Describe the action of a transformer.

An alternating current in the primary coil produces an alternating magnetic flux in the core, and that changing flux links the secondary coil and induces an alternating e.m.f. in it.

What is the purpose of the soft-iron core in a transformer?

Iron has a very high permeability, so it carries almost all the flux produced by the primary round to the secondary and greatly increases the flux linkage.

A transformer with 1150 primary turns is used on a 230 V supply. Calculate the number of secondary turns needed for an output of 12 V.

N₂ = N₁V₂/V₁ = 1150 × 12 ÷ 230 = 60 turns.

Write the equation relating the currents and turns of an ideal transformer.

For an ideal transformer I₂/I₁ = N₁/N₂, so the coil with fewer turns carries the larger current.

OCR A-Level Physics B H557

Planning

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Topic overview Particle accelerators and high-energy beams Scattering and the nuclear atom Paths of scattered particles and closest approach Discrete energy levels in atoms Electron standing waves and the confined electron Quarks, hadrons and leptons Conservation laws and nuclear equations Charged particles in detectors and magnetic fields Relativistic energy and the relativistic factor

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Topic overview The nature of ionising radiations Effects on living tissue and radiation safety Absorbed dose, effective dose and risk Binding energy and nuclear stability The Nuclear Valley and modes of decay Fission, fusion and nuclear power Activity, half-life and the decay constant Investigating absorption and half-life

Electromagnetism

The transformer

3 things to know
Components 01, 02 and 03

Overview

Recall questions on how a transformer works and why it needs alternating current, the soft-iron core, V₁/V₂ = N₁/N₂ and I₂/I₁ = N₁/N₂ with calculations, ideal transformers and power balance, step-up and step-down turns, high-voltage transmission and cable losses, a direct supply, changing the primary turns, and the primary current with no load.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Why must a transformer be supplied with an alternating current?

Key term

Write the equation relating the voltages and turns of an ideal transformer.

Numbers

An ideal transformer has 1150 primary turns and 60 secondary turns, and the secondary supplies 5.0 A. Calculate the primary current.

Questions from this subtopic

Each one with the model answer underneath.

1
Describe the action of a transformer.
Model answer
An alternating current in the primary coil produces an alternating magnetic flux in the core, and that changing flux links the secondary coil and induces an alternating e.m.f. in it.
2
Why must a transformer be supplied with an alternating current?
Model answer
Only a changing flux induces an e.m.f., so the primary current must keep changing to keep the flux linkage of the secondary changing.
3
What is the purpose of the soft-iron core in a transformer?
Model answer
Iron has a very high permeability, so it carries almost all the flux produced by the primary round to the secondary and greatly increases the flux linkage.
4
Write the equation relating the voltages and turns of an ideal transformer.
Model answer
For an ideal transformer V₁/V₂ = N₁/N₂, where subscript 1 refers to the primary and subscript 2 to the secondary.
5
A transformer with 1150 primary turns is used on a 230 V supply. Calculate the number of secondary turns needed for an output of 12 V.
Model answer
N₂ = N₁V₂/V₁ = 1150 × 12 ÷ 230 = 60 turns.
6
Write the equation relating the currents and turns of an ideal transformer.
Model answer
For an ideal transformer I₂/I₁ = N₁/N₂, so the coil with fewer turns carries the larger current.

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