Quantum Physics

Answers to Quick Quizzes, Odd-Numbered Conceptual Questions and Problems A.2315. If an external battery is acting to increase the currentin the inductor, an emf is induced in a direction tooppose the increase of current. Likewise, if we attempt toreduce the current in the inductor, the emf that is set uptends to support the current. Thus, the induced emfalways acts to oppose the change occurring in the circuit,or it acts in the “back” direction to the change.17. (a) clockwise (b) The net force exerted on the bar must bezero because it moves at constant speed. The componentof the gravitational force down the incline is balanced by acomponent of the magnetic force up the incline.19. from left to rightCONCEPTUAL QUESTIONS1. For best reception, the length of the antenna should be parallelto the orientation of the oscillating electric field. Be-PROBLEMS1. 5.9 10 2 T m 23. 7.71 10 1 T m 25. (a) B,net 0 (b) 07. (a) 3.1 10 3 T m 2 (b) B,net 09. 0.18 T11. 94 mV13. 2.7 T/s15. (a) 4.0 10 6 T m 2 (b) 18 V17. 10.2 V19. 0.763 V21. (a) toward the east (b) 4.58 10 4 V23. (a) from left to right (b) from right to left25. (a) F N 2 B 2 w 2 v/R to the left (b) 0(c) F N 2 B 2 w 2 v/R to the left27. into the page29. (a) from right to left (b) from right to left (c) from leftto right (d) from left to right31. 1.9 10 11 V33. (a) 18.1 V (b) 035. (a) 60 V (b) 57 V (c) 0.13 s37. 20 mV39. (a) 2.0 mH (b) 38 A/s43. 12 mH45. 1.92 47. 0.140 J49. (a) 18 J (b) 7.2 J51. negative (V a V b )53. (a) 20.0 ms (b) 37.9 V (c) 1.52 mV (d) 51.8 mA55. 1.20 C57. (a) 0.500 A (b) 2.00 W (c) 2.00 W59. 115 kV61. (a) 0.157 mV (end B is positive) (b) 5.89 mV (end A ispositive)63. (a) 9.00 A (b) 10.8 N (c) b is at the higher potential(d) No65. v t mgRB 2 2Chapter 21ANSWERS TO QUICK QUIZZES1. (c)2. (b)3. (b)4. (b), (c)5. (b), (d)cause of atmospheric variations and reflections of the wavebefore it arrives at your location, the orientation of this fieldmay be in different directions for different stations.3. The primary coil of the transformer is an inductor. Whenan AC voltage is applied, the back emf due to the inductancewill limit the current in the coil. If DC voltage is applied,there is no back emf, and the current can rise to ahigher value. It is possible that this increased current willdeliver so much energy to the resistance in the coil that itstemperature rises to the point at which insulation on thewire can burn.5. An antenna that is a conducting line responds to the electricfield of the electromagnetic wave—the oscillatingelectric field causes an electric force on electrons in thewire along its length. The movement of electrons alongthe wire is detected as a current by the radio and is amplified.Thus, a line antenna must have the same orientationas the broadcast antenna. A loop antenna responds to themagnetic field in the radio wave. The varying magneticfield induces a varying current in the loop (by Faraday’slaw), and this signal is amplified. The loop should be inthe vertical plane containing the line of sight to thebroadcast antenna, so the magnetic field lines go throughthe area of the loop.7. The flashing of the light according to Morse code is adrastic amplitude modulation—the amplitude is changingfrom a maximum to zero. In this sense, it is similar tothe on-and-off binary code used in computers and compactdisks. The carrier frequency is that of the light, onthe order of 10 14 HZ. The frequency of the signaldepends on the skill of the signal operator, but it is on theorder of a single hertz, as the light is flashed on and off.The broadcasting antenna for this modulated signal is thefilament of the lightbulb in the signal source. The receivingantenna is the eye.9. The sail should be as reflective as possible, so that themaximum momentum is transferred to the sail from thereflection of sunlight.11. Suppose the extraterrestrial looks around your kitchen.Lightbulbs and the toaster glow brightly in the infrared.Somewhat fainter are the back of the refrigerator and theback of the television set, while the television screen isdark. The pipes under the sink show the same weak glowas the walls, until you turn on the faucets. Then the pipeon the right gets darker and that on the left develops agleam that quickly runs up along its length. The food onthe plates shines, as does human skin, the same color forall races. Clothing is dark as a rule, but your seat and thechair seat glow alike after you stand up. Your face appearslit from within, like a jack-o’-lantern; your nostrils and theopenings of your ear canals are bright; brighter still arethe pupils of your eyes.13. Radio waves move at the speed of light. They can travelaround the curved surface of the Earth, bouncing betweenthe ground and the ionosphere, which has an altitude thatis small compared with the radius of the Earth. Thedistance across the lower 48 states is approximately5 000 km, requiring a travel time that is equal to (5 10 6 m)/(3 10 8 m/s) 10 2 s. Likewise, radio wavestake only 0.07 s to travel halfway around the Earth. Inother words, a speech can be heard on the other side ofthe world (in the form of radio waves) before it is heard atthe back of the room (in the form of sound waves).