Direct and Alternating Potential Difference

Potential difference tells us how much energy is transferred by each coulomb of charge in a circuit. It is measured in volts, V.

For GCSE Physics, you need to know that there are two main types of potential difference: direct and alternating.

A direct potential difference stays the same way round.

This means one terminal of the supply is always positive and the other terminal is always negative. A cell or battery provides a direct potential difference.

Because the terminals stay the same way round, the current flows in one direction only. This is called direct current, or d.c.

An alternating potential difference keeps changing direction.

This means the positive and negative sides of the supply swap over again and again. The current also changes direction again and again. This is called alternating current, or a.c.

The mains electricity supply in the UK is alternating. It is about 230 V and has a frequency of 50 Hz.

A frequency of 50 Hz means the potential difference changes direction 50 times every second.

A simple way to remember it is this:

A battery gives a direct potential difference, so the current flows one way.

The mains supply gives an alternating potential difference, so the current changes direction repeatedly.

The key difference is whether the potential difference stays the same way round or keeps reversing.