A coil of 50\ turns has a flux through each turn changing at 0.020\ Wb s^-1. Find the induced emf. [2 marks]

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How does a changing magnetic flux induce an electromotive force, and what do Faraday's and Lenz's laws tell us?

Electromagnetic induction: magnetic flux and flux linkage, Faraday's law of induction, Lenz's law and energy conservation, and the operation of a simple generator and transformer.

A focused answer to the Eduqas A-Level Physics Component 3 electromagnetic induction content, covering magnetic flux and flux linkage, Faraday's law of induction, Lenz's law and its link to energy conservation, and the operation of a simple AC generator and a transformer.

Generated by Claude Opus 4.813 min answerUpdated 2026-06-02

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What this dot point is asking

Eduqas wants you to define magnetic flux and flux linkage, state and apply Faraday's law of electromagnetic induction, state Lenz's law and explain it through conservation of energy, and describe the operation of a simple AC generator and a transformer.

The answer

Magnetic flux and flux linkage

Faraday's law

Lenz's law and conservation of energy

Generators and transformers

Induced emf in a rotating coil

A coil of $120\ \text{turns}$ and area $2.5 \times 10^{-3}\ \text{m}^2$ in a field of $0.40\ \text{T}$ is rotated so that its flux linkage reverses (from $+NBA$ to $-NBA$ ) in $0.015\ \text{s}$ . Find the average induced emf.

step 1: Find the flux linkage at one extreme

$N\Phi = NBA = 120 \times 0.40 \times 2.5 \times 10^{-3} = 0.12\ \text{Wb turns}$ .

step 2: Find the total change as it reverses

The flux linkage goes from $+0.12$ to $-0.12$ , a change of magnitude $0.24\ \text{Wb turns}$ .

step 3: Apply Faraday's law

$\varepsilon = \dfrac{\Delta(N\Phi)}{\Delta t} = \dfrac{0.24}{0.015} = 16\ \text{V}$ .

Examples in context

Electromagnetic induction is the basis of almost all electricity generation: power-station generators, wind turbines and bicycle dynamos all rotate coils in fields. Transformers raise voltage for low-loss transmission across the grid and lower it again for homes. Induction also drives induction hobs, contactless phone charging, metal detectors, electric guitar pickups and regenerative braking in electric vehicles, where the motor acts as a generator to recover energy.

Try this

Q1. State Faraday's law of electromagnetic induction. [1 mark]

Cue. The induced emf is equal to the rate of change of flux linkage.

Q2. A coil of $50\ \text{turns}$ has a flux through each turn changing at $0.020\ \text{Wb s}^{-1}$ . Find the induced emf. [2 marks]

Cue. $\varepsilon = N\frac{\Delta\Phi}{\Delta t} = 50 \times 0.020 = 1.0\ \text{V}$ .

Q3. State Lenz's law and the principle behind it. [2 marks]

Cue. The induced current opposes the change producing it; this follows from conservation of energy.

Exam-style practice questions

Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Eduqas 20204 marksA coil of

250\ \text{turns}

and area

4.0 \times 10^{-3}\ \text{m}^2

sits in a magnetic field perpendicular to its plane. The flux density falls uniformly from

0.60\ \text{T}

to zero in

0.020\ \text{s}

. Calculate the average electromotive force induced.

Show worked answer →

The flux linkage changes from $N\Phi = NBA$ to zero.

Initial flux linkage: $N\Phi = 250 \times 0.60 \times 4.0 \times 10^{-3} = 0.60\ \text{Wb turns}$ .

Faraday's law: $\varepsilon = \dfrac{\Delta(N\Phi)}{\Delta t} = \dfrac{0.60 - 0}{0.020} = 30\ \text{V}$ .

Markers reward calculating the flux linkage $NBA$ , applying $\varepsilon = \frac{\Delta(N\Phi)}{\Delta t}$ , and the induced emf $30\ \text{V}$ .

Eduqas 20224 marksA bar magnet is pushed north pole first into a coil connected to a sensitive ammeter. State and explain, using Lenz's law, the direction of the induced current and how the magnet experiences a force.

Show worked answer →

By Lenz's law, the induced current opposes the change producing it. As the north pole approaches, the induced current flows so that the end of the coil facing the magnet becomes a north pole, repelling the incoming magnet.

This means work must be done to push the magnet in against the repulsion, and that work is the source of the electrical energy generated, consistent with conservation of energy. (If Lenz's law were reversed, the magnet would be attracted and accelerate, creating energy from nothing.)

Markers reward the induced current opposing the change (coil face becomes a north pole), the resulting repulsion, and linking the work done against this force to conservation of energy.

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Sources & how we know this

Eduqas GCE AS/A Level Physics specification (A720QS) — WJEC Eduqas (2015)