Pre-AP Physics Electrical Energy/Capacitance/Current/Power Review Sheet
1. A uniform electric field with a magnitude of 345 N/C is directed in the positive x direction. A 10.1 µC charge moves from the origin to a point 54.0 cm to the right of origin. What is the change in electrical potential energy of the system? [-1.88 x10-3 J]
2. Two charges are located along the x-axis. One has a charge of 80.2 µC, and the second has a charge of -13.8 µC. If the electrical potential energy associated with the pair of charges is 0.075 J, what is the distance between the charges? [132.811 m]
3. A point charge of 11.1 x 10-9 C is located at the origin. A positive charge of 4.3 x 10-9 C is brought in from infinity to a point such that the electrical potential energy associated with the two charges is 7.16 x 10-7 J. How far apart are the charges at this time? [0.600 m]
4. Find the potential difference between a point infinitely far away and a point 0.25 m from a proton with charge, 1.6 x 10-19 C. [5.76 x 10-9 V]
5. In a particular television tube, the beam current is 75.0 µA. How long does it take for 6.3 x 1012 electrons to strike the screen? One electron’s charge is -1.6 x 10-19 C. [0.0134 sec]
6. A steady current of 3.8 A flows in a wire for 5 minutes. How much charge passes through any point in the circuit? [1140 C]
7.A calculator is rated at 0.21 W when connected to a 1.5 V battery. What is the resistance of this device? [10.714 Ώ]
8. An electric heater is operated by applying a potential difference of 75.0 V across a nichrome wire of total resistance 9.25 Ώ. Find:
a. the current in the wire [8.108 A]
b. the power rating of the heater. [608.1 W]
9. A 4.0 µF capacitor is connected to a 12 V battery. What is the magnitude of the charge on each plate of the capacitor? [4.8 x 10-5 C]
b. How much electrical potential energy is stored in the capacitor? [2.88 x 10-4 J]
10. The compressor on an air conditioner draws 45.0 A when it starts up. If the start-up time is 0.75 s, how much charge passes a cross-sectional area of the circuit in this time? [33.75 C]
11. A total charge of 11.0 mC passes through a cross-sectional area of nichrome wire in 4.2 s.
a. What is the current in the wire? [2.619 x 10-3 A]
b. If the number of charges that pass through the cross-sectional area during the given time interval triples, what is the resulting current? [7.857 x 10-3 A]
12. A1300 W electric toaster is operated on a household circuit of 120 V. What is the resistance of the wire that makes up the heating element of the toaster? [11.077 Ώ]
13. A 5.6 µF capacitor is connected to a 12 V battery. What is the charge on each plate of the capacitor? [6.72 x 10-5C]
b. If this same capacitor is connected to a 1.50 V battery, how much electrical potential energy is stored? [6.3 x 10-6 J]
14. The current in a certain resistor is 0.62 A when it is connected to a potential difference of 120 V. What is the current in this same resistor if:
a. the operating potential difference is 75 V? [0.388 A]
b. the operating potential difference is 135 V? [0.698 A]
15. Assuming electrical energy costs $0.12 per kWh, calculate the cost of running each of the following appliances for 24 hours if 120 V is supplied to each:
a. a 95 W stereo. [$0.27]
b. an electric oven that draws 32.0 A of current. [$11.06]
c. a television with a resistance of 76Ώ. [$0.54]
16. A 12 V battery is connected to a device and 24mA of current flows through it. If the device obeys Ohm’s law, how much current will flow when a 24 V battery is used? [0.048 A]
17. The damage caused by an electric shock depends on the current flowing through the body; 1mA can be felt; 5 mA is painful. Above 15 mA, a person loses muscle control, and 70 mA can be fatal. A person with dry skin has a resistance from one arm to the other of about 1 x 105 Ώ. When skin is wet, the resistance drops to about 5 x 103 Ώ.
a. What is the minimum voltage placed across the arms that would produce current that could be felt by a person with dry skin? [100 V]
b. What effect would the same voltage have if the person had wet skin? [0.02 A] (0.015 A causes loss of muscle control!)
c. What would be the minimum voltage that would produce a current that could be felt when the skin is wet? [5 V]
18. Both a 3.3 µF and a 6.8 µF capacitor are connected across a 15 V potential difference. Which capacitor has a greater charge? What is it? [6.8 µF capacitor; 1.02 x 10-4 C]
19. A capacitor is connected to a 45.0 V source contains 90 µC of charge. What is its capacitance? [2 x 10-6 F]
20. If you have a parallel-plate capacitor with a capacitance of 1.50 pF and a plate separation of 1.8 mm, what would be the required surface area of each plate? [3.05 x 10-4 m2]