Electromotive force

A source, such as a battery, supplies energy to move electric charge around a complete circuit. Its electromotive force (e.m.f.) is the electrical work it does per unit charge moved. Despite its name, e.m.f. is not a force: it describes energy supplied per unit charge.

Here, $$\mathcal{E}$$ is the e.m.f., $$W$$ is the work done by the source, and $$Q$$ is the charge moved.

E.m.f. is measured in volts ($$\mathrm{V}$$). One volt is one joule per coulomb: $$1,\mathrm{V} = 1,\mathrm{J/C}$$. For example, a source with an e.m.f. of $$6,\mathrm{V}$$ supplies $$6,\mathrm{J}$$ of energy for each coulomb of charge moved around the circuit.

Potential difference

As charge passes between two points, work is done. Potential difference (p.d.) is the work done per unit of charge passing between those points.

Across a component, the potential difference tells us how much work is done by each coulomb of charge as it passes through the component.

Here, $$V$$ is the potential difference, $$W$$ is the work done in joules and $$Q$$ is the charge in coulombs.

Potential difference is measured in volts ($$\mathrm{V}$$). One volt is one joule per coulomb: a potential difference of $$1,\mathrm{V}$$ means that $$1,\mathrm{J}$$ of work is done for each $$1,\mathrm{C}$$ of charge passing between the points.

For example, if $$2,\mathrm{C}$$ of charge passes through a component and does $$6,\mathrm{J}$$ of work, the potential difference across the component is $$6/2 = 3,\mathrm{V}$$.

Using a voltmeter

A voltmeter measures the potential difference between two points in a circuit. To measure the potential difference across a component, connect the voltmeter in parallel: attach one meter connection to each side of the component.

Reading a voltmeter

An analogue voltmeter uses a pointer moving over a scale. Read the scale using the range selected on the meter. For example, if the selected range is 0–5 volts, a pointer halfway across that range indicates 2.5 volts.

A digital voltmeter displays the reading as a number. The displayed value must still be interpreted using the selected range and units.

Selecting a range

Choose a range that includes the expected potential difference. If the voltage is unknown, start on a high range. Once you have a reading, select a lower range that still includes it; this can give a clearer, more precise reading.

Polarity for direct current

For a direct-current measurement, connect the meter's positive connection to the more positive point and its negative connection to the more negative point. If the connections on a digital voltmeter are reversed, the display shows a negative reading.