Basic Electricity Practice Exam

AC voltages and currents vary with time, so we use a single number that reflects the actual power they can deliver. That number is the RMS (root-mean-square) or effective value. It’s defined so that the power in a resistor with AC voltage equals the same as with a steady DC voltage of the same magnitude, i.e., P = V_rms^2 / R = I_rms^2 R. For a sinusoidal signal, V_rms is the peak voltage divided by sqrt(2); the same relation holds for current. If we used peak values, power ratings wouldn’t have a consistent basis; the average value over a full cycle for a sine wave is zero, which isn’t useful for power. Instantaneous values describe the exact momentary reading, but ratings and comparisons require a stable measure, hence RMS. Devices and systems are thus specified by effective (RMS) values.