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Chapter 18 (Circuits)
1. A metallic conductor has a resistivity of 18 ´ 10‑6 W‑m. What is the resistance of a piece that is 30 m long and has a uniform cross sectional area of 3.0 ´ 10‑6 m2?
a. 0.056 W b. 180 W c. 162 W d. 90 W
2. If a metallic wire of cross sectional area 3.0 ´ 10-6 m2 carries a current of 6 A and has a mobile charge density of 4.24 ´ 1028 carriers/m3 what is the average drift velocity of the mobile charge carriers? (charge value = 1.6 ´ 10‑19 Coul)
a. 3.4 ´ 103 m/s b. 1.7 ´ 103 m/s c. 1.5 ´ 10‑4 m/s d. 2.9 ´ 10‑4 m/s
3. You measure a 25 V potential difference across a 5 W resistor. What is the current flowing through it?
a. 125 A b. 5 A c. 4 A d. 1 A
4. A 20 W platinum wire at 20˚C with a temperature coefficient of resistivity of 3.9 ´ 10‑3/C˚ will have what resistance at 100˚C?
a. 14 W b. 20 W c. 26 W d. 28 W
5. A superconducting wire’s chief characteristic is which of the following?
a. an extremely great length b. a large cross sectional area c. an extremely high temperature d. no resistance 6. If a 6 V battery, with negligible internal resistance, and a 12 W resistor are connected in series, what is the amount of electrical energy transformed to heat per Coul of charge that flows through the circuit?
a. 0.50 J b. 3.0 J c. 6.0 J d. 72 J
7. A 60W light bulb is in a socket supplied with 120 V. What is the current in the bulb?
a. 0.50 A b. 2.0 A c. 60 A d. 7200 A
8. A resistor is connected to a battery. If you replace the resistor with one that has twice the resistance, by what factor does the power dissipated in the circuit change?
a. 0.5 b. 1/4 c. 4.0 d. 2.0
9. Replacing a wire resistor with another of the same material and length but with three times the diameter will have the effect of changing the resistance by what factor?
a. 0.33 b. 1/9 c. 3.0 d. 9.0
10. The size of the electric current in a electrical conductor is a function of which of the following?
a. velocity of charge carriers b. conductor thickness c. density of charge carriers d. All of the above choices are valid
11. The current in an electron beam in a cathode‑ray tube is measured to be 70 µA. How many electrons hit the screen in 5 seconds? (qe = 1.6 ´ 10‑19 C)
a. 2.19 ´ 1011 electrons b. 8.75 ´ 1013 electrons c. 2.19 ´ 1015 electrons d. 8.75 ´ 1018 electrons 12. The unit of electric current, the ampere, is dimensionally equivalent to which of the following?
a. Volt ´ Ohm b. Volt /Ohm c. Ohm ´ meter d. Volt /sec
13. The unit of electric resistance, the ohm, is dimensionally equivalent to which of the following?
a. Volt /Amp b. Volt ´ meter c. Amp /sec d. Amp /meter
14. When a superconductor’s temperature drops below the critical temperature, its resistence:
a. equals that of a semiconductor of equal dimensions b. increases by two c. drops to zero d. reduces to one half
15. The quantity, electromotive force, is dimensionally equivalent to which of the following?
a. Joule ´ meter b. Joule ´ Coulomb c. Coulomb/Ohm d. Joule/Coulomb
16. The unit for rate of energy transformation, the Watt, in an electric circuit is dimensionally equivalent to which of the following?
a. Volt/sec b. Ampere ´ Ohm c. Volt ´ Ampere d. Volt/Ohm
17. When an electric current exists within a conducting wire, which of the following statements describes the condition of any accompanying electric field?
a. must be zero b. must be parallel to current flow c. must be anti‑parallel to current flow d. must be perpendicular to current flow 18.A wire carries a steady current of 0.1 A over a period of 20 s. What total charge passes through the wire in this time interval?
a. 200 C b. 20 C c. 2.0 C d. 0.005 C
19. A 0.20 m long metal wire has a radius of 1 cm and a resistance of 3.2 ´ 10‑5 W. What is the resistivity of the metal?
a. 1.6 ´ 10‑8 W‑m b. 5.0 ´ 10‑8 W‑m c. 16.0 ´ 10‑8 W‑m d. 160 ´ 10‑8 W‑m
20. A nichrome wire has a radius of 0.50 mm and a resistivity of 1.5 ´ 10‑6 W‑m. What is the resistance per unit length of this wire?
a. 0.0015 W‑m b. 0.95 W‑m c. 1.6 W‑m d. 1.9 W‑m
21. A nichrome wire has a radius of 0.50 mm and a resistivity of 1.5 ´ 10‑6 ohm‑m. If the wire carries a current of 0.5 A, what is the potential difference per unit length along this wire?
a. 0.003 V/m b. 0.95 V/m c. 1.6 V/m d. 1.9 V/m
22. A metal wire has a resistance of 25.00 W under room temperature conditions of 25˚C. When the wire is heated to 85˚C the resistance increases by 0.75 W. What is the temperature coefficient of resistivity of this metal?
a. 5.0 ´ 10-4/C˚ b. 1.27 ´ 10-3/C˚ c.1.5 ´ 10-3/C˚ d. 2.5 ´ 10-3/C˚ 23. A metal wire has a resistance of 10.00 W at a temperature of 20˚C. If the same wire has a resistance of 10.55 W at 90˚C, what is the resistance when its temperature is ‑20˚C?
a. 0.7 W b. 9.69 W c. 10.31 W d. 13.8 W
24. If a 500 W heater carries a current of 4.0 A, what is the voltage across the ends of the heating element?
a. 2000 V b. 125 V c. 250 V d. 0.008 V
25. If a 500 W heater carries a current of 4.0 A, what is the resistance of the heating element?
a. 85.7 W b. 42.8 W c. 31.3 W d. 11.2 W
26. A 500 W heater carries a current of 4.0 A. How much does it cost to operate the heater for 30 min if electrical energy costs 6 cents per kW‑hr?
a. 1.5 cents b. 9.0 cents c. 18.0 cents d. 36.0 cents
27. By what factor is the resistance of a copper wire changed when its temperature is increased from 20˚C to 120˚C? (the temperature coefficient of resistivity for copper = 3.9 ´ 10‑3/C˚)
a. 0.72 b. 1.06 c. 1.39 d. 1.44
28. If a lamp has resistance of 120 W when it operates at 100 W, what is the applied voltage?
a. 110 V b. 120 V c. 125 V d. 220 V 29. If a lamp has a resistance of 120 W when it operates at 100 W, what current does it carry?
a. 2.10 A b. 1.2 A c. 0.91 A d. 0.83 A
30. An electric toaster requires 1100 W at 110 Volts. What is the resistance of the heating coil?
a. 7.5 W b. 9.0 W c. 10.0 W d. 11.0 W
31. An electric clothes dryer draws 15 A at 220 V. If the clothes put into the dryer have a mass of 7 kg when wet and 4 kg dry, how long does it take to dry the clothes? (Assume all heat energy goes into vaporizing water, Lvap = 2.26 ´ 106 J/kg.)
a. 55.1 min b. 34.2 min c. 20.0 min d. 15.6 min
32. A steam turbine at an electric power plant delivers 4500 kW of power to an electrical generator which converts 95% of this mechanical energy into electrical energy. What is the current delivered by the generator if it delivers at 3600 V?
a. 0.66 ´ 103 A b. 1.0 ´ 103 A c. 1.19 ´ 103 A d. 5.9 ´ 103 A
33. The heating coil of a hot water heater has a resistance of 20 ohm and operates at 210 V. How long a time is required to raise the temperature of 200 kg of water from 15˚C to 80˚C? (the specific heat for water = 103 cal/kg‑C˚ and 1.0 cal = 4.186 J)
a. 1.7 hr b. 3.8 hr c. 5.1 hr d. 6.9 hr 34. The heating coil of a hot water heater has a resistance of 20 ohm and operates at 210 V. If electrical energy costs 5.5 cents per kW‑hr, what does it cost to heat 200 kg water from 15˚C to 80˚C? (the specific heat of water = 103 cal/kgC˚ and 1.0 cal = 4.186 J)
a. 48 cents b. 83 cents c. 16 cents d. 80 cents
35. Number 10 copper wire (radius = 1.3 mm) is commonly used for electrical installations in homes. What is the voltage drop in 40 meters of #10 copper wire if it carries a current of 10 Amps? (The resistivity of copper is 1.7 ´ 10‑8 W‑m.)
a. 1.28 volts b. 0.77 volts c. 0.50 volts d. 0.13 volts
36. A flashlight bulb operating at a voltage of 4.5 V has a resistance of 8 ohm. How many electrons pass through the bulb filament per second (qe = 1.6 ´ 10‑19 Coul)
a. 3.7 ´ 1016 b. 1.8 ´ 1021 c. 9.4 ´ 1017 d. 3.5 ´ 1018
37. In a certain material there is a current of 16 A flowing through a surface to the right, and there is an equal amount of positive and negative charge passing through the surface producing the current. How much negative charge passes through the surface?
a. 8 C/s toward the right b. 8 C/s toward the left c. 16 C/s toward the right d. 16 C/s toward the left 38. A lightbulb has resistance of 240 W when operating at 120 V. Find the current in the lightbulb.
a. 2.0 A b. 1.0 A c. 0.5 A d. 0.2 A
39. When you flip a switch to turn on a light, the delay before the light turns on is determined by:
a. the number of electron collisions per second in the wire. b. the drift speed of the electrons in the wire. c. the speed of the electric field moving in the wire. d. none of these, since the light comes on instantly. 40. If a certain resistor obeys Ohm's law, its resistance will change:
a. as the voltage across the resistor changes b. as the current through the resistor changes c. as the energy given off by the electrons in their collisions changes. d. none of the above, since resistance is a constant for a given resistor.
41. Consider some material that has been cooled until it has become a superconductor. If it is cooled even further its resistance will:
a. increase b. decrease c. stay constant and non‑zero d. none of the above
42. A certain material is in a room at 27˚C. If the absolute temperature (K) of the material is doubled, its resistance also doubles. (Water freezes at 273K.) What is the value for a, the temperature coefficient of resistivity?
a. 1/C˚ b. 2/ C˚ c. 0.0033/C˚ d. 0.038/C˚
43. In a carbon microphone, the change in resistance in the microphone as the sound wave moves the diaphragm back and forth is caused by the:
a. change in length of the carbon resistance. b. change in temperature of the carbon resistance. c. change in voltage across the carbon resistance. d. change in velocity of the carbon resistance.
44. When current is flowing in a superconductor, which statement is not true?
a. A battery keeps the current going. b. Electrical charges are moving. c. The resistance is zero. d. No power is given off in the form of heat.
45. Ten Coulombs of charge start from the negative terminal of a battery, flow through the battery and then leave the positive terminal through a wire, flow through a resistor and then return to the starting point on this closed circuit. In this complete process, the ten coulombs:
a. do positive work on the battery. b. receive heat energy from the resistor. c. have a net loss of potential energy. d. have no net change in potential energy.
46. Which process will double the power given off by a resistor?
a. Doubling the current while doubling the resistance b. Doubling the current by making a resistance half as big c. Doubling the current by doubling the voltage d. Doubling the current while making the voltage half as big
47. Which is a unit of power?
a. Kilowatt‑hour b. Watt per sec c. Ampere Ohm d. Joule per sec
48. Which is not a force?
a. Gravity b. Electrical force c. Electromotive force d. Friction
49. Some superconductors are capable of carrying a very large quantity of current. If the measured current is 105 A, how many electrons are moving through the superconductor per second? (qe = 1.6 ´ 10‑19 C)
a. 6.25 ´ 1023 b. 3.12 ´ 1022 c. 6.25 ´ 1021 d. 3.12 ´ 1020
50. A platinum wire is utilized to determine the melting point of indium. The resistance of the platinum wire is 2 W at 20˚C and increases to 3.072 W as the indium starts to melt. aplatinum = 3.92 ´ 10‑3/C˚. What is the melting temperature of indium?
a. 137˚C˚ b. 157C˚ c. 351C˚ d. 731C˚
51. The resistance of a platinum wire is to be calibrated for low‑temperature work. A platinum wire with resistance 1 W at 20˚C is immersed in liquid nitrogen at 77 K (‑196˚C). If the temperature response of the platinum wire in linear, what is the expected resistance of the platinum wire at ‑196˚C? (qplatinum = 3.92 ´ 10-3/C˚)
a. 0.153 W b. 0.232 W c. 1.768 W d. 1.847 W 52. A water pump draws about 3.8 A when connected to 240 V. What is the cost (with electrical energy at 9˘/kWh) of running the pump for 10 hours?
a. 8.0 ˘ b. 15 ˘ c. 82 ˘ d. 95 ˘
53. A high voltage transmission line of diameter 2 cm and length 200 km carries a steady current of 1000 A. If the conductor is copper with a free charge density of 8 ´ 1028 electrons/m3, how long does it take one electron to travel the full length of the cable? (q = 1.6 ´ 10‑19 C)
a. 8 ´ 102 s b. 8 ´ 104 s c. 8 ´ 106 s d. 8 ´ 108 s
54. A high‑voltage transmission line carries 1000 A at 700,000 V. What is the maximum power carried in the line?
a. 700 MW b. 370 MW c. 100 MW d. 70 MW
55. A high‑voltage transmission line carries 1000 A at 700 kV for a distance of 100 miles. If the resistance in the wire is 1 W/mile, what is the power loss due to resistive losses?
a. 10 kW b. 100 kW c. 10 MW d. 100 MW
56. A copper cable needs to carry a current of 300 A with a power loss of only 2 Watts per meter. What is the required radius of the copper cable? (The resistivity of copper is 1.7 ´ 10‑8 W‑m)
a. 0.8 cm b. 1.6 cm c. 3.2 cm d. 4.0 cm 57. An electric car is designed to run off a bank of 12‑V batteries with total energy storage of 2 ´ 107 J. If the electric motor draws 8000 W,what current will be delivered to the motor?
a. 666 A b. 444 A c. 222 A d. 111 A
58. An electric car is designed to run off a bank of 12‑V batteries with total energy storage of 2 ´ 107 J. If the electric motor draws 8000 W in moving the car at a steady speed of 20 m/s, how far will the car go before it is “out of juice?”
a. 25 km b. 50 km c. 100 km d. 150 km
59. A solar panel measures 80 cm ´ 50 cm. In direct sunlight, the panel delivers 3.2 A at 15 V. If the intensity of sunlight is 1000 W/m2, what is the efficiency of the solar panel in converting solar energy into electrical energy?
a. 24.0% b. 18.0% c. 12.0% d. 6.0%
60. Suppose that a voltage surge produces 140 V for a moment in a 120 V‑line. What will temporarily be the output of a 100 W lightbulb assuming its resistance does not change?
a. 109 W b. 118 W c. 127 W d. 136 W Chapter 18 - Answers
# Ans Difficulty # Ans Difficulty
1. B 2 31. B 2 2. D 2 32. C 2 3. B 1 33. D 3 4. C 2 34. B 3 5. D 1 35. A 3 6. C 1 36. D 2 7. A 1 37. B 2 8. D 1 38. C 1 9. B 1 39. C 2 10. D 1 40. D 1 11. C 1 41. D 1 12. B 1 42. C 2 13. A 1 43. A 1 14. C 1 44. A 1 15. D 1 45. D 2 16. C 1 46. B 2 17. B 1 47. D 1 18. C 1 48. C 1 19. B 2 49. A 2 20. D 2 50. B 2 21. B 2 51. A 2 22. A 2 52. C 2 23. B 2 53. D 3 24. B 1 54. A 2 25. C 1 55. D 2 26. A 1 56. B 2 27. C 2 57. A 2 28. A 1 58. B 2 29. C 1 59. C 3 30. D 2 60. D 2
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