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Cambridge IGCSE Physics · 0625

Chapter 4: Electricity and magnetism — Part 4

Topic 4.2.3 · Electromotive force and potential difference

e.m.f. and p.d.

Electromotive force (e.m.f.) is the electrical work done by a source in moving a unit charge around a complete circuit. It is measured in volts (V).

Potential difference (p.d.) is the work done by a unit charge passing through a component. It is also measured in volts.

E = W / Q and V = W / Q with W in joules (J) and Q in coulombs.

1 V means 1 J of work per coulomb.

A lamp transfers 36 J of energy when 4.0 C of charge passes through it. Calculate the p.d. across the lamp.

Worked example: potential difference is 36 joules over 4.0 coulombs equals 9.0 volts
V = W / Q = 9.0 V.

Exam Traps

  • e.m.f. belongs to the source (the whole circuit). p.d. belongs to a component or between two points. Do not call the reading across a lamp an e.m.f.

Voltmeters

A voltmeter measures p.d. Connect it in parallel with the component (or across the two points). Analogue and digital meters have different ranges — choose a range that fits the expected voltage.

Cell, lamp and voltmeter: the voltmeter in parallel across the lamp measuring p.d.; e.m.f. marked as the source driving charge around the complete circuit.
Diagram 1: Cell, lamp and voltmeter: the voltmeter in parallel across the lamp measuring p.d.; e.m.f. marked as the source driving charge around the complete circuit.

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