A direct current (DC) is the unidirectional flow of electrons. The current flow may vary and fall towards zero, but as long as the direction of electron flow does not change, this remains a direct current. DC can be represented graphically in many ways; one example is shown below.

AC can be converted to DC by a rectifier circuit (shown below). Diodes are used in rectifier circuits because they conduct one way (behaving as one-way flow valves for the electric current). This property of the diode is the fundamental principle of designing DC power supplies from AC.

Applied science

How can we compare AC to DC?

Measuring the voltage of DC can be done directly, but measuring the average voltage of AC is not as simple. Because the waveform is sinusoidal, AC does not have a steady voltage. It has equal deflections on both the positive and the negative axis, so the mean voltage is zero. The magnitude of an AC can be expressed as peak voltage or a root mean squared (RMS) value. In the UK, the domestic mains supply is 240 V RMS, but the peak voltage is actually 339 V (240√2).

The root mean squared (RMS) value allows comparison of the equivalent amounts of AC and DC power required to produce the same heating effect.

A sinusodial waveform is represented by $\text{[math]}$ . To calculate the RMS, the square root of the mean of the squares of all the amplitudes for θ from 0° to 360° is calculated. The mean of sine²θ from 0° to 360° equals 1/2. Therefore, RMS is given by $\text{[math]}$ .