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Household outlets in many countries supply 120 V AC. What does it mean that this 120 V figure is a time-averaged value?
AThe 120 V figure is measured once a year and is then never rechecked
BThe voltage is exactly 120 V at every single instant, with no variation
CThe alternating voltage varies continuously, and 120 V is its time-averaged magnitude
DThe 120 V figure applies only to direct current appliances, never AC ones
Answer & Solution
Correct answer: C. The alternating voltage varies continuously, and 120 V is its time-averaged magnitude
1. Household power is explicitly identified as AC, not DC.
2. AC voltage alternates back and forth rather than staying at one fixed value.
3. The commonly quoted 120 V is described as the time-averaged value of this alternating supply, not a constant instantaneous value.
4. So a device connected to this supply experiences a voltage that varies over time, averaging to 120 V rather than sitting fixed at 120 V.
_Source: OpenStax Physics (CC BY 4.0), Ch 19 "Electrical Circuits", section Find the Resistance of a Lightbulb_
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