MDCAT Physics · Physics · Alternating current. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
An alternating current (AC) changes its size and direction again and again, following a sine curve: V = V0 sin ωt. Here V0 is the peak value and ω = 2πf. One full cycle takes the period T = 1/f. The peak-to-peak value is 2V0.
The average of a sine wave over a full cycle is zero, so we use the rms value. It is the steady direct current or voltage that gives the same heating in the same resistor, and it equals the peak value divided by √2.
Power depends on the phase angle φ between current and voltage. Only resistance uses up energy. A pure inductor or capacitor has φ = 90°, so its average power is zero.
Putting the peak voltage straight into P = V2/R gives twice the true power. Change peak (or peak-to-peak) values to rms first, and remember that in a circuit with reactance only the resistance dissipates power.
Answer: Frequency 60 Hz, average power 200 W.
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