Physics 106 - How Things Work II - Spring 2007
Problem Set #4
1. What would happen if you included a p-n junction (a diode) in the AC circuit that
connects the power company to your table lamp?
a. The lamp would no longer light.
b. The lamp would light, but would be roughly half as bright.
c. The lamp would light and have the same brightness.
d. The lamp would light and would actually be slightly brighter.
2. Your stereo’s amplifier obtains power from the power line. Since the power line delivers
no power during the moments when its alternating current reverses direction, the
stereo’s power supply must store energy. How can it do this?
a. The capacitor stores energy.
b. The diodes store energy.
c. The transformer stores energy.
d. The resistors store energy.
3. A step-down transformer transfers power from the 120 Volt alternating current passing through its primary coil to the 12 Volt alternating current passing through its
secondary coil. If you interchange the primary and secondary coil, and send the 120
Volt alternating current through the new primary coil,
a. a 12 Volt alternating current will pass through its new secondary coil.
b. a 1.2 Volt direct current will pass through its new secondary coil.
c. a 1.2 Volt alternating current will pass through its new secondary coil.
d. a 1200 Volt alternating current will pass through its new secondary coil.
4. Which of the following would significantly lessen the extent to which your hair would stand on end when placing
your hand on a Van de Graaff generator?
a. If you were wearing shoes with insulating rubber soles.
b. If the Van de Graaff generator had a positive charge
instead of a negative charge.
c. If you had washed your hair with a product that made
it less conducting.
d. If the humidity of the air were quite low.
- 50,000V
5. If the commutator of a brushed DC motor is spun around, thus making the motor
function as a generator, what will be the nature of the current produced?
a. More-or-less constant DC current.
b. Standard AC current.
c. Voltage similar to what we had with a “half-wave rectifier” (a single diode). This
was one of the circuits we looked at as we prepared to discuss the power adaptor
circuit during lecture 19.
d. Voltage similar to what we had with a “full-wave rectifier” (a four-diode bridge),
the full power adaptor circuit we discussed during lecture 19 when it did not
include a capacitor.
Problem Set #4 - continued
6. What happens if you increase the (AC) voltage to an AC synchronous motor?
a. The motor rotates more quickly.
b. The rotation frequency remains the same, but the torque increases.
c. It will not function. AC synchronous motors are very sensitive to the voltage
level.
d. Essentially nothing. The rotation frequency and the torque remain essentially
unchanged.
7. In many grocery checkout counters, a conveyor belt carries food to the register but
stops when the food reaches the end and blocks a beam of light. How might a photoconductor be used to sense this blockage?
8. Consider the circuit that is shown schematically below. The circuit includes a 12 V
battery, a blow dryer, a space heater, and a light bulb, all hooked up in series. The
resistance of each device is shown. The resistance of all the devices, as seen by the
battery, is given by: Rtotal = Rblow dryer + Rspace heater + Rlight bulb . You may neglect
the resistance of the wires and of the battery.
1 Ohm
Blow dryer
2 Ohms
3 Ohms
Space heater
12 Volts
a. Using Ohm’s Law, how much current will flow in the circuit?
b. How much power is produced by the battery?
c. How much power is consumed by each of the blow dryer, the space heater, and
the light bulb individually?