alternating current (AC)

Alternating current is electricity that keeps changing its mind about which way to flow. Instead of marching steadily in one direction like a river, the charge sloshes back and forth many times a second — in most homes, 50 or 60 round trips every second. Picture a crowd doing the wave in a stadium: nobody actually travels around the arena, yet the wave of motion races along the seats. AC carries energy the same way, by wiggling charge in place.

The grid uses AC because its voltage is wonderfully easy to change. A transformer — two coils of wire sharing a magnetic field — can step voltage way up for long-distance travel (so less is lost as heat) and back down to a safe level near your house. Direct current (DC), the steady one-way flow, can't pass through a simple transformer like that. So power plants ship AC down the lines, while batteries and the chips inside your phone run on DC.