In the United States, household alternating current has a frequency of 60 Hz. At what frequency does a light bulb on this AC supply actually flicker on and off?
A30 Hz
B60 Hz
C120 Hz
D240 Hz
Answer & Solution
Correct answer: C. 120 Hz
1. An AC voltage crosses zero twice during each full cycle -- once going positive-to-negative and once negative-to-positive.
2. Each of these two zero crossings corresponds to a moment when essentially no current (and hence no light) flows.
3. The text states: "the frequency of alternating current is 60 Hz, so the lights flicker on and off at a frequency of 120 Hz."
4. So the flicker frequency is double the AC supply frequency: 2 x 60 Hz = 120 Hz, not 30, 60, or 240 Hz.
_Source: OpenStax Physics Ch 20 "Magnetism", p.688 §20.2 Motors, Generators, and Transformers_
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