AC power refers to the current whose current magnitude and direction change periodically with time, and the running average in one cycle is zero. Unlike direct current, its direction changes over time, while direct current does not change periodically.
Usually the alternating current (AC) waveform is sinusoidal. AC power can transmit power efficiently. But in fact, other waveforms are applied, such as triangular waves and square waves. The utility power used in life is an alternating current with a sinusoidal waveform.
According to the principle of Fourier series, the periodic function can be expanded into an infinite series composed of a sine function and a cosine function. Any non-harmonic AC can also be decomposed into a series of simple harmonic cosine AC.
The frequency of alternating current refers to the number of periodic changes in its unit time, in Hertz, which is inversely related to the period. The frequency of alternating current in daily life is generally 50 Hz or 60 Hz, and the frequency of alternating current involved in radio technology is generally large, reaching a measure of kilohertz (KHz) or even millions of hertz (MHz). Power systems in different countries have different AC frequencies, usually 50 Hz or 60 Hz.