Physics 106 - How Things Work II - Spring 2007 Final Exam ...

Physics 106 - How Things Work II - Spring 2007
Final Exam — May 4, 2007 — Cover Page

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Physics 106 - How Things Work II - Spring 2007
Final Exam — May 4th, 2007 — Page 1

1. The colors that you see when sunlight strikes a cut crystal wineglass or a cut diamond
are a result of the fact that two light waves with
a. the same wavelength in air can have different frequencies in a material.
b. the same wavelengths can have different colors.
c. the same frequency in air can have different wavelengths in a material.
d. different wavelengths travel at different speeds in most materials.

2. If you put a heavy, steel marble into a microwave oven, along with a bowl of raw
broccoli, and turn the oven on at full power for a minute, the most likely result is that
a. the magnetron will burn out and the microwave will stop working altogether.
b. sparks will leap out of the marble and may burn the nearby broccoli.
c. the marble will become red hot and the broccoli will remain raw.
d. almost nothing will have happened to the marble and the broccoli will be partly
cooked.

3. Red light has a longer wavelength than blue light, so you know that, in empty space,
a. red light travels faster than blue light.
b. blue light has a higher frequency than red light.
c. blue light travels faster than red light.
d. red light has a higher frequency than blue light.

4. You place a bowl of soup on a ceramic saucer and put the two into a microwave. After
a minute or two of cooking, the soup is very hot but the saucer is still cold because
a. the saucer contains no water molecules and doesn’t absorb microwaves.
b. the oven’s microwaves are directed downward from above and are absorbed by
the soup before they can reach the saucer.
c. the saucer is horizontal and the microwaves are horizontally polarized. A hori-
zontal object can’t absorb horizontally polarized electromagnetic waves.
d. the saucer is horizontal and the microwaves are vertically polarized. A horizontal
object can’t absorb vertically polarized electromagnetic waves.

5. Two people are loading suitcases onto a truck. The suitcases have equal weights and
roll easily on wheels. The man pushes a suitcase up a 2 meter ramp with a force of
100 newtons while the woman pushes a suitcase up a 4 meter ramp with a force of 50
newtons. Which person is doing the most work?
a. The woman is doing more work because she is pushing the suitcase a longer
distance while raising the suitcase to the height of the truck.
b. They are both doing zero work on their suitcases because the suitcases roll freely
on wheels.
c. They are both doing the same amount of work because the product of force times
distance is the same in each case.
d. The man is doing more work because he is raising the suitcase to the height of
the truck with a larger force.

Physics 106 - How Things Work II - Spring 2007
Final Exam — May 4th, 2007 — Page 2

6. You get on your bicycle and begin pedaling. You travel forward faster and faster and
soon reach full speed. As you are picking up speed, friction between the ground and
the bicycle wheels is exerting
a. a forward force on the bicycle.
b. zero force on the bicycle.
c. a backward force on the bicycle.
d. an upward force on the bicycle.

7. Fluorescent brighteners are added to laundry detergents to increase the amount of
blue light emitted by aging, yellowed fabrics. Brighteners obtain the energy they need
to emit this additional blue light from
a. ultraviolet light.
b. radio waves and microwaves.
c. infrared light.
d. red light.

8. The fallout from a nuclear explosion and the waste from a nuclear reactor contain
radioactive isotopes. The atoms of these isotopes differ from stable atoms in that
a. they are chemically unstable but their nuclei are indistinguishable from stable
atoms.
b. their nuclei are unstable but they are chemically indistinguishable from stable
atoms.
c. each atom emits a steady stream of X-ray radiation.
d. their nuclei are unstable and they are chemical unstable.

9. Exposure to sunlight creates a small quantity of three-atom oxygen molecules, a chem-
ical called “ozone.” These ozone molecules absorb short-wavelength ultraviolet light
and reduce the amount of that light striking the earth’s surface. The presence of the
ozone is a good thing for us because short-wavelength ultraviolet light
a. heats our skin quickly, causing thermal damage and burns.
b. causes damage to the atomic nuclei in our skin.
c. is absorbed in packets with enough energy to do chemical damage to molecules
in your skin.
d. exerts strong forces on electric charges in our skin, causing large and damaging
currents to flow through us.

10. Any rotating object that is experiencing zero torque will
a. gradually increase its rate of rotation.
b. gradually slow its rotation until it begins to reverse directions.
c. continue rotating with constant angular momentum.
d. gradually slow its rotation until it comes to a complete stop.

Physics 106 - How Things Work II - Spring 2007
Final Exam — May 4th, 2007 — Page 3

11. You are moving two file cabinets. One cabinet is empty and is resting directly on the
floor. The other cabinet is full of documents but is resting on a dolly with wheels.
You push each cabinet forward 5 meters by exerting a forward force of 100 newtons
on it. At the end of this pushing,
a. the rolling cabinet has about 500 joules of kinetic energy while the sliding cabinet
has much less than 500 joules of kinetic energy.
b. the rolling cabinet and the sliding cabinet each have much less than 500 joules of
kinetic energy.
c. the rolling cabinet and the sliding cabinet each have about 500 joules of kinetic
energy.
d. the sliding cabinet has about 500 joules of kinetic energy while the rolling cabinet
has much less than 500 joules of kinetic energy.

12. Which of the following creates a magnetic field?
a. Only a constant electric field.
b. Only an electric field that changes with time.
c. Only a moving electric charge.
d. Both a moving electric charge and an electric field that changes with time.

13. When you see rainbow-like colors in reflections from soap bubbles, you are seeing the
effects of
a. refraction, since different colors of light are in general bent by different amounts
when the light passes from air into water (or vice versa).
b. light reflecting from the individual colored soap molecules in the bubble.
c. interference, as light reflected from the outer surface of the bubble interferes with
light reflected from the inner surface of the bubble.
d. interference, as light reflected from the bubble interferes with light that undergoes
Rayleigh scattering from the air.

14. In air, the laser beam used in a CD player has a frequency of about 380,000,000,000,000
Hz and a wavelength of about 780 nanometers. In the plastic CD disk, that same beam
has
a. a shorter wavelength and a higher frequency.
b. a shorter wavelength but the same frequency.
c. a longer wavelength and a higher frequency.
d. a longer wavelength and a lower frequency.

15. A skydiver leaps out of an airplane and begins to fall. After a few seconds, he reaches
a constant downward speed of about 300 kilometers-per-hour (190 miles-per-hour).
The net force he is experiencing at that point is
a. downward, because the only force acting on him is gravity. He is in free fall.
b. upward, because he is in free fall and the only force he experiences is an upward
centrifugal force.
c. downward, because his velocity is downward and net force is proportional to
velocity.
d. zero, because the upward drag force exerted on him by the air is equal to his
weight.

Physics 106 - How Things Work II - Spring 2007
Final Exam — May 4th, 2007 — Page 4

16. Which of the following electronic devices allows a very small electrical signal to control
a large electrical current?
a. A capacitor.
b. A diode.
c. A resistor.
d. A transistor.

17. An acorn falls 20 meters from the high branch of an oak tree to the ground. This fall
takes about 2 seconds. After it has fallen for only 1 second, the acorn is still falling
and remains
a. 10 meters above the ground.
b. slightly less than 10 meters above the ground.
c. substantially less than 10 meters above the ground.
d. substantially more than 10 meters above the ground.

18. A battery
a. creates positive charge.
b. pumps positive charge from its positive terminal to its negative terminal.
c. pumps positive charge from its negative terminal to its positive terminal.
d. creates negative charge.

19. As a nucleus of 235U, the light isotope of uranium, falls apart, potential energy con-
tained in the nucleus is converted to kinetic energy in the fragments. Before the
nucleus falls apart, that potential energy is stored in the
a. repulsive electrostatic force between protons in the nucleus.
b. attractive nuclear force between particles in the nucleus.
c. attractive electrostatic force between protons in the nucleus.
d. repulsive nuclear force between particles in the nucleus.

20. The electrical power to a neighborhood is usually delivered by wires with a voltage
difference of about 5000 volts. Normal household electrical outlets have a voltage
difference of about 120 volts. Electrical power is delivered to the neighborhood at the
high voltage because that arrangement reduces the
a. number of cycles per second that are needed to carry the power to the neighbor-
hood.
b. amount of energy carried by each electron traveling through the wires to the
neighborhood.
c. amount of electrical power that the neighborhood needs by a factor of more than
40.
d. amount of current that must flow through the wires to the neighborhood.

Physics 106 - How Things Work II - Spring 2007
Final Exam — May 4th, 2007 — Page 5

21. Of the following types of electromagnetic waves, the type with the highest frequency
is
a. visible light.
b. ultraviolet light.
c. microwaves.
d. infrared light.

22. Electrostatic air cleaners use very thin wires to transfer electric charge to dust particles
primarily because very thin wires
a. heat up more quickly and easily than thick wires.
b. break easily, require frequent repair, and are a boon to the manufacturer’s service
department.
c. can be spaced more closely together so that sparks can jump more easily from
one wire to the other.
d. will have such a concentration of charges that their mutual repulsion will push
some off the wire and into the surrounding air and dust.

23. When you throw a ball straight upwards, the momentum of the ball immediately after
it leaves your hand is in the upward direction. When the ball reaches its maximum
height, however, the momentum of the ball is zero. Given the law of conservation of
momentum, how can this be?
a. The upward momentum is converted into kinetic energy.
b. The upward momentum is transferred to the earth.
c. The upward momentum is converted into random thermal energy.
d. In this case, the conservation of momentum does not apply.

24. In a xerographic copier, charge is sprinkled uniformly over the surface of a photocon-
ductor. The charge is then removed in some places by illuminating the photoconductor
with a pattern of light. The charge is removed when light
a. causes the photoconductor to become insulating.
b. causes the photoconductor to become conducting.
c. causes the photoconductor to become a diode.
d. ionizes atoms.

25. The explosion of a uranium-based nuclear weapon involves a chain reaction. Which
of the following is necessary for the chain reaction to successfully take place.
a. Over 90% of the uranium must be 238U.
b. The total mass of uranium in the bomb must be less than the “critical mass”.
c. The bomb must also contain an amount of cadmium equal to the amount of
uranium.
d. On average, the neutrons released from each fission reaction must induce at least
one additional fission reaction.

Physics 106 - How Things Work II - Spring 2007
Final Exam — May 4th, 2007 — Page 6

26. A gymnast doing a double back flip leaps off the floor with her arms and legs extended
and then pulls herself into a very compact position. In her compact shape, she rotates
very rapidly and completes two full rotations before opening back up to land on the
floor. During the time that she is not touching the floor, the one aspect of her motion
that is constant is her
a. velocity.
b. momentum.
c. angular velocity.
d. angular momentum.

27. The reason why 238U, the heavy isotope of uranium, cannot be used by itself in a
nuclear reactor is that 238U
a. has more electrons than 235U, the light isotope of uranium, and wastes most of
its energy heating those electrons.
b. does not contain very much nuclear energy and thus cannot create much heat.
c. repels an approaching proton while 235U, the light isotope of uranium, does not.
d. rarely undergoes fission when struck by a neutron.

28. You are working in a factory for the summer. Lifting a barrel of salt directly upward
1 meter from the parking lot to the loading dock requires 2000 joules of energy. If you
create a ramp with a 5 meter long board, you can roll the barrel slowly and steadily
up the ramp with an uphill force of
a. 10,000 newtons.
b. 50,000 newtons.
c. 400 newtons.
d. 2,000 newtons.

29. Your neighbor’s house loses power during a windstorm. To help out, you run a long
extension cord from your house to your neighbor’s house. Your neighbor’s appliances
don’t run properly when plugged into this cord because too much power is wasted in
its thin wires. As current goes from you house to your neighbors, its
a. current and voltage both gradually decrease along the length of the cord
b. voltage gradually decreases along the length of the cord.
c. current gradually decreases along the length of the cord.
d. voltage gradually decreases along the length of the cord and current gradually
increases to partly compensate.

30. You jump off of a diving board into a swimming pool. As you fall toward the water,
your velocity
a. remains constant because your acceleration is zero.
b. remains constant because your acceleration is constant.
c. increases, but your acceleration remains constant.
d. increases because your acceleration increases.

Physics 106 - How Things Work II - Spring 2007
Final Exam — May 4th, 2007 — Page 7

31. When two pieces of semiconductor touch to form a p-n junction, a depletion region
forms around that junction. In the depletion region, all the valence levels are filled
and all the conduction levels are empty. The depletion region is
a. electrically conducting.
b. filled only with spin-up electrons.
c. electrically insulating.
d. magnetic.

32. If you double the DC voltage supplied to a “brushed” DC motor, what will happen?
a. The rotor’s rotation speed will roughly double.
b. The rotor’s rotation speed will remain the same, but the motor will have more
torque.
c. The rotor will not turn at all, as the motor needs to be carefully designed around
a single voltage.
d. The rotor’s rotation speed will remain the same, as will the motor’s torque, but
the motor will quickly overheat.

33. An AM radio station transmits its signal from a tall antenna because
a. only a tall antenna can produce the large amplitude signals required for
amplitude-modulated transmissions.
b. a tall antenna will transmit the signal farther.
c. the band of frequencies set aside for AM transmissions have long wavelengths.
d. the band of frequencies set aside for AM transmissions have short wavelengths.

34. An iron bar is wrapped with several hundred turns of wire. What happens when a
constant DC current is run through the wire?
a. Positive charge accumulates on one end of the bar and negative charge accumu-
lates on the other end of the bar.
b. One end of the bar becomes a magnetic north pole and the other end of the bar
becomes a magnetic south pole.
c. One end of the bar both accumulates positive charge and becomes a magnetic
north pole, and the other end of the bar accumulates negative charge and becomes
a magnetic south pole.
d. The iron bar is heated, but neither accumulates charge nor becomes magnetic.

35. You are given four light emitting diodes (LED’s) that emit blue light, yellow light,
red light, and infrared light. Each of the voltages given below represent the required
forward bias voltage for one of the four LED’s. Which is the voltage that must be
used with the blue LED.
a. 1.5 Volts.
b. 1.8 Volts.
c. 2.2 Volts.
d. 3.6 Volts.

Physics 106 - How Things Work II - Spring 2007
Final Exam — May 4th, 2007 — Page 8

36. If you “cook” a CD or compact disc briefly in a microwave oven,
a. it becomes permanently magnetized.
b. it vibrates back and forth violently in response to the fluctuating electric field.
c. nothing significant happens because it contains no organic materials.
d. it becomes hot enough to soften and sparks leap about its surface.

37. When an electron with a great deal of kinetic energy passes very near the massive
nucleus of a large atom, the electron’s direction suddenly changes. The electron emits
an X-ray because
a. the passing electron causes a current to flow in the nucleus, releasing the electro-
magnetic wave.
b. an accelerating electric charge emits an electromagnetic wave and this very abrupt
acceleration produces a very high frequency wave.
c. sliding friction causes its temperature to increase and it begins to glow white hot.
It becomes so hot that it emits X-rays.
d. a moving electric charge emits a visible electromagnetic wave but this wave be-
comes an X-ray when it is emitted near an atomic nucleus.

38. Which of the following best describes what it is that you are looking at when you view
an image made using MRI:
a. The degree to which X-rays can pass through the body.
b. A reconstruction of the degree to which X-rays can pass through the body using
images made from many different angles.
c. The degree to which radio waves can pass through the body.
d. The distribution of tissues containing hydrogen, such as tissues containing a high
fraction of water.

39. When light of a particular frequency causes an atom to undergo a transition and emit
a photon of the same frequency this process is known as:
a. Spontaneous emission.
b. Stimulated emission.
c. Photoemission.
d. Photoelectric effect.

40. Which two socks are most likely to stick to one another when coming out of the dryer?
a. Two wool socks.
b. Two polyester socks.
c. A wool sock and a polyester sock.
d. Any of these socks are equally likely to stick to one another.

Physics 106 - How Things Work II - Spring 2007
Final exam — May 4, 2007 — Page 9

41. You and a friend, who is half your weight, are on a seesaw. You have no trouble
balancing, however, as long as you sit so that the distance between you and the pivot
is about half of the distance between your friend and the pivot.
a. Explain why this is the case.

b. You and your friend are now sitting on an identical seesaw, only you are doing so
in the recreational area of a permanent settlement on the moon. You are both
sitting in the same positions on the seesaw as was the case in part a, and you are
in the down position. You push off with your feet. On Earth, when doing this,
it took one second for your friend to find herself in the down position. How long
will it take on the moon? You may assume that the force with which you push
and the time over which you push is identical in both cases.

Physics 106 - How Things Work II - Spring 2007 A.
Final exam — May 4, 2007 — Page 10 B.

42. Shown at right is a diagram representing an AC syn-
chronous motor. Notice that at the moment depicted
in the figure, the north pole of the rotor has just ro-
tated to roughly the “upwards” position. This hap-
pened, in part, because there was an attractive force
between the north pole of the rotor and the portion
of the iron core marked “A”.
a. Why was the North pole of the rotor attracted
to the portion of the iron core marked “A”?

b. Why will the rotor continue to rotate instead of staying in the position shown?

43. Consider a p-n junction.
a. Is the p-type half of the p-n junction electrically neutral, positive, or negative?
Explain your reasoning.

Physics 106 - How Things Work II - Spring 2007
Final exam — May 4, 2007 — Page 11

43. Continued.
b. In which direction does the electric field point in the middle of a p-n junction?
Again explain your reasoning.

44. You have a portable battery-powered MOSFET-based amplifier that drives a small
set of speakers.
a. When you connect your iPod to the amplifier, it sends current back and forth
through the input. As a result, charge moves onto and off of what critical control
element in the amplifier?

b. When you plug the speakers directly into your iPod, you can hardly hear the
music at all, but when plugged into the amplifier, the speakers are reasonably
loud. What is it within the amplifier that supplies the power that ultimately
results in the louder music?

Physics 106 - How Things Work II - Spring 2007
Final exam — May 4, 2007 — Page 12

45.
a. An AM radio station has a frequency of 500 kilohertz and a wavelength of roughly
600 meters. A second radio station has a frequency of 1,000 kilohertz. What is
the wavelength of the second radio station?

b. You are some distance North of the trans- S W
mitter of one of the radio stations referred E N
to above. Your radio has a built-in an-
tenna consisting of several loops of wire as
shown in the figure at right. How should
you orient your radio to get the best re-
ception? A north/south orientation as
shown? An east/west orientation? Per-
haps the the coils should be horizontal, i.e.
tip the radio on its side? Explain your an-
swer.