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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
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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)
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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
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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.
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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.
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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.
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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
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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.
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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
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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?
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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
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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.