10.33-10.36 - Direct and alternating supplies

Direct and alternating supplies

A torch powered by cells and a kettle connected to a wall socket both need a potential difference, but their supplies behave differently. Potential difference, or voltage, is measured between two points. Voltage does not flow; charge moves when a potential difference acts around a complete circuit.

A direct voltage keeps the same polarity: one terminal stays at a higher potential than the other. In a complete circuit supplied by a cell or battery, charge therefore moves in one direction only. This is direct current (d.c.).

An alternating voltage repeatedly reverses polarity. It drives charge one way and then the other, so in alternating current (a.c.) the movement of charge changes direction. The charge in the appliance's wires moves back and forth; it is not used up by the appliance.

[DIAGRAM: asset_name: 45_1PH0-P2-10G_10.33-10.42 - Mains electricity and safety - diagram 01; asset_slug: 45_1ph0-p2-10g_10-33-10-42-mains-electricity-and-safety_diagram_01; recommended_method: vector; description: Exact 50 Hz AC waveform plotted over 0–40 ms beside constant direct voltage. Vertical axis is voltage divided by peak magnitude; a complete cycle is 20 ms; 230 V is the effective mains value, not peak.]

The UK domestic supply is a.c. at about 230 V and 50 Hz. The 230 V value is the potential difference between live and neutral used for domestic appliances. A frequency of 50 Hz means 50 complete a.c. cycles every second; each complete cycle contains both directions of charge movement. There are two direction reversals in a cycle, but frequency counts complete cycles, not reversals. A 50 Hz cycle lasts 1/50 s = 0.020 s = 20 ms.