AC reverses direction periodically — DC does not

Alternating Currents — AQA A-Level Physics

Key Definition

Direct current (DC) — An electric current that flows in one direction only. The magnitude may be constant or varying, but the direction never reverses.

Key Definition

Alternating current (AC) — An electric current that periodically reverses direction. The current repeatedly changes between flowing one way and flowing the opposite way.

A battery produces DC. The currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A). always flows from the positive terminal through the circuit to the negative terminal.
The UK mains supply is AC: 230 V RMS at 50 Hz. The currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A). direction reverses 100 times per second (50 complete cycles).
AC is produced by rotating a coil in a magnetic field. As the coil rotates, the flux linkageThe product of magnetic flux and the number of turns of a coil. Measured in weberThe SI unit of magnetic flux. One weber is the flux through an area of 1 m² when the magnetic flux density is 1 T perpendicular to the area.-turns (Wb turns). through it changes sinusoidally, inducing a sinusoidal EMFElectromotive force. The energy transferred per unit charge by a source in driving charge around a complete circuit. Measured in volts (V).. This is how generators work.
The output of a generator is a sine wave. The voltageThe energyThe capacity to do work. Measured in joules (J). transferred per unit chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). between two points. Measured in volts (V). Informal term for potential difference. rises to a peak, falls back through zero, goes to a negative peak, and returns to zero. That's one complete cycle.
Most exam questions deal with sinusoidal AC. In practice, other waveforms exist (square waves, triangular waves), but the AQA spec focuses on sinusoidal.

Common Mistake MEDIUM

Students often: Thinking DC always means constant current.
Instead: DC means the current flows in one direction. It can still vary in magnitude. A phone charger outputs DC, but the current changes as the battery charges. The defining feature of DC is one-way flow, not constant value.