Electromotive Force and Potential DifferenceCambridge IGCSE Physics: Revision notes
Section 1
What Is Electromotive Force (e.m.f.)?
The electromotive force (e.m.f.) of a source, such as a cell or battery, is a measure of how much electrical work it does on the charge passing through it.
- E.m.f. is defined as the electrical work done by a source in moving a unit charge around a complete circuit.
- E.m.f. is measured in volts (V), using the equation E = W / Q, where E is e.m.f., W is work done (in joules) and Q is charge (in coulombs).
E.m.f. describes the energy given to charge by the source itself — it is what drives the current around the whole circuit.
Section 2
What Is Potential Difference (p.d.)?
Potential difference (p.d.), also called voltage, describes the work done by a unit charge as it passes through a single component.
- P.d. is defined as the work done by a unit charge passing through a component.
- Like e.m.f., p.d. is measured in volts (V), using the equation V = W / Q.
- P.d. is measured using a voltmeter, which is available in analogue and digital forms with different ranges.
A voltmeter is connected in parallel across a component, not in series — this is the opposite of how an ammeter is connected.
Section 3
E.m.f. and p.d. — What's the Difference?
E.m.f. and p.d. are both measured in volts and both relate work done to charge, but they describe different things.
| Quantity | Describes | Equation |
|---|---|---|
| E.m.f. (E) | Energy supplied by the source per unit charge, around the whole circuit | E = W / Q |
| P.d. (V) | Energy transferred to a component per unit charge | V = W / Q |
- The e.m.f. of a source is the total electrical energy given to each unit of charge that leaves it.
- The p.d. across a component is the electrical energy transferred to that component (e.g. into heat or light) for each unit of charge that flows through it.
A cell with an e.m.f. of 6 V gives 6 joules of energy to every coulomb of charge that passes through it; a lamp with a p.d. of 6 V across it transfers 6 joules of energy from every coulomb of charge that flows through it.
Must Know
- E.m.f. is the electrical work done by a source in moving a unit charge around a complete circuit; e.m.f. is measured in volts using E = W / Q.
- P.d. is the work done by a unit charge passing through a component; p.d. is measured in volts using V = W / Q.
- Voltmeters measure p.d. and are connected in parallel across a component.
- E.m.f. relates to the whole circuit and the source; p.d. relates to a single component.
That's the notes covered.
Carry on to the next subtopic.