# A transformer is used to change the voltage of an alternating current supply. Explain why a transformer does not work with a direct current supply.

> Cambridge International IGCSE Physics (0625) — 4.5 Electromagnetic effects · Explain · 4 marks

> Transformers are widely used in the national grid to step up or step down voltages. They consist of two coils of wire wound around a soft iron core.

## Mark scheme (4 marks)

1. A direct current produces a magnetic field that does not change / is constant / is steady
2. A changing magnetic field is required to induce a voltage / e.m.f. in the secondary coil
3. Alternating current continuously changes direction, so the magnetic field continuously changes
4. Therefore no voltage / e.m.f. is induced in the secondary coil with d.c., so no current flows in the secondary circuit

## Key terms

- [alternating current](https://www.gradenine.co.uk/glossary/alternating-current)
- [direct current](https://www.gradenine.co.uk/glossary/direct-current)

## Related

- [Revision notes for Cambridge International IGCSE Physics (0625)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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