UK Domestic Supply

• it is alternating current (a.c.);
• its frequency is 50 Hz;
• its potential difference is about 230 V.

Why the supply is called a.c.

A potential difference pushes charge around a complete circuit. In an a.c. supply, the direction of this push repeatedly reverses. Consequently, when an appliance is connected, the charge movement through it also repeatedly changes direction.

This differs from direct current, or d.c., which travels in one direction. Cells and batteries provide d.c. The detailed functions of the mains wires are considered separately; here, the essential point is that the domestic supply alternates rather than maintaining one direction.

What 50 Hz means

The frequency tells us how many complete cycles occur each second. A frequency of 50 Hz means that the alternating pattern completes 50 cycles every second. One cycle contains movement in both directions, so 50 Hz should not be described as merely 50 individual reversals.

A graph can represent the changing current. Values on one side of zero indicate one direction; values on the other side indicate the opposite direction. The following simplified graph shows two cycles in 0.040 s, consistent with a 50 Hz supply. Its vertical scale is relative rather than a measurement of mains current.

Data for A 50 Hz alternating-current pattern

What about 230 V means

The domestic supply has a potential difference of about 230 volts. Potential difference measures the energy transferred per unit charge: one volt is one joule per coulomb. The figure 230 V describes the supply’s potential difference, whereas 50 Hz describes how rapidly its alternating pattern repeats. The two numbers therefore represent different physical quantities and must be written with different units.