relationship between heat added to a system heat added to a system and the internal energy and

external work done by the system? Amount of heat added = increases in internal energy + work done

8. If work is done adiabatically on a system, will the internal energy of the system increase or decrease? If

work is done by a system, will the internal energy of the system increase or decrease?

Increase; decrease

9. What condition is necessary for a process to be adiabatic?

No heat enters or leaves while the process occurs

10. What happens to the temperature of the air when it is adiabatically compressed? When it adiabatically

expands? Increases; decreases

11. What generally happens to the temperature of rising air? Decreases, if adiabatic

12. What generally happens to the temperature of sinking air? Increases if adiabatic

13. How does the second law of thermodynamics relate to the direction of heat flow?

It defines the direction of heat flow from hot to cold

14. What three processes occur in every heat engine?

Energy absorption, conversion of some energy to work, expulsion of the rest

15. What is thermal pollution? Unwanted exhaust

16. If all friction could be removed form a heat engine, would it be 100% efficient? Explain.

No, efficiency depends on input and output temperatures

17. What is the ideal efficiency of a heat engine that operates with its hot reservoir at 500 k and its sink at 300

K? 0.4 or 40%

18. Why are heat engines intentionally run at high operating temperatures? To increase efficiency

19. Give at least two examples to distinguish between organized energy and disorganized energy.

Electricity converting to heat, car braking to a stop

20. How much of the electrical energy transformed by a common light bulb becomes heat energy? 100%

21. With respect or orderly and disorderly states, what do natural systems tend to do? Can a disorderly state

ever transform to an orderly state? Explain. Become disordered; yes, but only with work input

22. What is the physicist’s term for a measure of messiness? Entropy

23. Under what condition can entropy decrease in a system? Only with work or other organized energy input

24. What is the relationship between the second law of thermodynamics and entropy?

Entropy increases in natural systems

25. Distinguish between the first and second laws of thermodynamics in terms of whether or not exceptions

occur. The first law has no exceptions, the second law may have some exceptions

Plug and Chug 26. Calculate the ideal efficiency of a heat engine that takes in energy at 800K and expels heat to a reservoir at

300K. IE = (Thot – Tcold) / Thot – (800 K – 300 K) / (800 K) = 0.63

27. Calculate the ideal efficiency of a ship’s boiler when steam comes out at 530 K, pushes through a steam

turbine, and exits into a condenser that is kept at 290 K by circulating seawater.

IE = (Thot – Tcold) / Thot – (530 K – 290 K) / (530 K) = 0.45 28. Calculate the ideal efficiency of a steam turbine that has a hot reservoir of 112ºC high-pressure steam and a

sink at 27ºC. IE = (Thot – Tcold) / Thot – ( (112 + 273) K – (27+273) K) / ( (112 + 273) K) = 0.22

29. In a heat engine driven by ocean temperature differences, the heat source (water near the surface) is at 293

K and the heat sink (deeper water) is at 283 K. Calculate the ideal efficiency of the engine.

IE = (Thot – Tcold) / Thot – (293 K – 283 K) / (293 K) = 0.034

Think and Explain (Extra Credit) 30. A friend said the temperature inside a certain oven is 600 and the temperature inside a certain star is 60,000.

You’re unsure about whether your friend meant K or °C. How much difference does it make in each case?

An oven at 600°°°°C (873 K) is hotter than an oven at 600 K by 45% in absolute temperature; a star at

60,273 K is hotter than a star at 60,000K only by 0.45% 31. When you pump a tire with a bicycle pump, the cylinder of the pump becomes hot. Give two reasons why

this is so. Compression of air and friction in the piston on the inner wall of the chamber

32. Is it possible to entirely convert a given amount of heat into mechanical energy? Is it possible to entirely

convert a given amount of mechanical energy into heat? Cite examples to illustrate your answers. No; yes;

if you drag a block across a floor, you produce heat but heat cannot drag the block back.

33. We know that warm air rises. SO it might seem that the air temperature should be higher at the top of

mountains than down below. But the opposite is more often the case. Why? Rising air undergoes adiabatic

expansion and cools 34. Will the efficiency of a car engine increase, decrease, or remain the same if the muffler is removed? If the

car is driven on a very cold day? Defend your answers. Increase; back pressure is reduced. Efficiency

also increases on a cold day due to the increase in the temperature difference between the hot

reservoir in the engine and its surroundings (the sink) 35. The combined molecular kinetic energies of molecules in a very large container of cold water are greater

than the combined molecular kinetic energies in a cup of hot tea. Pretend you partially immerse the teacup

in the cold water and that the tea absorbs 10 joules of energy from the water and becomes hotter, while the

water that gives up 10 joules of energy becomes cooler. Would this energy transfer violate the first law of

thermodynamics? The second law of thermodynamics? Explain. No, energy is conserved; you, internal

energy will not freely transfer from a cooler to a warmer object. 36. A mixture of fuel and air is burned rapidly in a combustion engine to push a piston in the engine that in turn

propels the vehicle. In a jet engine a mixture of fuel and air is burned rapidly and, instead of pushing

pistons, pushes the aircraft itself. Which do you suppose is more efficient? Jet engine; it saves a step and

so is more efficient 37. Suppose one whishes to cool a kitchen by leaving the refrigerator door open and closing the kitchen door

and windows. What will happen to the room temperature? Why? After a very brief decrease due to the

mixing of warm and cold air, the room temperature will increase because the room is the heat sink. 38. In buildings that are being heated electrically, is it wasteful to turn on all the lights. Is turning on all the

lights wasteful if the building is being cooled by air conditioning ? Defend your answers. No, the lights

help heat; in air conditioning it is wasteful because you must get rid of the heat they produce 39. Water put into a freezer compartment in your refrigerator goes to a state of less molecular disorder when it

freezes. Is this an exception to the entropy principle? Explain. No, work has been put into the

refrigeration system

40. ON a cold 10°C day, your friend who likes cold weather says she wishes it were twice as cold. Taking this

to mean she wishes the air had half the internal energy, what temperature would this be? Half its absolute

temperature or (1/2)(10+273) K = 141.5 K = -131.5°°°°C 41. Why is “thermal pollution” a relative term? It refers to an undesirable by produce of some process, and

its desirability is relative. 42. Is it possible to construct a heat engine that produces no thermal pollution? Defend your answer. Yes. If

the exhausted heat is desirable then no thermal pollution is produced. 43. What happens to the efficiency of a heat engine when the temperature of the reservoir into which heat

energy is rejected is lowered? Increases (substitution a smaller Tcold into the efficiency equation)

Unit 2 Optics Review Questions & Solutions Chapter 29 -

1. Why do smooth metal surfaces make good mirrors? They reflect almost all the colors of visible light.

2. What becomes of a wave’s energy when the wave is totally reflected at a boundary? When it is partially

reflected at a boundary? It reverses direction and goes back through the original medium; part goes

into the second medium. 3. When light strikes perpendicular to the surface of a pane of glass, how much light is reflected and how

much is transmitted? 4%; 96% (at its surface)

4. What is meant by normal to a surface? Any line that is perpendicular to the surface

5. What is the law of reflection? Angle of incidence – angle of reflection

6. When you view your image in a plane mirror, how far behind the mirror is your image compared with

your distance in front of the mirror? Same distance

7. Does the law of reflection hold for curved mirrors? Yes, but the normals at different points are not

parallel to one another. 8. Does the law of reflection hold for diffuse reflection? Explain. Yes, each single ray obeys the law of

reflection 9. What is meant by the idea that a surface may be polished for some waves and rough for others? The

surface is considered polished if its irregularities are less than (1/8) of a wavelength of the incident

wave. 10. Distinguish between an echo and a reverberation. Echo – Single Reflection; Reverberation –

Multiple Reflections 11. Does the law of reflection hold for both sound waves and light waves? Yes, and for all other types of

waves. 12. Distinguish between reflection and refraction. Reflection – waves travel back into the original

medium; Refraction – waves enter a new medium. 13. When a wave crosses a surface at an angle from one medium into another, why does it “pivot” as it

moves across the boundary into the new medium? Different parts of the wave change speed at

different times. 14. What is the orientation of a wave in relation to the wave front of a wave? Perpendicular

15. Give an example where refraction is abrupt, and another where refraction is gradual. Abrupt – light

traveling from air to water (sharp boundary); gradual – light traveling through the atmosphere 16. Does refraction occur for both sound waves and light waves? Yes, and for all other types of waves

17. If light had the same speed in air and in water, would light be refracted in passing from air into water?

No, refraction depends on change in wave speed.

18. If you can see the face of a friend who is underwater, can she also see you? Yes (eyes at least). The

directions of rays are reversible 19. Does refraction tend to make objects submerged in water seem shallower or deeper than they really are?

Shallower 20. Is a mirage the result of refraction or reflection? Explain. Refraction. It only appears to be a

reflection. 21. Is daytime a bit longer or a bit shorter because of atmospheric refraction? Longer

22. As light passes through glass or water, do the high or low frequencies of light interact more in the

process of absorption and reemission and therefore lag behind? High frequencies

23. Why does blue light refract at grater angles than red light in transparent materials? Blue interacts more

and slows down more than red 24. What conditions are necessary for viewing a rainbow in the sky? The observer must be between a low

sun and the waters drops 25. How is a raindrop similar to a prism? Both refract and disperse light.

26. What is the critical angle in terms of refraction and total internal reflection? The angle at which light

doesn’t refract, but reflects 27. Why are optical fibers often called light pipes? They literally pipe light along the fiber

Think and Explain (Extra Credit)

28. Suppose that a mirror and three lettered cards are set up as in the figure. If a person’s eye is at point P,

which of the lettered cards will be seen reflected in the mirror? Explain. By the law of reflection, only

light from card B reaches his or her eyes. 29. Why is the lettering on the front of some vehicles “backward” as seen here? It will be seen in proper

form in the rearview mirrors of cars in front. 30. Trucks often have signs on their backs that say, “If you can’t see my mirrors, I can’t see you.” Explain

the Physics here. Light that takes a path from one point to another will take the same path when it

goes in the reverse direction 31. Contrast the types of reflection from a rough road and from the smooth surface of a wet road to explain

why it is difficult for a motorist to see the roadway ahead when driving on a rainy night. A dry road

causes diffuse reflection of headlight beams and only a small part of the reflected light returns to

the driver’s eyes. A wet road acts more like a plane mirror, so most of the light is reflected ahead

(causing glare for oncoming motorists!) 32. Cameras with automatic focus bounce a sonar (sound) beam from the object being photographed, and

compute distance from the time interval between sending and receiving the signal. Why will these

cameras not focus properly for photographs of mirror images? Sound, like any wave, spreads as it

travels and is diluted with distance. 33. Why is an echo weaker than the original sound? Sound, like any wave, spreads as it travels and is

diluted with distance. 34. Does the reflection of a scene in calm water look exactly the same as the scene itself only upside down?

(Hint: place a mirror on the floor between you and a table. Do you see the top of the table in the

reflected image?) No, the reflected view of an object is seen from a lower angle, as from a point as

far below the reflecting surface as the viewer is above it. The geometrical difference is most

noticeable when closer objects and their reflections are viewed. 35. If you were spearing a fish with a spear, would you aim above, below are directly at the observed fish to

make a direct hit? Would your answer be the same if you used laser light to “spear” the fish? Defend

your answer. Below. The fish appears to be closer to the surface than it really is, because of the

refraction of light that leaves the fish and travels to your eye in the air above the water; No, laser

light will travel back along the same path as the light from the fish, so you would have to aim

directly at the fish’s image. 36. A rainbow viewed from an airplane may form a complete circle. Will the shadow of the airplane appear

at the center of the circle? Explain with the help of figure 29.26. Yes, your head is directly between

the sun and the center of the bow. 37. The photo below shows two identical cola bottles, each with the same amount of cola. The right bottle

is in air, and the left bottle is encased in solid plastic that has nearly the same “index of refraction” as

glass (the speed of light in the plastic and the glass are nearly the same). Which bottle shows an illusion

of the amount of cola? How does the other bottle give a truer view of its contents? Both show illusions,

but the encased bottle shows a truer view (refraction at curved bottle shrinks cola a bit). More

refraction occurs fro the bottle in air, which sows more cola than exists.

Light & Color Questions & Solutions Chapter 28 1. List the order of colors in the color spectrum. Red, orange, yellow, green, blue, violet

2. Are black and white real colors, in the sense that red and green are? Explain.

No, black is the absence of light and white is a combination of all the colors of light.

3. A vibrating tuning fork emits sound. What is emitted by the vibrating of atoms. Light

4. What happens to light of a certain frequency that encounters atoms of the same resonant frequency?

It is absorbed

5. Why does the color of an object look different under a fluorescent lamp light from the way it looks under an

incandescent lamp?

Incandescent light – more lower frequencies; fluorescent – more higher frequencies

6. What color(s) of light does a transparent red object transmit? What color(s) does it absorb?

Red; Non-Red colors

7. What is the function of a pigment? Pigments absorb light of specific colors

8. Why are more and more fire engines being painted yellow green instead of red?

The human eye is most sensitive to yellow-green

9. How can yellow be produced on a screen if only red light and green light are available?

Overlap the red and the green 10. What is the name of the color produced by a mixture of green and blue light? Cyan

11. What colors of spots are lit on a television tube to give full color? Red, Green and Blue

12. What are complementary colors? Two colors tat when added produce white

13. What color is the complement of blue? Yellow

14. The process of producing a color by mixing pigments is called color mixing by subtraction. Why do we say

“subtraction” instead of “addition” in this case? Pigments absorb or “subtract” colors from light

15. What colors of ink are used to print full-color pictures in books and magazines?

Cyan, Yellow, Magenta, Black

16. What is light scattering? Redirection of light in multiple directions

17. Do tiny particles in the air scatter high or low frequencies of light? What frequencies do large particles

scatter? High; Low

18. Why is the sky blue? Molecules in the air scatter high-frequency light

19. Why is the sky sometimes whitish? Larger particles scatter low frequencies too

20. Why are clouds white? The mixture of droplet sizes scatters many frequencies

21. Why are sunsets red? Only low frequencies are not scattered on the long path through the atmosphere

22. Why is water greenish blue? Water absorbs red light

23. What is a spectroscope, and what is its function? A device that separates colors

24. Does the red light from glowing neon gas have only one frequency or a mixture of frequencies?

A mixture

25. Why might atomic spectra be considered the “fingerprints” of atoms?

Each element has its own characteristic spectrum.

Think and explain- extra credit 26. What is the color of common tennis balls, and why?

Yellow-green tennis balls are highly visible. Our eyes are most sensitive to yellow-green.

27. Shine red light on a rose. Why will the temperature of the leaves increase more than the temperature of the

red petals? The temperature of the green leaves will increase more because they absorb the red

light while the red petals reflect the red light.

28. Why are the interiors of optical instruments painted black?

The black paint absorbs errant light rays, resulting in a better image.

29. ON a TV screen, red, green and blue spots of fluorescent materials are illuminated at a variety of relative

intensities to produce a full spectrum of colors. What dots are activated to produce yellow? Magenta?

White? Yellow- red and green; magenta- red and blue; white- red, blue, and green.

30. Suppose two beams of white light are shone on a white screen, one beam through a pane of red glass and the

other through a pane of green glass. What color appears on the screen where the two beams overlap? What

occurs if instead the two panes of glass are placed in the path of a single beam?

Yellow; all the light is absorbed.

31. When white light is shone on red ink dried on a glass plate, the color that is transmitted is red. But the color

that is reflected is not red. What is it? The reflected color is white minus red, or cyan.

32. In a dress shop that has only fluorescent lighting, a customer insists on taking a garment into the daylight at

the doorway. Is she being reasonable? Explain. Yes, outside there is a full range of colors to be

reflected.

33. What color would a yellow cloth appear if illuminated with sunlight? With yellow light? With blue light?

Yellow; yellow; black

34. A spotlight is coated so that it won’t transmit blue from its white-hot filament. What color is the emerging

beam of light?

When blue is subtracted from white light, the emerging color is the complementary color, yellow.

35. How could you use the spotlights at a play to make the yellow clothes of the performers suddenly change to

black? If the yellow clothes are illuminated with blue light, no color will be reflected and they will

appear black.

36. A stage performer stands where beams of red and green light cross.

a. What is the color of her white shirt under this illumination?

b. What are the colors of the shadows she casts on the stage floor?

a. Yellow, the addition of red and green

b. Where both shadows overlap, black will result, since no light appears there. But the part of the

shadow from the green light source that is illuminated with red will appear red. Similarly, the part of

the shadow from the red light source that is illuminated with green will appear green.

37. What colors of ink do color ink-jet printers use to produce the colors you see? Do the inks form colors by

color addiction or by color subtraction? Magenta, yellow, and cyan; subtraction.

38. On a photographic print, you dearest friend is seen wearing a red sweater. What color is the sweater on the

negative? The opposite of red, cyan.

39. Why can’t we see a laser beam going across the room unless there is fog, chalk dust, or a mist in the air?

We see the laser beam only when it scatters from fog, chalk dust, or some other particles in the air.

40. Very big particles, such as droplets of water, absorb more radiation than they scatter. How does this fact

help to explain why rain clouds appear dark? Rain clouds are composed of large light-absorbing

droplets.

41. If the sky on a certain planet in the solar system were normally orange, what color would sunset be?

Orange indicates scattering of low frequencies. At sunset, when the path is longer, very little low-

frequency light would get through so the sunset would be bluish.

42. What causes the beautiful colors sometimes seen in the burning of various materials in a fireplace?

Atoms of the material are heated to glowing, and different kinds of atoms give off their own

characteristic colors.

43. What is the evidence for the claim that iron exists in the atmosphere of the sun?

Iron spectral lines are found in the solar spectrum.

44. The only light to reach very far beneath the surface of the ocean is greenish blue. Objects at these depths

either reflect greenish blue of reflect no color at all. If a ship that is painted red, green, and white sinks to

the bottom of the ocean, how will these colors appear?

The red will appear black, the green will appear green, and the white will appear greenish blue.

45. A lamp filament is made of tungsten. When made to flow it emits a continuous spectrum- all the colors of

the rainbow. When tungsten gas is made to glow, however, the light is a composite of very discrete colors.

Why is there a difference in spectra?

In a solid, atoms vibrate with a broad range of frequencies dictated by the temperature. When atoms

are far apart in the form of a gas, the emitted light is determined by the structure of the individual

atoms. The frequencies are discrete and depend on which elements are present in the gas.

Unit 4 Electricity Questions & Review Solutions Chapter 32

Review Questions 1. Which force – gravitational or electrical – repels as well as attracts? Electrical; Gravitational force only

attracts

2. Gravitational forced depend on the property called mass. What comparable property underlies electrical

forces? Charge

3. How do protons and electrons differ in their electric charge? Same magnitude but opposite charge

4. Is an electron in a hydrogen atom them same as an electron in a uranium atom? Yes, all electrons are

identical

5. Which has more mass – a proton or an electron? Proton – more than 1800 times greater than the

electron

6. In a normal atom, how many electrons are there compared with protons? Same number, no net charge

7. How do like charges behave? How do unlike charges behave? Repel Each other; attract each other

8. How does a negative ion differ from a positive ion? A negative ion has extra electron(s); a positive ion

has lost electron

9. If electrons are rubbed from cat’s fur onto a rubber rod, does the rod become positively or negatively

charged? How about the cat’s fur? Negatively; positively

10. What does it mean to say that charge is conserved? It is neither created nor destroyed, only transferred

11. How is Coulomb’s law similar to Newton’s law of gravitation? How are the two laws different? Both are

inverse-square laws; one depends on mass and one depends on charge and coulomb’s law works for

attraction and repulsion.

12. The SI unit of mass is the kilogram. What is the SI unit of charge? Coulomb

13. The proportionality constant k in Coulomb’s law is huge in ordinary units, whereas the proportionality

constant G in Newton’s law of gravitation is tiny. What does the mean in terms of the relative strengths of

these two forces? Electrical force is relatively much greater

14. Why does the weaker force of gravity dominate over electrical forces for astronomical objects? It is not

diminished by repelling forces and acts on large masses

15. Why do electrical forces dominate between atoms that are close together? There is a slight imbalance

between attracting and repelling forces

16. What is the difference between a good conductor and a good insulator? Electrons are free to move in a

good conductor

17. Why are metals good conductors? Why are materials such as rubber and glass good insulators? Free

electrons; Bound electrons

18. What is a semiconductor? Material that can behave as either an insulator or a conductor

19. What is a superconductor? A conductor with zero resistance

20. What are the three main methods of charging objects? Which method involves no touching? Contact,

friction, and induction; induction

21. What is lightning? Electrical discharge from cloud to cloud or cloud to ground

22. What is the function of a lightning rod? To prevent discharge and to conduct charge to the ground

23. What does it mean to say an object is electrically polarized? Negative on one side, and positive on the

other

24. When a charged object polarizes another, why is there an attraction between the objects? The oppositely

charges side is a little closer

25. What is an electric dipole? A molecule in which the distribution of charge is uneven

Think and Explain (Extra Credit) 26. Electrical forces between charges are enormous relative to gravitational forces. Yet, we normally don’t

sense electrical forces between us and our environment, while we do sense our gravitational interaction with

the earth. Why is this so? Charges can cancel, while masses cannot.

27. By how much is the electrical force between a pair of ions reduced when their separation distance is

doubled? Tripled? To ¼; to 1/9

28. If you scuff electrons from your hair onto a comb, are you positively or negatively charged? How about the

comb? Positive; negative

29. An electroscope is a simple device. It consists of a metal ball that is attached by a conductor to two fine

gold leaves that are protected from air disturbances in a jar, as sown in the sketch. When the ball is touched

by a charged object, the leaves that normally hang straight down spring apart. Why? (Electroscopes are

useful not only as charge detectors, but also for measuring the amount of charge: the more charge

transferred to the ball, the more the leaves diverge.) The leaves have like charges, and repel each other

30. Would it be necessary for a charged object to actually touch the leaves of an electroscope (see question 29)

for the leaves to diverge? Defend your answer. No, Charging by induction will also charge the leaves.

31. If a glass rod that is rubbed with a plastic dry cleaners bag acquires a certain charge, why does the plastic

bag have exactly the same amount of opposite charge? Charge is transferred. No net charge is ever

created or destroyed 32. Why is a good conductor of electricity also a good conductor of heat? Both types of conduction are via

free moving electrons 33. Explain how an object that is electrically neutral can be attracted to an object that is charged. The side

having opposite sign of charge is closer to the charged object. The attraction between the opposite

charges is greater than the repulsion between the like charges. 34. If electrons were positive and protons negative, would Coulomb’s law be written the same or differently?

The same. Coulomb’s law does not distinguish between positive and negative charges. 35. The five thousand billion billion feely moving electrons in a penny repel one another. Why don’t they fly

out of the penny? The electrons are attracted to the same number of protons in the penny

36. Imagine a proton at rest a certain distance from a negatively charged plate. It is released and collides with

the plate. Then imagine the similar case of an electron at rest, the same distance away, from a plate of equal

and opposite charge. In which case would the moving particle have the greater speed when the collision

occurs? Why? Electron. The force on both will be the same but the electron will have more

acceleration and therefore more speed because of lesser mass.

Magnetism Review Questions & Solutions Chapters 36 & 37 Chapter 36 Review Questions 1. What do electric charges have to do with magnetic poles? (36.1) They both attract and repel

2. What is a major difference between electric charges and magnetic poles? (36.1)

Charges can be isolated, whereas poles cannot

3. What is a magnetic field, and what is its source? (36.2)

Altered space around a magnet; moving electric charges

4. Every spinning electron is a tiny magnet. Since all atoms have spinning electrons, why are not all atoms

tiny magnets? (36.3) Electrons spin in opposite directions, so their fields cancel out

5. What is so special about iron that makes each iron atom a tiny magnet? (36.3)

Fields of iron’s electrons do not entirely cancel

6. What is a magnetic domain? (36.4) A cluster of aligned atoms

7. Why do some pieces of iron behave as magnets, while other pieces of iron do not? (36.4)

Domains in some are aligned

8. How can a piece of iron be induced to become a magnet? For example, if you place a paper clip near a

magnet, it will itself become a magnet. Why? (35.4) Align the domains with the magnetic field

9. Why will dropping or heating a magnet weaken it? (36.4) The domains are jostled out of alignment

10. What is the shape of the magnetic field that surrounds a current-carrying wire? (36.5)

Concentric Circles

11. If a current-carrying wire is bent into a loop, why is the magnetic field stronger inside the loop than outside?

(36.5) The fields overlap inside the loop

12. What must a charged particle be doing in order to experience a magnetic force? (36.6)

Moving across magnetic field lines

13. With respect to an electric and a magnetic field, how does the direction of a magnetic force on a charged

particle differ from the direction of the electric force? (36.6)

Magnetic – force is perpendicular to the field; electric – force is parallel to the field.

14. What role does Earth’s Magnetic field play in cosmic ray bombardment? (36.6)

It deflects incoming charged cosmic rays.

15. How does the direction in which a current-carrying wire is forced when in a magnetic field compare with

the direction that moving charges are forced? (36.7) Same

16. How do the concepts of force, field and current relate to a galvanometer? (36.8)

Force acts on the current in the field and deflects the galvanometer coil

17. Why is it important that the current in the armature of a motor that uses a permanent magnet periodically

change direction? (36.8) So it will do more that rotate 180°

18. What is meant by magnetic declination? (36.9)

Difference between geographic north and magnetic north

19. According to most geophysicists, what is the probably cause of Earth’s magnetic field? (36.9)

Currents in Earth’s molten interior

20. What are magnetic pole reversals, and what evidence is there that Earth’s magnetic field has undergone pole

reversals throughout history? (36.9) N Changes to S, S to N; magnetism found in rock strata

Think and Explain (Extra Credit) 21. What kind of field surrounds a stationary electric charge? A moving electric charge? An electric field;

electric and magnetic fields 22. Why can iron be made to behave as a magnet, while wood cannot? Iron, unlike wood, has magnetic

domains that can be induced into alignment. 23. Since iron filings are not themselves magnets, by what mechanism do they align themselves with a

magnetic field as sown in figure 36.6? Magnetic domains in the filings are induced into alignment

along the external magnetic field lines. This produces poles in the filings, which are then acted on

by the external field, resulting in a torque on each filing. The torque produces a “couple” that

aligns the filing with the field. 24. A strong magnet and a weak magnet attract each other. Which magnet exerts the stronger force 0 the

strong one or the weak one? (Could you have answered this way back in chapter 6?) The forces in a

force pair in any interaction are equal and opposite 25. Why will the magnetic field strength be further increased inside a current-carrying coil if a piece of iron

is placed in the coil? The alignment of domains adds to the field strength

26. A cyclotron is a device for accelerating charged particles to high speeds in circuloar orbits of ever-

increasing radius. The charged particles are subjected to both an electric field and a magnetic field. One

of these fields increases the speed of the particles, and the other field holds them in a circular path.

Which field performs which function? The electric field increases the spped of an electric charge

(force on the charge can be in the direction of motion). The magnetic force always acts

perpendicular to the motion of the charge, sot he force changes the direction of the charged

particles path 27. A magnetic field can deflect a beam of electrons, but it cannot do work on them to speed them up.

Why? (Hint: consider the direction of the force relative to the direction in which the electrons move)

There is no component of magnetic force in the direction of motion 28. In what direction relative to a magnetic field does a charged particle travel in order to experience

maximum magnetic force? Minimum magnetic force? Perpendicular to the field lines; parallel to the

field lines 29. Pigeons have multiple-domain magnetite magnets within their skulls that are connected through a large

number of nerves to the pigeons brain. How does this aid the pigeon in navigation? (magnetic material

also exists in the abdomen of bees) It likely allows them to sense the direction of the Earth’s

magnetic field. 30. What changes in cosmic ray intensity at Earth’s surface would you expect during periods in which

Earth’s magnetic field passed through a zero phase while undergoing pole reversals? (A widely held

theory, supported by fossil evidence, is that the periods of no protective magnetic field may have been as

effective in changing life forms as X-rays have been in the famous heredity studies of fruit flies.)

Increased intensity of cosmic rays 31. Earth’s core is probably composed of iron and nickel, excellent metals for making permanent magnets.

Why is it unlikely that Earth’s core is a permanent magnet? It is too hot for iron and nickel to be

magnetic 32. Can an electron be set into motion with a magnetic field? With an electric field? Explain. No, no force

on a stationary charge; yes

Chapter 37

Review Questions 1. What did Michael Faraday and Joseph Henry discover?

Electric current can be produced in a wire by motion of a magnet

2. How can voltage be induced in a wire with the help of a magnet?

By moving the magnet past the wire or into the coil

3. A magnet moved into a coil of wire will induce voltage in the coil. What is the effect of moving a

magnet into a coil of wire with more loops?

Greater induced voltage

4. Why is it more difficult to move a magnet into a coil of more loops that is connected to a resistor?

The coil becomes a stronger electromagnet and repels more

5. Current, as well as voltage, is induced in a wire by electromagnetic induction. Why is Faraday’s law

expressed in terms of induced voltage and not induced current?

Current depends on resistance of a circuit

6. How does the frequency of a changing magnetic field compare with the frequency of the alternating

voltage that is induced?

Same

7. What is a generator and how does it differ from a motor?

A motor converts electricity to work, whereas a generator converts work to electricity 8. Why is alternating voltage induced in the rotating armature of a generator?

The magnetic field increases and decreases each turn.

9. The armature of a generator must rotate in order to induce voltage and current. What causes the

rotation?

External source of energy such as fuel, wind or water

10. A motor is characterized by three main ingredients: magnetic field, moving charges and magnetic force.

What are the three main ingredients that characterize a generator?

Same

11. How can a change in voltage in a coil of wire (the primary) be transferred to a neighboring coil of wire

(the secondary) without physical contact?

Induction by changing the magnetic field

12. Why does an iron core that extends inside and connects the primary and secondary coils intensify

electromagnetic induction?

The domains of the iron core align and increase the number of magnetic field turns?

13. What does a transformer actually transform – voltage, current, or energy?

Voltage and current, but not energy

14. What does a step up transformer step up – voltage, current or energy?

Voltage

15. How does the relative number of turns on the primary and the secondary coil in a transformer affect the

step-up or step-down voltage factor?

More turns on the secondary coil step up the voltage

16. If the number of secondary turns is 10 times the number of primary turns, and the input voltage to the

primary is 6 volts, how many volts will be induced in the secondary coil?

60 V

17. In a transformer, how does the power input to the primary coil compare with the power output of the

secondary coil? How does the produce of voltage and current in the primary compare with the product

of voltage and current in the secondary?

Same; Same (as long as power loss is negligible)

18. Why is it advantageous to transmit electric power long distances at high voltages?

Lower current results in less energy loss through the heating of the wires, it is also much safer in

terms of human and animal life and fire safety

19. What fundamental quantity underlies the concepts of voltages and currents?

Electric Field

20. Distinguish between Faraday’s Law expressed in terms of fields and Maxwell’s counterpart to Faraday’s

law. How are the two laws symmetrical?

Faraday’s Law – Electricity induced by moving magnetism; Maxwell’s – Magnetism induced by

moving electricity; they are inverses or exact opposites of each other.

21. How do wave speeds compare for high-frequency and low-frequency electromagnetic waves?

The speeds are the same – c (see chapter 27 for review of electromagnetic wave speeds)

22. What is light?

Electromagnetic waves whose frequency range is from 400-700 nm

Think and Explain (Extra Credit)

23. A common pickup for an electric guitar consists of a coil of wire around a permanent magnet. The

permanent magnet induces magnetism in the nearby guitar string. When the string is plucked, it

oscillates above the coil, thereby changing the magnetic field that passes through the coil. The rhythmic

oscillations of the string produce the same rhythmic changes in the magnetic field in the coil, which in

turn include the same rhythmic voltages in the coil, which when amplified and sent to a speaker produce

music! Why will this type pickup not work with nylon strings? Nylon is nonmagnetic, has no

magnetic domains, and is not magnetized by the permanent magnet

24. What is the basic difference between an electric generator and an electric motor? None, they simply

operate in opposite manners 25. With no magnets around, why will current flow in a coil of wire waved around in the air? Waving the

coil moves it through the earth’s magnetic field, inducing voltage and hence current 26. Why is it important that the core of a transformer pass through both coils? To ensure the maximum

number of magnetic field lines produced in the primary coil and intercepted by the secondary coil 27. Why can a hum often be heard when a transformer is operating? The hum is a same frequency force

vibration of the iron slaps in the transformer core as their magnetic polarities alternate 28. If a bar magnet is thrown into a coil of wire, it will slow down. Why? The magnet will induce a

current in the loop. The field produced by the current tends to repel the magnet as it approaches

and attract it as it leaves. 29. What is the source of all electromagnetic waves? Accelerating electric charges

30. Why is a generator armature more difficult to rotate when it is connected to and supplying electric

current to a circuit? The repulsion of the electromagnets opposes the rotation of the armature. The

greater the current, the greater the repulsion, and the more work that must be done to spin the

armature. The answer is implied by energy conservation. Work done in turning the armature

goes into the electrical energy supplied to the external circuit. 31. Some bicycles have electric generators that are made to turn when the bike wheel turns. These

generators provide energy for the bike’s lamp. Will a cyclist coast further if the lamp connected to the

generator is turned off? Explain. When the lamp is on, the energy that goes into lighting th lamp

comes at the expense of the KE of the moving bicycle. The extra KE saved by not lighting the

lamp makes the bicycle go farther. 32. An electric hair drier running at normal speed draws a relatively small current. But if somehow the

motor shaft is prevented from turning, the current dramatically increases and the motor overheats.

Why? A running motor always draws less net current than a stalled motor. If the motor jams or is

somehow prevented from turning, then the back current is no longer generated and the net

current in the motor windings is greater. This overheats the motor. 33. When a piece of plastic tape coated with iron oxide that is magnetized more in some parts than others is

moved past a small coil of wire, what happens in the coil? What is the practical application of this?

Variations in voltage, the principle that underlies the operation of a tape recorder. 34. When a strip of magnetic material, variably magnetized, is embedded in a plastic card that is moved past

a small coil of wire, what happens in the coil? What is a practical application of this? Voltage is induce

in the coil, the principle that underlies the operation of the device that checks a credit card. 35. If a car made of iron and steed moves over a wide closed loop of wire embedded in a road surface, will

the magnetic field of Earth in the look be altered? Will this produce a current pulse? (Can you think of a

practical application of this?) Yes, Yes, triggering automobile traffic lights.

36. How could you move a conducting loop of wire through a magnetic field without inducing voltage in the

loop? Move it in such a way that the number of field lines doesn’t change

37. Why does a transformer require alternating voltage? AC provides the change needed for

electromagnetic induction 38. Can an efficient transformer step up energy? Defend your answer. No. To do so would violate the law

of energy conservation 39. What is wrong with this scheme? To generate electricity without fuel, arrange a motor to run a

generator that will produce electricity that is stepped up with a transformer so that the generator can run

the motor while furnishing electricity for other uses. It violates the law of energy conservation.

Because of inherent inefficiencies, the generator will produce less electric power than is used to

run the adjoining motor. A transformer will not step up voltage and current simultaneously.