Average and RMS Values

Because an alternating voltage or current continuously changes both its magnitude and its sign throughout every cycle, it is often convenient to describe it using a single representative number rather than the full time-varying expression -- much as we might describe a fluctuating daily temperature using just its average, or a runner's fluctuating speed using just their average speed. Two such representative values are standard for AC: the AVERAGE (or mean) value, and the RMS (root-mean-square, also called the effective or virtual) value. As the next two sections show, these two values answer different practical questions and are computed in quite different ways -- the average deliberately over only a HALF cycle (since the full-cycle average of any symmetric AC waveform is identically zero, and so useless as a descriptor), and the rms …