Conceptual Questions:
1. Why don't the free electrons in a metal fall to the bottom due to gravity, or move to the surface due to conduction and mutual repulsion?
2. Since the actual charges move pretty slowly through a copper wire (drift speed about 3 x10-4 m/s) why does a light bulb light up right away when you turn on the switch?
Numerical Problems: answers link to a help page.
1. A 60 W light bulb is in a socket supplied with 120 V. What is the current in the bulb?
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?
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)?
4. 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
5. An electric toaster requires 1100 W at 110 Volts. What is the resistance of the heating coil?
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.
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.
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)
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?"
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?