UniversityPhysicsVolume2-Ch09  Old Dominion University PHYS 232N

University Physics Volume II
Unit 2: Electricity and Magnetism
Chapter 9: Current and Resistance
Conceptual Questions
1. Can a wire carry a current and still be neutral—that is, have a total charge of zero? Explain.
Solution
If a wire is carrying a current, charges enter the wire from the voltage source’s positive terminal
and leave at the negative terminal, so the total charge remains zero while the current flows
through it.
2. Car batteries are rated in ampere-hours ( A h  ). To what physical quantity do ampere-hours
correspond (voltage, current, charge, energy, power,…)?
Solution
( A h  ) is a measure of charge.
3. When working with high-power electric circuits, it is advised that whenever possible, you
work “one-handed” or “keep one hand in your pocket.” Why is this a sensible suggestion?
Solution
Using one hand will reduce the possibility of “completing the circuit” and having current run
through your body, especially current running through your heart.
4. Incandescent light bulbs are being replaced with more efficient LED and CFL light bulbs. Is
there any obvious evidence that incandescent light bulbs might not be that energy efficient? Is
energy converted into anything but visible light?
Solution
An incandescent light bulb gives off both light and heat. Since the purpose of the light bulb is to
supply light, the fact that heat is also produced means that all the energy is not being used to
produce light and the bulb may not be very efficient at producing light.
5. It was stated that the motion of an electron appears nearly random when an electrical field is
applied to the conductor. What makes the motion nearly random and differentiates it from the
random motion of molecules in a gas?
Solution
Even though the electrons collide with atoms and other electrons in the wire, they travel from the
negative terminal to the positive terminal, so they drift in one direction. Gas molecules travel in
completely random directions.
6. Electric circuits are sometimes explained using a conceptual model of water flowing through a
pipe. In this conceptual model, the voltage source is represented as a pump that pumps water
through pipes and the pipes connect components in the circuit. Is a conceptual model of water
flowing through a pipe an adequate representation of the circuit? How are electrons and wires
similar to water molecules and pipes? How are they different?
Solution
Water is pumped into the pipe, much like electrons are “pumped” into a circuit by a battery or
another voltage source. Unlike water in a pipe, the flow of charges require a complete circuit. If
the end of the pipe filled with water, the water spills out, but if the wire is not connected to
anything, the charges do not spill out.
7. An incandescent light bulb is partially evacuated. Why do you suppose that is?
Solution
In the early years of light bulbs, the bulbs are partially evacuated to reduce the amount of heat
conducted through the air to the glass envelope. Dissipating the heat would cool the filament,
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