E.m.f., potential difference and cells
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The teaching part (1 min) is free to watch. A free account opens the whole lecture: 3 worked examples, 4 practice questions, 2 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- The push and the payoff
- Electromotive force
- Potential difference
- The equation
Read the transcript of the teaching part
Last time
Before we start: last time, in Charge, current, e.m.f. and potential difference, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
The push and the payoff
A cell is like a pump. It gives energy to each coulomb of charge, and pushes it round the circuit. Every component then takes some of that energy back, as heat or light. The two quantities that measure this are the e.m.f. and the potential difference, and the exam loves to ask which is which.
Electromotive force
The electromotive force, e.m.f., of a source is the electrical work done by the source in moving unit charge around a complete circuit. It is the energy the source gives to each coulomb. It is measured in volts. A cell labelled 1.5 V gives 1.5 joules to every coulomb.
Potential difference
The potential difference, p.d., across a component is the work done in driving unit charge through that component. It is the energy transferred by each coulomb as it passes through the component. It is also measured in volts, using a voltmeter connected in parallel with the component.
The equation
Potential difference equals work done divided by charge. V equals W over Q. Rearranged: the energy transferred equals charge times potential difference. W equals Q V. Work in joules, charge in coulombs, potential difference in volts.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: E.m.f. and potential difference · E.m.f. and potential difference.
The whole chapter: Electrical Quantities, Electrical Quantities.
Chapter 4.2: Electrical quantities
- Charge and electric fields 5 min
- Electric current, d.c. and a.c. 5 min
- E.m.f., potential difference and cells 6 min
- Resistance and Ohm's law 5 min
Also for O Level Physics 5054
Which topics come up most in O Level Physics 5054 past papers, counted from every paper since 2004.