The metal contacts which are used to connect a battery into a circuit are called its terminals.

For this reason, when the voltage of a battery is measured, we often describe the result as the terminal potential difference of the battery.

A battery converts chemical energy into electrical energy.

The electrical energy given to each Coulomb of charge is called the emf^*, , of the battery.

In the following circuits, the voltmeter is assumed to have infinite resistance (a modern digital voltmeter has a resistance of around 10⁷W).

If we measure the terminal p.d. of a battery which is not supplying any current, the result is equal to the emf of the battery.

The substances of which the battery is made have some resistance to the flow of electric current. This is called the internal resistance of the battery.

A more complete symbol to represent a battery is shown below.

The resistor, r, represents the internal resistance of the battery.

We now consider measuring the terminal p.d. of a battery which is supplying current.

The energy supplied to each Coulomb of charge by the chemical reactions in the battery is .

The energy taken from each Coulomb of charge by the internal resistance of the battery is Ir. This is often called the "lost voltage".

Therefore, the terminal p.d. of the battery, V, (the reading of the voltmeter) will be given by

The terminal potential difference is only equal to the emf of the battery if the current flowing through the battery is zero.

Also, V = IR, where R is the external resistance.

Therefore, we have

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