The magnetic flux through each turn of a 250-turn coil decreases uniformly from 4.8 x 10−4 Wb to 1.2 x 10−4 Wb in 0.015 s. Calculate the magnitude of the induced emf.
[3 marks]
Total for this question: 3
All equation deep-dives
Electromagnetic induction
Magnetic fields
AQA 7408
In the data booklet
ORIGINAL
epsilon = N d(Phi) / d(t)
Faraday's law uses the rate of change of flux linkage, not just the magnetic flux.
Board and spec code confirmed against AQA 7408 · registry checked 2026-07-11How this checking works
Know the equation
| Symbol | Quantity | Unit |
|---|---|---|
| epsilon | magnitude of induced emf | V |
| N | number of turns | no unit |
| Phi | magnetic flux through one turn | Wb |
| t | time | s |
| B | magnetic flux density | T |
| A | area normal to the field | m2 |
Rearrangements
- d(Phi) = epsilon d(t) / N
- d(t) = N d(Phi) / epsilon
- N = epsilon d(t) / d(Phi)
- Phi = B A cos(theta)
Apply it — mark your own working
Work each one out on paper first, then reveal the mark scheme and tick the marks you actually earned. That is exactly how you should mark past papers.
Q1
Do the calculation on paper first — then mark it.
Q2
An 80-turn coil has area 3.0 x 10−3 m2. Its plane is perpendicular to a uniform magnetic field that rises from 0.20 T to 0.85 T in 0.050 s. Calculate the magnitude of the induced emf.
[4 marks]
Total for this question: 4
Do the calculation on paper first — then mark it.
Where the marks get lost
- Forgetting the factor N: Faraday's law uses flux linkage N Phi.
- Using the final flux rather than the change in flux.
- Using sin(theta) in Phi = B A cos(theta); theta is measured between B and the normal to the coil.
Exam tip: If the question asks for direction or polarity, use Lenz's law separately: the induced effect opposes the change that produced it.
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