Edexcel IAL Physics revision · A2 — Fields
Electromagnetic induction
The IGCSE hand-waving becomes two laws with names: Faraday's, which sets the size of the induced e.m.f., and Lenz's, which sets its direction — and turns out to be conservation of energy wearing a magnetic disguise.
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What the syllabus demands
- —Define magnetic flux Φ = BA and flux linkage NΦ; the weber
- —State and use Faraday's law: induced e.m.f. = rate of change of flux linkage
- —State and use Lenz's law; relate it to conservation of energy
- —Explain induction experiments: moving magnets, rotating coils, changing fields
Definitions that earn marks
Clear definitions to practise — check your course mark scheme
- Magnetic flux (Φ)
- The product of the flux density and the area at right angles to the field: Φ = BA. Units: webers (Wb).
- Flux linkage
- The product of the number of turns and the flux through the coil: NΦ.
- Faraday's law
- The induced e.m.f. is equal to the rate of change of magnetic flux linkage: E = −d(NΦ)/dt.
- Lenz's law
- The direction of the induced e.m.f. (and current) is such as to oppose the change producing it.
The equations
More equations to practise: the Edexcel IAL formula sheet.
Where the marks die
Common mistakes to check
- 01
Saying the e.m.f. is proportional to the flux. It is proportional to the RATE OF CHANGE of flux linkage — a huge steady flux induces nothing. The word 'rate' is the mark.
- 02
Lenz's law answers without the energy argument: the induced current opposes the change, so work must be done against the opposition — that work is the source of the electrical energy. Without opposition, energy would appear from nowhere.
- 03
Forgetting the N. For a coil, it is flux LINKAGE (NΦ) that matters; dropping the number of turns loses a factor of several hundred.
- 04
Flux at an angle: Φ = BA cos θ, where θ is measured between B and the NORMAL to the coil — not the plane of the coil. Half of all flux errors are this angle.
One worked example, done properly
Question
A 500-turn coil of area 4.0 × 10⁻³ m² sits in a field of 0.20 T perpendicular to its plane. The field falls to zero in 0.10 s. Find the average induced e.m.f.
Method
- 1.Initial flux linkage = NΦ = 500 × 0.20 × 4.0 × 10⁻³ = 0.40 Wb-turns.
- 2.E = Δ(NΦ) ÷ Δt = 0.40 ÷ 0.10.
E = 4.0 V