Physics A2 · Alternating currents. 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 changes direction again and again. For a sinusoidal supply the current and voltage follow a sine curve. The peak value is the largest value in either direction. The period is the time for one complete cycle and the frequency is the number of cycles per second.
The power in a resistor changes all the time, between zero and a maximum. Its mean value is half the maximum.
The r.m.s. value of an alternating current is the value of the steady direct current that would give the same mean power in the same resistor. Mains voltages are quoted as r.m.s. values.
Students put a peak value into P = V2/R and call the result the mean power. That gives the maximum power. Use r.m.s. values for mean power, and set the calculator to radians for sin ωt.
Answer: 60 Hz and 80 W
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