Electromagnetic induction
Cambridge A-Level Physics (9702) · Unit 20: Magnetic fields · 7 flashcards
Electromagnetic induction is topic 20.5 in the Cambridge A-Level Physics (9702) syllabus , positioned in Unit 20 — Magnetic fields , alongside Concept of a magnetic field, Force on a current-carrying conductor and Force on a moving charge. In one line: Magnetic flux (Φ) is the measure of the quantity of magnetism, being the number of magnetic field lines passing through a surface. It's calculated as Φ = BA, where B is the magnetic flux density and A is the area perpendicular to the field.
Marked as A2 Level: examined at A Level in Paper 4 (A Level Structured Questions) and Paper 5 (Planning, Analysis and Evaluation). It is not tested on the AS-only papers (Papers 1, 2 and 3).
The deck below contains 7 flashcards — 3 definitions, 3 key concepts and 1 calculation — covering the precise wording mark schemes reward. Use the 3 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
Magnetic flux (Φ) and provide its formula
Magnetic flux (Φ) is the measure of the quantity of magnetism, being the number of magnetic field lines passing through a surface. It's calculated as Φ = BA, where B is the magnetic flux density and A is the area perpendicular to the field.
What the Cambridge 9702 syllabus says
2025-2027 syllabus, in our words · A2 LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- Define magnetic flux: the magnetic flux density multiplied by the cross-sectional area at right angles to the direction of that flux density.
- Recall Φ = BA and use it.
- Understand what magnetic flux linkage means and use the idea.
- Understand and explain experiments demonstrating three things: an e.m.f. is induced in a circuit when the magnetic flux through it changes; the induced e.m.f. acts to oppose the change that produces it; and the factors that determine its size.
- Recall Faraday's law and Lenz's law of electromagnetic induction and use them.
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9702 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in Electromagnetic induction
- › Always follow the logical sequence: changing flux linkage induces an e.m.f., which then causes a current if the circuit is complete.
- › Explain that the induced current creates a magnetic field that interacts to produce an opposing force.
- › Induced e.m.f. equals the gradient of the flux linkage-time graph, so it is largest where the graph is steepest.
Define magnetic flux (Φ) and provide its formula.
Magnetic flux (Φ) is the measure of the quantity of magnetism, being the number of magnetic field lines passing through a surface. It's calculated as Φ = BA, where B is the magnetic flux density and A is the area perpendicular to the field.
What is magnetic flux linkage and how does it relate to magnetic flux?
Magnetic flux linkage (NΦ) is the product of the number of turns (N) in a coil and the magnetic flux (Φ) through the coil. It represents the total magnetic flux interacting with all the turns of the coil. NΦ = NBA
Describe an experiment to demonstrate that a changing magnetic flux can induce an e.m.f. in a circuit.
Move a magnet in and out of a coil connected to a galvanometer. The galvanometer deflects, indicating an induced e.m.f., only when the magnet is moving, showing that a changing magnetic flux induces an e.m.f.
Explain Lenz's Law and its significance in electromagnetic induction.
Lenz's Law states that the direction of the induced e.m.f. is such that it opposes the change producing it. This opposition is a consequence of energy conservation; the induced current creates a magnetic field that resists the change in the original magnetic flux.
State Faraday's Law of electromagnetic induction.
Faraday's Law states that the magnitude of the induced e.m.f. in a circuit is proportional to the rate of change of magnetic flux linkage through the circuit. Mathematically, e.m.f. = - d(NΦ)/dt.
List three factors that affect the magnitude of the induced e.m.f.
The magnitude of the induced e.m.f. is affected by: 1) the number of turns in the coil (N), 2) the strength of the magnetic field (B), and 3) the speed of the relative motion between the coil and the magnetic field, thereby changing the rate of change of flux linkage (d(NΦ)/dt).
A coil with 500 turns is placed in a magnetic field. The magnetic flux through the coil changes from 0.02 Wb to 0.05 Wb in 0.5 seconds. Calculate the average induced e.m.f. in the coil.
Using Faraday's Law: e.m.f. = -N * (dΦ/dt) = -500 * ((0.05 - 0.02) / 0.5) = -30 V. The negative sign indicates the direction of the e.m.f. opposes the change in flux.
Review the material
Read full revision notes on Electromagnetic induction — definitions, equations, common mistakes, and exam tips.
Read NotesMore topics in Unit 20 — Magnetic fields
Electromagnetic induction sits alongside these A-Level Physics decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Electromagnetic induction deck
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