Using Electricity in the HomeOxford AQA IGCSE Physics: Revision notes

Direct current (dc) always flows in the same direction. It is produced by cells and batteries.

Alternating current (ac) repeatedly changes direction, reversing back and forth at a set frequency.

Mains electricity in the UK/GCC and most countries is supplied as an ac supply, with a set frequency and voltage.

For a supply shown as a trace against time (such as on an oscilloscope):

• The period is the time taken for one complete cycle of the wave
• The frequency is the number of complete cycles per second (frequency = 1 ÷ period)
• For a dc supply, the potential difference stays constant, so it can simply be read off
• For an ac supply, the peak potential difference is the maximum height of the wave from the centre line (zero) to its highest point

Several features keep household electrical circuits and users safe:

• Earthing: a low-resistance wire connecting the metal casing of an appliance to the ground; if the casing becomes live due to a fault, current flows harmlessly to earth rather than through a person touching it
• Fuses and circuit breakers: disconnect the circuit if the current becomes too large
• Double insulation: appliances with plastic (non-conducting) casing and no exposed metal parts do not need an earth wire, since there is no way for the casing to become live

A fuse contains a thin wire that melts and disconnects the live wire if an excessive current flows through the circuit, breaking the circuit and stopping current flow.

A circuit breaker is an electromagnetic switch that trips (opens) if the current is too high — it works faster than a fuse and, unlike a fuse, can simply be reset rather than replaced.