Alternating current (commonly abbreviated AC) is a characterization of electricity in which the voltage or current changes rapidly. (Except in unusual circumstances, if the voltage changes the current changes and vice-versa; however, the term "alternating current" is always used to describe this situation.) If the voltage is constant, the situation is called direct current or DC.
There are two common situations in which this occurs:
- The voltage might be undulating in a regular manner; the rate at which the voltage repeats itself is called the frequency. This is almost universally done for electrical power transmission, the reason being that this makes it possible for voltages to be changed by transformers, devices which only work on AC. The frequency is measured in Hertz (abbreviated Hz). For power distribution, the frequency is almost always 60 Hz, or 60 complete cycles (zero, positive, zero, negative) per second.
- Regularly undulating voltages are also used for all forms of electromagnetic transmission—radio, television, cell phone, etc. In this case the frequency is often measured in Megahertz or Gigahertz.
- Any type of varying signal, such as an audio signal, is often characterized as "AC". Audio signals don't generally have a specific frequency, they are described as being contained in a "band" of frequencies, but they are still called AC. If the long-term average voltage of a signal is nonzero, that average is often called the "DC component", while the signal with the DC component subtracted is called the "AC component".