E.m.f., internal resistance and V-I graphs
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The teaching part (1 min) is free to watch. A free account opens the whole lecture: 3 practice questions, 4 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
- Why headlights dim at start-up
- E.m.f.
- Internal resistance
Read the transcript of the teaching part
Last time
Before we start: last time, in Resistivity of wires and LDR circuits, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
Why headlights dim at start-up
When a car engine is started, the headlights dim. The starter motor draws a huge current from the battery. A real battery has resistance of its own inside it, so a large current wastes part of its voltage inside the battery itself.
E.m.f.
The electromotive force, e.m.f., of a source is the energy transferred from other forms to electrical energy per unit charge, as the charge goes round the complete circuit. It is measured in volts. It is a property of the source, and does not depend on the current drawn. The p.d. across a component is the opposite way round: energy transferred from electrical to other forms per unit charge.
Internal resistance
Charge flowing through the source also flows through its internal resistance r, wasting energy as heat inside. So the p.d. across the external circuit is less than the e.m.f. The 'lost volts' are I r. E equals V plus I r, where V is the terminal p.d. across the external resistor. With external resistance R, V equals I R, so E equals I times R plus r.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Internal resistance.
The whole chapter: D.C. Circuits, D.C. Circuits.
Chapter 10.1: Cells
- E.m.f., internal resistance and V-I graphs 6 min
Also for A Level Physics 9702
Which topics come up most in A Level Physics 9702 past papers, counted from every paper since 2004: Paper 1 topic frequency, Paper 2 topic frequency.