EnglandPhysicsSyllabus dot point

How do magnetic fields exert forces and generate EMFs?

Magnetic flux density and the force on a current-carrying conductor, the force on a moving charge, magnetic flux and flux linkage, and Faraday's and Lenz's laws of electromagnetic induction.

A focused answer to the Edexcel 9PH0 magnetic fields content, covering magnetic flux density and the motor effect, the force on a moving charge, magnetic flux and flux linkage, and Faraday's and Lenz's laws of induction.

Generated by Claude Opus 4.812 min answerUpdated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

Have a quick question? Jump to the Q&A page

Jump to a section

What this dot point is asking
The answer
Examples in context
Try this

What this dot point is asking

Edexcel wants you to define magnetic flux density, calculate the force on a current-carrying conductor ( $F = BIL$ ) and on a moving charge ( $F = BQv$ ), define magnetic flux and flux linkage, and apply Faraday's and Lenz's laws to find the size and direction of induced EMFs.

The answer

Force on a current-carrying conductor

The force is maximum when the current is perpendicular to the field ( $\sin\theta = 1$ ) and zero when parallel. Fleming's left-hand rule gives the direction: thumb for force (motion), first finger for field, second finger for current. This motor effect is the basis of motors, loudspeakers and moving-coil meters.

Force on a moving charge

A moving charge is a current, so it too feels a force in a field:

Because the force is always perpendicular to the velocity, it does no work and the speed stays constant: the charge circles at constant speed. This principle underlies the cyclotron, the mass spectrometer, and the bending magnets in particle accelerators.

Magnetic flux and flux linkage

Flux is the amount of field passing through a loop. It is greatest when the loop faces the field squarely and zero when the loop plane lies along the field. Changing $B$ , the area $A$ , or the orientation all change the flux and hence can induce an EMF.

Faraday's and Lenz's laws

Lenz's law is conservation of energy: if the induced current aided the change, it would create energy from nothing. So a magnet pushed into a coil induces a current whose field repels the magnet, and you must do work to push it in, which becomes electrical energy. For a coil rotating in a field, the flux linkage varies sinusoidally and the induced EMF is a maximum when the coil is moving fastest through the field (flux changing quickest), which is when the flux itself is momentarily zero.

Charged particle in a field

A proton ( $m = 1.67 \times 10^{-27}$ kg, $Q = 1.6 \times 10^{-19}$ C) moves at $2.0 \times 10^{6}$ m per second perpendicular to a $0.40$ T field. Find the radius of its circular path.

step 1: Equate magnetic and centripetal force

$BQv = \frac{mv^2}{r}$ , so $r = \frac{mv}{BQ}$ .

step 2: Substitute

$r = \frac{1.67 \times 10^{-27} \times 2.0 \times 10^{6}}{0.40 \times 1.6 \times 10^{-19}} = \frac{3.34 \times 10^{-21}}{6.4 \times 10^{-20}}$ .

step 3: Evaluate

$r = 5.2 \times 10^{-2}$ m, about $5.2$ cm.

Examples in context

A bicycle dynamo and a power-station generator both use a coil rotating in a field; Faraday's law sets the output EMF. Induction hobs heat a pan by inducing eddy currents in its base. A transformer transfers energy between coils through changing flux linkage in an iron core. Maglev trains and metal detectors rely on induced eddy currents and Lenz's law to provide braking or detection. The mass spectrometer bends ions on circular paths whose radius reveals their mass-to-charge ratio.

Try this

Q1. State the unit of magnetic flux density and one equivalent definition. [1 mark]

Cue. The tesla; the flux density giving one newton per metre on a wire carrying one amp at right angles to the field.

Q2. A coil of $50$ turns and area $0.010$ m squared has the field through it changed from $0.20$ T to zero in $0.10$ s. Find the average induced EMF. [3 marks]

Cue. $\varepsilon = N\frac{\Delta(BA)}{\Delta t} = 50 \times \frac{0.20 \times 0.010}{0.10} = 1.0$ V.

Q3. Explain how Lenz's law is a statement of conservation of energy. [2 marks]

Cue. The induced current opposes the change causing it, so work must be done against it; if it aided the change, energy would be created from nothing.

Exam-style practice questions

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

Edexcel 20183 marksA wire of length

0.25

m carries a current of

4.0

A at right angles to a magnetic field of flux density

0.30

T. Calculate the force on the wire.

Show worked answer →

The motor-effect force on a conductor perpendicular to the field is $F = BIL$ .

$F = 0.30 \times 4.0 \times 0.25 = 0.30$ N.

Markers reward $F = BIL$ , the perpendicular condition (full $\sin 90 = 1$ ), and the value $0.30$ N.

Edexcel 20215 marksA coil of

200

turns and area

4.0 \times 10^{-3}

m squared sits in a field that falls uniformly from

0.50

T to zero in

0.020

s. Calculate the average EMF induced and explain the direction of the induced current using Lenz's law.

Show worked answer →

Faraday's law: induced EMF $\varepsilon = -N\frac{\Delta\Phi}{\Delta t}$ , where flux linkage is $N\Phi = NBA$ .

Change in flux linkage: $\Delta(N\Phi) = N A \Delta B = 200 \times 4.0 \times 10^{-3} \times 0.50 = 0.40$ Wb turns.

$\varepsilon = \frac{0.40}{0.020} = 20$ V (magnitude).

Lenz's law: the induced current flows in the direction that opposes the decrease in flux, so it acts to maintain the original field through the coil.

Markers reward $N\Phi = NBA$ , the rate of change, the value $20$ V, and the Lenz's law reasoning.

Related dot points

Sources & how we know this

Pearson Edexcel A-Level Physics (9PH0) specification — Pearson Edexcel (2015)