A rod 0.30\ m long moves at 5.0\ m s^-1 across a 0.20\ T field. Find the induced emf. [2 marks]

OCR OCR 2019-style practice: A coil of 250 turns and cross-sectional area 1.2 × 10^-3\ m^2 sits in a magnetic field that falls uniformly from 0.40\ T to zero in 0.020\ s. The field is perpendicular to the plane of the coil. Calculate the average induced emf.

Change in flux linkage: (N) = N × (BA) = 250 × (0.40 - 0)(1.2 × 10^-3). = 250 × (4.8 × 10^-4) = 0.12\ Wb turns. Faraday's law (magnitude): = ((N))/( t) = (0.12)/(0.020) = 6.0\ V. Markers reward the change in flux linkage, = ((N))/( t), and the value 6.0\ V.

OCR OCR 2022-style practice: An ideal transformer has 1200 turns on the primary and 80 turns on the secondary. The primary is connected to a 230\ V mains supply and the secondary delivers a current of 3.0\ A. Calculate the secondary voltage and the primary current.

Turns ratio: (V_s)/(V_p) = (N_s)/(N_p), so V_s = V_p × (N_s)/(N_p) = 230 × (80)/(1200) = 15.3\ V. For an ideal transformer power is conserved: V_p I_p = V_s I_s, so I_p = (V_s I_s)/(V_p) = (15.3 × 3.0)/(230) = 0.20\ A. Markers reward the turns-ratio equation giving about 15\ V, power conservation, and the primary current about 0.20\ A.

EnglandPhysicsSyllabus dot point

How does a changing magnetic field generate an electromotive force, and how do transformers work?

Electromagnetic induction: magnetic flux and flux linkage, Faraday's law of induction, Lenz's law, the emf induced in a moving conductor, and the operation of transformers.

A focused answer to the OCR H556 content on electromagnetic induction, covering magnetic flux and flux linkage, Faraday's law relating induced emf to the rate of change of flux linkage, Lenz's law and energy conservation, the emf induced in a moving conductor, and the operation of the ideal transformer.

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

Quick answer

Magnetic flux through a coil is $\Phi = BA\cos\theta$ , and flux linkage is the flux times the number of turns, $N\Phi$ , measured in weber turns. Faraday's law states that the induced emf equals the rate of change of flux linkage, $\varepsilon = \frac{\Delta(N\Phi)}{\Delta t}$ . Lenz's law gives the direction: the induced emf (and current) opposes the change producing it, which is why there is a minus sign and why induction conserves energy. A conductor of length $L$ moving at speed $v$ across a field induces $\varepsilon = BLv$ . A transformer uses a changing flux in a shared iron core to change voltage; for an ideal transformer $\frac{V_s}{V_p} = \frac{N_s}{N_p}$ and $V_p I_p = V_s I_s$ .

Jump to a section

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

What this dot point is asking

OCR wants you to define magnetic flux and flux linkage, state and use Faraday's law of induction, state Lenz's law and relate it to energy conservation, find the emf induced in a moving conductor, and explain the operation of a transformer.

The answer

Magnetic flux and flux linkage

Faraday's law and Lenz's law

Lenz's law follows from conservation of energy: if the induced current aided the change, it would create energy from nothing, so it must oppose it, which is why work must be done to move a magnet into a coil.

Emf in a moving conductor

Transformers

Examples in context

Electromagnetic induction generates almost all the world's electricity, as turbines spin coils in magnetic fields in power stations. Transformers step the voltage up for efficient long-distance transmission (reducing current and power loss) and down again for safe domestic use, which is why the grid uses alternating current. Induction underlies microphones, electric guitars, contactless charging, induction hobs and the regenerative braking that recharges electric-car batteries.

Try this

Q1. State Faraday's law of electromagnetic induction. [2 marks]

Cue. The induced emf is equal to the rate of change of flux linkage, $\varepsilon = \frac{\Delta(N\Phi)}{\Delta t}$ .

Q2. A rod $0.30\ \text{m}$ long moves at $5.0\ \text{m s}^{-1}$ across a $0.20\ \text{T}$ field. Find the induced emf. [2 marks]

Cue. $\varepsilon = BLv = (0.20)(0.30)(5.0) = 0.30\ \text{V}$ .

Q3. Explain why a transformer does not work with a direct current. [2 marks]

Cue. A steady current produces a constant flux, so there is no change in flux linkage and no induced emf in the secondary; induction requires a changing flux.

Exam-style practice questions

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

OCR 20194 marksA coil of

250

turns and cross-sectional area

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

sits in a magnetic field that falls uniformly from

0.40\ \text{T}

to zero in

0.020\ \text{s}

. The field is perpendicular to the plane of the coil. Calculate the average induced emf.

Show worked answer →

Change in flux linkage: $\Delta(N\Phi) = N \times \Delta(BA) = 250 \times (0.40 - 0)(1.2 \times 10^{-3})$ .

$= 250 \times (4.8 \times 10^{-4}) = 0.12\ \text{Wb turns}$ .

Faraday's law (magnitude): $\varepsilon = \frac{\Delta(N\Phi)}{\Delta t} = \frac{0.12}{0.020} = 6.0\ \text{V}$ .

Markers reward the change in flux linkage, $\varepsilon = \frac{\Delta(N\Phi)}{\Delta t}$ , and the value $6.0\ \text{V}$ .

OCR 20224 marksAn ideal transformer has

1200

turns on the primary and

80

turns on the secondary. The primary is connected to a

230\ \text{V}

mains supply and the secondary delivers a current of

3.0\ \text{A}

. Calculate the secondary voltage and the primary current.

Show worked answer →

Turns ratio: $\frac{V_s}{V_p} = \frac{N_s}{N_p}$ , so $V_s = V_p \times \frac{N_s}{N_p} = 230 \times \frac{80}{1200} = 15.3\ \text{V}$ .

For an ideal transformer power is conserved: $V_p I_p = V_s I_s$ , so $I_p = \frac{V_s I_s}{V_p} = \frac{15.3 \times 3.0}{230} = 0.20\ \text{A}$ .

Markers reward the turns-ratio equation giving about $15\ \text{V}$ , power conservation, and the primary current about $0.20\ \text{A}$ .

Related dot points

Sources & how we know this

OCR AS and A Level Physics A (H156, H556) specification — OCR (2015)