Voltage

Charges move round a circuit only when there is a difference between its two ends, the electric potential difference or voltage, as water flows only from a higher tank to a lower one. A battery makes it: a chemical reaction inside sends electrons to its negative terminal and away from its positive one, turning chemical energy into electrical energy. The voltage tells how much energy each bit of charge is given. It is measured in volts with a voltmeter, connected across the part being measured. Cells placed one after another add their voltages. A torch cell gives 1.5 volts, a car battery 12, and household mains 220, which can kill.