6.3FieldsStretch

Electromagnetic Induction

Push a magnet into a coil and, with no battery anywhere, a current appears. This reverse of the motor effect — electromagnetic induction — is the working principle behind every generator on the planet.

45 min Video by Physeeks Module 6.3 Magnetic fields

Faraday’s Law, Lenz’s Law — A Level PhysicsWatch the full walkthrough before the notes below.

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What you'll be able to do

Define magnetic flux and flux linkage
Use Faraday’s law of electromagnetic induction
State and apply Lenz’s law
Describe the operation of a simple generator

Magnetic flux and flux linkage

Magnetic flux, $Φ = B A$ , is the product of flux density and (perpendicular) area. For a coil of $N$ turns, flux linkage is $N Φ$ .

Φ = B A flux linkage = N Φ

Magnetic flux and flux linkage.

1Flux:

Φ = B A = 0.25 \times 0.0040 = 1.0 \times 1 0^{- 3}

Wb.

2Flux linkage:

N Φ = 150 \times 1.0 \times 1 0^{- 3}

Answer0.15 Wb-turns

Faraday’s law

Induced e.m.f. equals the rate of change of flux linkage — a constant flux, however large, induces nothing.

ε = - \frac{Δ ( N Φ )}{Δ t}

Faraday’s law.

1Change:

Δ (N Φ) = 0.045 - 0.015 = 0.030

ε = \frac{0.030}{0.30}

Answer

ε = 0.10

Lenz’s law and generators

Lenz’s law states the induced current opposes the change producing it — a consequence of conservation of energy. A rotating coil in a field (simple generator) produces alternating e.m.f.; use Fleming’s RIGHT-hand rule for induced current direction.

Tip — Motor effect (force from current): LEFT hand. Generator effect (current from motion): RIGHT hand.

Equation recap

Φ = B A

Magnetic flux.

ε = - \frac{Δ ( N Φ )}{Δ t}

Faraday’s law.

Common mistakes to avoid

Thinking a large constant flux induces a large e.m.f.

Only a CHANGING flux linkage induces an e.m.f. — a constant flux induces nothing.

Mixing up Fleming’s left and right hand rules.

LEFT for the motor effect (force from current); RIGHT for the generator effect (current from motion).

Key takeaways

Magnetic flux Φ=BA; flux linkage = NΦ.
Faraday’s law: ε = −Δ(NΦ)/Δt.
Lenz’s law: induced current opposes its cause (conservation of energy).
A rotating coil generator produces alternating e.m.f.; use Fleming’s right-hand rule.

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