Alternating Current (AC)

Definition

Electric current that flows back and forth, reversing direction many times per second — what comes out of wall outlets.

$I(t) = I_0\sin(\omega t)$

Symbol: $I(t)$ · Unit: A

Technical definition

Current whose direction (and usually magnitude) varies periodically, typically sinusoidally: I(t) = I₀sin(ωt). Mains AC is 50 or 60 Hz depending on country. AC enables efficient voltage transformation for long-distance power transmission.

Example

US wall outlets supply 120 V AC at 60 Hz — the current reverses direction 120 times per second.

Common confusion

AC does not mean current flows ‘both ways at once’ — at any instant it flows one way, then reverses. And ‘120 V AC’ is an RMS value; the peak voltage is actually about 170 V.