4.2.3 Electromotive force and potential difference

Potential Difference (p.d.)

Potential difference is the difference in electric potential between two points. It is what drives the flow of electric charge in a circuit.

The potential difference between two points is measured in volts (V).

A volt is defined as a unit of potential energy per unit charge. One volt is equal to one joule per coulomb (J/C).

Electromotive Force (e.m.f.)

Electromotive force (e.m.f.) is the energy supplied by a source to each unit charge of electric current flowing through it. It is the force that pushes the electrons around a circuit.

The e.m.f. is also measured in volts (V).

Sources of e.m.f. include batteries and generators.

Relationship between Potential Difference and Electromotive Force

For an ideal source, the electromotive force (e.m.f.) is equal to the potential difference (p.d.) across its terminals.

$$ \text{e.m.f.} = \text{p.d.} $$

Measuring Potential Difference

A voltmeter is used to measure potential difference. It is connected in parallel with the component across which the potential difference is to be measured.

The voltmeter has a high resistance to minimise the current drawn from the circuit.

Table Summary

Quantity	Symbol	Unit	Description
Potential Difference	$V$	Volt (V)	Difference in electric potential between two points.
Electromotive Force	$ε$ or $E$	Volt (V)	Energy supplied by a source per unit charge.
Instrument Used for Measurement	Voltmeter	V	Connected in parallel to measure potential difference.