1. A 60 W light bulb is in a socket supplied with 120 V. What is the current in the bulb?
ans: 0.50 A; use P = I*V, solve for I.
2. A light bulb is connected to a battery. If you replace the bulb with one that has twice the resistance, by what factor does the power dissipated in the circuit change?
ans: 0.5; use P = V*V/R.
3. The current in an electron beam in a TV cathode-ray tube is measured to be 70 ľA. How many electrons hit the screen in 5 seconds (qe = -1.6 x10-19 C)?
ans: 2.19 x1015 electrons; divide 70 x10-6 by qe to get number of charges per second, then x5.
5. An electric toaster requires 1100 W at 110 Volts. What is the resistance of the heating coil?
ans: 10.0 Ohms; use P = V*V/R.
6. 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, with Lvap = 2.26 x106 J/kg.
ans: 34 min; use Lvap*3kg to get total energy E in joules, then use P = I*V to get joules/second provided. The time will be E/P.
7. 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 x10-8 Ohm*m.
ans: 1.28 volts; use V = I*R; total R of 40 meters of wire is resistivity*length/(cross section area of wire).
8. 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 x1028 electrons/m3, how long does it take one electron to travel the full length of the cable? (qe = -1.6 x10-19 C)
ans: 8 x108 s; drift speed v = I/(n*q*A), t=L/v.
9. An electric car is designed to run off a bank of 12-V batteries with total energy storage of 2 x107 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?"
ans: 50 km; 8000 W is 8000 J/s, so the battery will last 20000000/8000 = 2500 s, and run 20*2500 meters.
10. A small solar panel measures 80 cm x 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 panel in converting solar energy into electrical energy?
ans: 12.0%; Power in sunlight is 1000*0.8*0.5 = 400 W. Power from the panel is I*V = 48 W. Efficiency is (P out)/(P in).