AP Physics Electricity and Magnetism #3 Capacitors, Resistors, Ohm s Law, Electric Power

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1 Name Period AP Physics Electricity and Magnetism #3 Capacitors, Resistors, Ohm s Law, Electric Power Dr. Campbell 1. The two plates of a capacitor hold µc and µc of charge, respectively, when the potential difference is 850 V. What is the capacitance? 2.9 x10-6 F 2. A0.20 F capacitor is desired. What area must the plates have if they are to be separated by a 2.2 mm air gap? 5.0 x 10 7 m 2 3. If a capacitor has opposite 5.2 µc charges on the plates, and an electric field of 2.0 kv/mm is desired between the plates, what must each plate s area be? 0.29 m 2 1

2 4. How strong is the electric field between the plates of a 0.80 µf air gap capacitor if they are 2.0 mm apart and each has a charge of 72 µc? 4.5 x 10 4 V/m 5. A hair dryer draws 7.5 A when plugged into a 120 V line. (a) What is its resistance? (b) How much charge pass through it in 15 min? (Assume direct current) 16Ω, 6800 C 6. A bird stands on a dc electric transmission line carrying 2800 A. The line has 2.5 x 10-5 Ω resistance per meter, and the bird s feet are 4.0 cm apart. What is the potential difference between the birds feet? 2.8 x 10-3 V 7. What is the resistance of a 3.5 m length of copper wire 1.5 mm in diameter? 3.3 x 10-2 Ω 2

3 8. Compute the voltage drop along a 26 m length of household no. 14 copper wire (used in 15A circuits). The wire has a diameter mm and carries a 12 A current. 2.5 V 9. Two copper wires have the same resistance. If one has twice the length of the other, what is the ratio of the diameter of the longer wire to the diameter of the shorter wire? 2 1/2 10. The heating element of an electric oven is designed to produce 3.3 kw of heat when connected to a 240 V source. What must be the resistance of the element? 17Ω 11. A 115 V fish tank heater is rated at 110 W. Calculate (a) the current through the heater when it is operating, and (b) its resistance A, 120 Ω 3

4 12. You buy a 75 Watt light bulb in Europe, electricity is delivered to homes at 240 V. If you used the light bulb in the United States at 120 V (assume its resistance does not change), how bright will it be relative to 75 W 120 V bulbs? [Assume roughly that brightness is proportional to power consumed.] 1/4 13. How many 100 W light bulbs, connected in series to 120 V outlets, can be used without blowing a 20 A fuse? 24 bulbs 14. The wiring in a house must be thick enough so it does not become so hot as to start a fire. What diameter must a copper wire be if it is to carry a maximum current of 35 A and produce no more than 1.8 W of heat per meter of length? 3.8 x 10-3 m 15. A 2200 W oven is hooked up to a 240 V source. (a) What is the resistance of the oven? (b) How long will it take to bring 120 ml of water from 15 C to 100 C assuming 75% efficiency? 26Ω, 26 s 4

5 16. An electric field E exists in the region between the two electrically charged parallel plates shown above. A beam of electrons of mass m, charge q, and velocity v enters the region through a small hole at position A. The electrons exit the region between the plates through a small hole at position B. Express your answers to the following questions in terms of the quantities m, q, E, θ, and v. Ignore the effects of gravity. a. i. On the diagram of the parallel plates above, draw and label a vector to show the direction of the electric field E between the plates. ii. On the following diagram, show the direction of the force(s) acting on an electron after it enters the region between the plates. iii. On the diagram of the parallel plates above, show the trajectory of an electron that will exit through the small hole at position B. b. Determine the magnitude of the acceleration of an electron after it has entered the region between the parallel plates. c. Determine the total time that it takes the electrons to go from position A to position B. d. Determine the distance d between positions A and B. e. Now assume that the effects of gravity cannot be ignored in this problem. How would the distance d change for an electron entering the region at A and leaving at B? Explain your reasoning. 5