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

Transcription

1 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:

2 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 horizontal 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.

3 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 chemical 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.

4 Final Exam May 4th, 2007 Page 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.

5 Final Exam May 4th, 2007 Page 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 235 U, the light isotope of uranium, falls apart, potential energy contained 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 neighborhood. 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.

6 Final Exam May 4th, 2007 Page 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 onewiretotheother. 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 photoconductor. 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 238 U. 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.

7 Final Exam May 4th, 2007 Page 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 238 U, the heavy isotope of uranium, cannot be used by itself in a nuclear reactor is that 238 U a. has more electrons than 235 U, 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 235 U, 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 uptherampwithanuphillforceof 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.

8 Final Exam May 4th, 2007 Page 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 accumulates 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.

9 Final Exam May 4th, 2007 Page 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 electromagnetic 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 becomes 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.

10 Final exam May 4, 2007 Page 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.

11 Final exam May 4, 2007 Page Shown at right is a diagram representing an AC synchronous motor. Notice that at the moment depicted in the figure, the north pole of the rotor has just rotated to roughly the upwards position. This happened, 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? 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.

12 Final exam May 4, 2007 Page 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?

13 Final exam May 4, 2007 Page 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 transmitter of one of the radio stations referred to above. Your radio has a built-in antenna consisting of several loops of wire as shown in the figure at right. How should you orient your radio to get the best reception? A north/south orientation as shown? An east/west orientation? Perhaps the the coils should be horizontal, i.e. tip the radio on its side? Explain your answer. S E W N