The frequency of alternating current (AC) is measured in hertz (Hz), which represents the number of cycles per second. For ordinary household electricity, the frequency is typically 60 Hz in the United States and some other countries, and 50 Hz in many other parts of the world. For 60 Hz alternating current, the time for a complete cycle (moving from one direction above zero to the opposite direction) is 1/60 of a second, which is approximately 0.0167 seconds or 16.7 milliseconds. For 50 Hz alternating current, the time for a complete cycle is 1/50 of a second, which is 0.02 seconds or 20 milliseconds. It is worth noting that the concept of "motion from one direction to another" can be better understood by communicating the positive and negative halves of the cycle rather than physical movement. In an AC circuit, the current changes direction, cycles back and forth rapidly and produces a sinusoidal waveform.