UniversityPhysicsVolume2-Ch15.Old Dominion University PHYS 232N

University Physics Volume II
Unit 2: Electricity and Magnetism
Chapter 15: Alternating-Current Circuits
Conceptual Questions
1. What is the relationship between frequency and angular frequency?
Solution
Angular frequency is 2 times frequency.
2. Explain why at high frequencies a capacitor acts as an ac short, whereas an inductor acts as an
open circuit.
Solution
The reactance of the capacitor goes to zero when the frequency is very high, thus acting like a
short or zero resistance. The reactance of the inductor goes to infinity as the frequency
approaches infinity, thus acting like an open or infinite resistance part of the circuit.
3. In an RLC series circuit, can the voltage measured across the capacitor be greater than the
voltage of the source? Answer the same question for the voltage across the inductor.
Solution
yes for both
4. For what value of the phase angle  between the voltage output of an ac source and the
current is the average power output of the source a maximum?
Solution
0
5. Discuss the differences between average power and instantaneous power.
Solution
The instantaneous power is the power at a given instant. The average power is the power
averaged over a cycle or number of cycles.
6. The average ac current delivered to a circuit is zero. Despite this, power is dissipated in the
circuit. Explain.
Solution
The current oscillates between positive and negative, resulting in an average value of zero.
7. Can the instantaneous power output of an ac source ever be negative? Can the average power
output be negative?
Solution
The instantaneous power can be negative, but the power output can’t be negative.
8. The power rating of a resistor used in ac circuits refers to the maximum average power
dissipated in the resistor. How does this compare with the maximum instantaneous power
dissipated in the resistor?
Solution
The maximum instantaneous power can be higher than the maximum power dissipated in the
resistor.
9. Why do transmission lines operate at very high voltages while household circuits operate at
fairly small voltages?
Solution
There is less thermal loss if the transmission lines operate at low currents and high voltages.
10. How can you distinguish the primary winding from the secondary winding in a step-up
transformer?
Solution
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