AC

Alternating current flows back and forth through a conductor, reversing direction at a fixed frequency. In North America and parts of Asia, this reversal happens 60 times per second (60 Hz); in Europe, Africa, and most of Asia, it happens 50 times per second (50 Hz). The voltage swings between positive and negative peaks in a smooth sine wave. This is different from direct current (DC), which flows in one direction only.

The main advantage of AC over DC is that transformers can step voltage up or down efficiently, making long-distance power transmission practical. A generator at a power plant produces AC by rotating a coil in a magnetic field. The rotating coil cuts through magnetic field lines, inducing a voltage that naturally alternates as the coil spins. This is why AC became the standard for grid electricity almost everywhere.

RMS voltage and power calculation

When you see a household voltage stated as 120 volts (US) or 230 volts (Europe), this is the root-mean-square (RMS) value, not the peak value. The actual peak voltage is about 1.41 times higher. RMS voltage is the equivalent DC voltage that would deliver the same power to a resistive load. This matters because power calculations use RMS: a 1500 watt space heater on 120V draws 12.5 amps RMS, not 12.5 amps at the peak.

AC motors are simpler and cheaper than DC motors for industrial use because they have no commutators or brushes. The rotating magnetic field produced by three-phase AC power (three voltage signals 120 degrees apart) creates smooth torque without electrical switching. This is why factories run on three-phase AC rather than single-phase.

The term 'AC' in informal speech often includes both current and voltage, even though technically they are different quantities measured in amps and volts respectively. When a technician says 'this circuit is AC', they mean it operates with alternating current and voltage at the standard grid frequency.