physics 106 - how things work ii - spring 2007 final exam...

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Physics 106 - How Things Work II - Spring 2007 Final Exam — May 4, 2007 — Cover Page PLEASE DO NOT LOOK AT THE CONTENTS OF THIS EXAM, OTHER THAN THIS COVER PAGE, UNTIL TOLD TO DO SO. Please write your name and student ID number on both the cover sheet of this exam as well as on the bubble sheet. Also, please sign the pledge below. Unsigned exams will not be graded. This midterm is closed book, closed notes, and silent. You may not consult any books, notes, computers, or calculators during the exam. You may not use head phones, microphones, or cellular phones during the exam. You may not have any additional paper on your desk during the exam. Once the exam has begun, you may not leave the exam room without permission. You may not take this exam out of this room. On multiple choice questions, please mark the correct answer for each question on the bubble sheet. To minimize the possibility of mistakes later, you should also circle your choice on the exam booklet. Note that there will be one choice and one choice only for each multiple choice question. On short answer questions (# 41 – #45) please write a brief answer contained within the box that is provided. Multiple choice questions are worth one point. Short answer questions are worth two points. Name: Student ID#: On my honor as a student of the University of Virginia I have neither given nor received aid on this exam. I have read all of the exam rules stated above, and have completed this exam honestly, according to those rules. I will report, to Professor Cates or an Honor Advisor, any act of dishonorable conduct and/or violation of the exam rules that I have witnessed during this exam. Signed: Date:

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

PLEASE DO NOT LOOK AT THE CONTENTS OF THIS EXAM,OTHER THAN THIS COVER PAGE, UNTIL TOLD TO DO SO.

• Please write your name and student ID number on both the cover sheet of this examas well as on the bubble sheet. Also, please sign the pledge below. Unsigned examswill not be graded.

• This midterm is closed book, closed notes, and silent.• You may not consult any books, notes, computers, or calculators during the exam.• You may not use head phones, microphones, or cellular phones during the exam.• You may not have any additional paper on your desk during the exam.• Once the exam has begun, you may not leave the exam room without permission.• You may not take this exam out of this room.

• On multiple choice questions, please mark the correct answer for each question on thebubble sheet.

• To minimize the possibility of mistakes later, you should also circle your choice onthe exam booklet. Note that there will be one choice and one choice only for eachmultiple choice question.

• On short answer questions (# 41 – #45) please write a brief answer contained withinthe box that is provided.

• Multiple choice questions are worth one point.• Short answer questions are worth two points.

Name:

Student ID#:

On my honor as a student of the University of Virginia I have neither given nor receivedaid on this exam. I have read all of the exam rules stated above, and have completed thisexam honestly, according to those rules. I will report, to Professor Cates or an HonorAdvisor, any act of dishonorable conduct and/or violation of the exam rules that I havewitnessed during this exam.

Signed:

Date:

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

1. The colors that you see when sunlight strikes a cut crystal wineglass or a cut diamondare 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 rawbroccoli, 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. Aftera 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 androll easily on wheels. The man pushes a suitcase up a 2 meter ramp with a force of100 newtons while the woman pushes a suitcase up a 4 meter ramp with a force of 50newtons. Which person is doing the most work?

a. The woman is doing more work because she is pushing the suitcase a longerdistance while raising the suitcase to the height of the truck.

b. They are both doing zero work on their suitcases because the suitcases roll freelyon wheels.

c. They are both doing the same amount of work because the product of force timesdistance is the same in each case.

d. The man is doing more work because he is raising the suitcase to the height ofthe truck with a larger force.

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

6. You get on your bicycle and begin pedaling. You travel forward faster and faster andsoon reach full speed. As you are picking up speed, friction between the ground andthe 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 ofblue light emitted by aging, yellowed fabrics. Brighteners obtain the energy they needto 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 containradioactive isotopes. The atoms of these isotopes differ from stable atoms in that

a. they are chemically unstable but their nuclei are indistinguishable from stableatoms.

b. their nuclei are unstable but they are chemically indistinguishable from stableatoms.

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 lightand reduce the amount of that light striking the earth’s surface. The presence of theozone 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 willa. 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 2007Final Exam — May 4th, 2007 — Page 3

11. You are moving two file cabinets. One cabinet is empty and is resting directly on thefloor. 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 newtonson it. At the end of this pushing,

a. the rolling cabinet has about 500 joules of kinetic energy while the sliding cabinethas much less than 500 joules of kinetic energy.

b. the rolling cabinet and the sliding cabinet each have much less than 500 joules ofkinetic energy.

c. the rolling cabinet and the sliding cabinet each have about 500 joules of kineticenergy.

d. the sliding cabinet has about 500 joules of kinetic energy while the rolling cabinethas 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 theeffects of

a. refraction, since different colors of light are in general bent by different amountswhen 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 380,000,000,000,000Hz and a wavelength of about 780 nanometers. In the plastic CD disk, that same beamhas

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 reachesa 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 2007Final Exam — May 4th, 2007 — Page 4

16. Which of the following electronic devices allows a very small electrical signal to controla 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 falltakes about 2 seconds. After it has fallen for only 1 second, the acorn is still fallingand 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 batterya. 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 thenucleus 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 voltagedifference of about 5000 volts. Normal household electrical outlets have a voltagedifference of about 120 volts. Electrical power is delivered to the neighborhood at thehigh 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 theneighborhood.

c. amount of electrical power that the neighborhood needs by a factor of more than40.

d. amount of current that must flow through the wires to the neighborhood.

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

21. Of the following types of electromagnetic waves, the type with the highest frequencyis

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 particlesprimarily 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 afterit leaves your hand is in the upward direction. When the ball reaches its maximumheight, however, the momentum of the ball is zero. Given the law of conservation ofmomentum, 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 photoconductorwith 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. Whichof 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 2007Final Exam — May 4th, 2007 — Page 6

26. A gymnast doing a double back flip leaps off the floor with her arms and legs extendedand then pulls herself into a very compact position. In her compact shape, she rotatesvery rapidly and completes two full rotations before opening back up to land on thefloor. During the time that she is not touching the floor, the one aspect of her motionthat 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 anuclear reactor is that 238U

a. has more electrons than 235U, the light isotope of uranium, and wastes most ofits 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 upward1 meter from the parking lot to the loading dock requires 2000 joules of energy. If youcreate a ramp with a 5 meter long board, you can roll the barrel slowly and steadilyup 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 longextension cord from your house to your neighbor’s house. Your neighbor’s appliancesdon’t run properly when plugged into this cord because too much power is wasted inits thin wires. As current goes from you house to your neighbors, its

a. current and voltage both gradually decrease along the length of the cordb. 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 2007Final Exam — May 4th, 2007 — Page 7

31. When two pieces of semiconductor touch to form a p-n junction, a depletion regionforms around that junction. In the depletion region, all the valence levels are filledand 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 becausea. 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 aconstant 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 barbecomes a magnetic south pole.

c. One end of the bar both accumulates positive charge and becomes a magneticnorth pole, and the other end of the bar accumulates negative charge and becomesa 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 requiredforward bias voltage for one of the four LED’s. Which is the voltage that must beused 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 2007Final 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 massivenucleus of a large atom, the electron’s direction suddenly changes. The electron emitsan 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 abruptacceleration 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 viewan 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 emita 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 2007Final exam — May 4, 2007 — Page 9

41. You and a friend, who is half your weight, are on a seesaw. You have no troublebalancing, however, as long as you sit so that the distance between you and the pivotis 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 soin the recreational area of a permanent settlement on the moon. You are bothsitting in the same positions on the seesaw as was the case in part a, and you arein 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 longwill it take on the moon? You may assume that the force with which you pushand the time over which you push is identical in both cases.

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

42. Shown at right is a diagram representing an AC syn-chronous motor. Notice that at the moment depictedin 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 forcebetween the north pole of the rotor and the portionof the iron core marked “A”.

a. Why was the North pole of the rotor attractedto the portion of the iron core marked “A”?

A.

B.

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 2007Final 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 smallset of speakers.

a. When you connect your iPod to the amplifier, it sends current back and forththrough the input. As a result, charge moves onto and off of what critical controlelement in the amplifier?

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

Physics 106 - How Things Work II - Spring 2007Final 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 isthe wavelength of the second radio station?

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

N

E

W

S