CHAPTER 22 Current and Resistance

Transcription

1 CHAPTER 22 Current and Resistance MULTIPLE CHOICE 1. The current in an electron beam in a cathode-ray tube is measured to be 70 µa. How many electrons hit the screen in 5.0 s? (e = C) a electrons b electrons c electrons d electrons 2. 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 C d C 3. 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 4. The charge flowing through a light bulb attached to a 12.0-V battery in 14.0 s is 30.0 C. On the average, how many electrons pass a point in the filament of the bulb per second? a. b. c. d. 5. The charge flowing through a light bulb attached to a 12.0-V battery in 14.0 s is 30.0 C. What is the total energy delivered to the filament during this process? a J b. 360 J c J d. 168 J

2 6. The charge flowing through a light bulb attached to a 12.0-V battery in 14.0 s is 30.0 C.What is the average power supplied by the battery during this process? a. 360 W b W c W d W 7. Materials having resistance changes as voltage or current varies are called: a. ohmic. b. inohmic. c. nonohmic. d. deohmic. 8. You measure a 25.0-V potential difference across a 5.00-Ω resistor. What is the current flowing through it? a. 125 A b A c A d A 9. The unit of electric current, the ampere, is equivalent to which of the following? a. V Ω b. V/Ω c. Ω m d. V/s 10. The unit of electric resistance, the ohm, is equivalent to which of the following? a. V/A b. V m c. A/s d. A/m 11. A flashlight bulb operating at a voltage of 4.5 V has a resistance of 8.0 Ω. How many electrons pass through the bulb filament per second (e = C)? a b c d 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.

3 d. none of the above, since resistance is a constant for the given resistor. 13. A metallic conductor has a resistivity of Ω m. What is the resistance of a piece that is 30 m long and has a uniform cross sectional area of m 2? a Ω b. 180 Ω c. 160 Ω d. 90 Ω 14. Two cylindrical resistors are made of the same material and have the same resistance. The resistors, R 1 and R 2, have different radii, r 1 and r 2, and different lengths, L 1 and L 2. Which of the following relative values for radii and lengths would result in equal resistances? a. r 1 = r 2 and L 1 = 2L 2 b. 2r 1 = r 2 and L 1 = 2L 2 c. r 1 = r 2 and 4L 1 = L 2 d. 2r 1 = r 2 and 4L 1 = L How long is a wire made from 100 cm 3 of copper if its resistance is 8.5 ohms? The resistivity of copper is Ω m. a. 7.1 m b m c m d m 16. Two wires with the same resistance have the same diameter but different lengths. If wire 1 has length L 1 and wire 2 has length L 2, how do L 1 and L 2 compare if wire 1 is made from copper and wire 2 is made from aluminum? The resistivity of copper is Ω m and the resistivity of aluminum is Ω m. a. L 1 = 1.7 L 2 b. L 1 = 0.60 L 2 c. L 1 = 2.8 L 2 d. L 1 = 0.36 L 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. 1/3 b. 1/9 c. 3 d. 9

4 18. A 0.20-m-long metal rod has a radius of 1.0 cm and a resistance of Ω. What is the resistivity of the metal? a Ω m b Ω m c Ω m d Ω m 19. A Nichrome wire has a radius of 0.50 mm and a resistivity of Ω m. What is the resistance per unit length of this wire? a Ω/m b Ω/m c. 1.6 Ω/m d. 1.9 Ω/m 20. A Nichrome wire has a radius of 0.50 mm and a resistivity of Ω m. If the wire carries a current of 0.50 A, what is the potential difference per unit length along this wire? a V/m b V/m c. 1.6 V/m d. 1.9 V/m 21. A copper cable needs to carry a current of 200 A with a power loss of only 3.0 W/m. What is the required radius of the copper cable? (The resistivity of copper is Ω m). a cm b cm c. 3.2 cm d. 4.0 cm 22. A 60-W light bulb is in a socket supplied with 120 V. What is the current in the bulb? a A b. 2.0 A c. 60 A d A 23. A resistor is connected to a battery with negligible internal resistance. If you replace the resistor with one that has twice the resistance, by what factor does the power dissipated in the circuit change? a b c. 4.0 d. 2.0

5 24. The quantity volt is equivalent to which of the following? a. J m b. J C c. C/Ω d. J/C 25. The unit for rate of energy transformation, the watt, in an electric circuit is equivalent to which of the following? a. V/s b. A Ω c. V A d. V/Ω 26. If a 500-W heater carries a current of 4.00 A, what is the voltage across the ends of the heating element? a V b. 125 V c. 250 V d V 27. If a 500-W heater carries a current of 4.00 A, what is the resistance of the heating element? a Ω b Ω c Ω d Ω 28. 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 6.0 cents per kwh? a. 1.5 cents b. 9.0 cents c. 18 cents d. 36 cents 29. If a lamp has resistance of 120 Ω when it operates at 100 W, what is the applied voltage? a. 110 V b. 120 V c. 125 V d. 220 V

6 30. If a lamp has a resistance of 120 Ω when it operates at 100 W, what current does it carry? a A b. 1.2 A c A d A 31. An electric toaster requires W at 110 V. What is the resistance of the heating coil? a. 7.5 Ω b. 9.0 Ω c Ω d Ω 32. A light bulb, sold as long-lasting, is rated 100 W at 130 V. The increased lifetime comes from using it at 120 V. Assuming negligible change in resistance at the different voltage, what is its power consumption at 120 V? a. 85 W b. 92 W c. 100 W d. 108 W 33. A steam turbine at an electric power plant delivers 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 V? a A b A c A d A 34. A light bulb has resistance of 240 Ω when operating at 120 V. Find the current in the light bulb. a. 2.0 A b. 1.0 A c A d A 35. 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

7 36. Which is a unit of power? a. kwh b. W/s c. A Ω d. J/s 37. Which is not a force? a. gravity b. electrical force c. voltage d. friction 38. A water pump draws about 3.8 A when connected to 240 V. What is the cost (with electrical energy at 9 cents per kwh) of running the pump for 10 h? a. 8.0 cents b. 15 cents c. 82 cents d. 95 cents 39. A high-voltage transmission line carries A at V. What is the power carried by the line? a. 700 MW b. 370 MW c. 100 MW d. 70 MW 40. An electric car is designed to run off a bank of 12-V batteries with total energy storage of J. If the electric motor draws W, what current will be delivered to the motor? a. 500 A b. 400 A c. 200 A d. 100 A 41. An 8.00-Ω resistor is dissipating 100 watts. What are the current through it and the difference of potential across it? a A, 28.3 V b A, 12.5 V c A, 28.3 V d A, 3.54 V

8 42. A light bulb with a tungsten filament is attached to a source of variable voltage. As the voltage is increased on the bulb, a. the bulb s resistance decreases. b. the bulb s resistance increases. c. the current in the bulb decreases. d. the power dissipated remains constant. 43. 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 44. Some superconductors are capable of carrying a very large quantity of current. If the measured current is A, how many electrons are moving through the superconductor per second? (e = C) a b c d A wire has resistance R. A second wire has twice the length, twice the diameter, and twice the resistivity of the first wire. What is its resistance? a. 8 R b. R c. R/4 d. The resistance is not given. 46. If a light bulb has half the resistance of a 100-W lightbulb, what would be its wattage? Assume both bulbs are attached to the same 120-V circuit. a. 200 W b. 50 W c. 25 W d. More information is needed.