Kirchhoff's laws and cells in parallel
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The teaching part (1 min) is free to watch. A free account opens the whole lecture: 1 worked example, 3 practice questions, 5 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
- Two rules that run every circuit
- The first law
- The second law
Read the transcript of the teaching part
Last time
Before we start: last time, in Internal resistance in networks, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
Two rules that run every circuit
Whatever circuit you meet, two ideas decide how current and voltage behave. What flows into a junction must flow out. And around any loop, the energy given by the sources equals the energy used up. Gustav Kirchhoff wrote them down in 1845.
The first law
Kirchhoff's first law: the sum of the currents entering a junction equals the sum of the currents leaving it. It follows from the conservation of charge. Charge cannot pile up at a junction. For three wires, with three amperes arriving and two leaving along one wire, the other wire carries one ampere out.
The second law
Kirchhoff's second law: around any closed loop, the sum of the e.m.f.s equals the sum of the potential differences. It follows from the conservation of energy. A charge returning to its starting point has gained as much as it has lost. Take care with signs: an e.m.f. is positive if you go through the source from negative to positive.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Kirchhoff's laws · Kirchhoff's second law.
The whole chapter: D.C. Circuits, D.C. Circuits.
Chapter 10.2: Kirchhoff's laws
- Kirchhoff's laws and cells in parallel 7 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.