What is an alternating current?

Direct current and alternating current

A direct current (labeled DC) always keeps the same direction and a constant intensity: this is the case for an electric battery. An alternating current (labeled AC) periodically changes direction and its intensity varies over time. This is the current found in household outlets.

A quantity that oscillates over time

The most common alternating voltage is sinusoidal. It is written in the form:

u(t) = Umax * sin(2pif*t + phi)

where Umax is the amplitude (maximum value), f the frequency in hertz (Hz), t the time in seconds, and phi the initial phase. The graph of u(t) is a curve that rises, falls, passes through zero, becomes negative, then starts again: this is a periodic phenomenon.

Why use alternating current?

The major advantage of alternating current is that it can easily be stepped up or down in voltage thanks to transformers, which is not possible with direct current. This allows electricity to be transported over long distances with low losses.

Common pitfall

Do not confuse the direction of the current (which changes) with its absolute value: an alternating current is not zero on average in absolute value of intensity, but its signed average value over a complete period is zero. Never say that an alternating current is "zero": it simply oscillates around zero.