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VoltageWhat actually pushes electricity round a circuit?

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Start with a question: what three things make up an electrical circuit? The answer is voltage, current and resistance.

Voltage is the electrical pressure that pushes electricity around a circuit. It is measured in volts, V.

Symbols
V in general formulas. U is how voltage is written in electrical books like BS 7671.
U₀
The UK nominal voltage, 230 V (line to earth).
U
The line-to-line voltage, 400 V (three-phase).
EMF
Electromotive force, the voltage a source produces (battery, generator, transformer winding). Still measured in volts, and written E in formulas. Don't think of it as a force in newtons. It's the push that drives electricity round the circuit.
Potential difference
The difference in voltage between 2 points. For example, 230 V on the line and 0 V on the neutral gives a PD of 230 V.
Waterfall analogy
Water at the top of a waterfall has a high potential, and the pool at the bottom a low one. The water only flows because there's a difference and a path between them. Another way to picture it is two cups, one at 230 V and one at 0 V: pour between them and there's a 230 V potential difference. If both cups are at the same level there's no difference, so nothing flows.
Key point: current can only flow when there is a potential difference.

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