# A transformer is used in the National Grid to step up the voltage from a power station before electricity is transmitted through overhead cables. Explain why a transformer only works with alternating current and not with direct current, and describe what happens inside the transformer when alternating current flows through the primary coil.

> Eduqas A-Level Physics — 4.5 Electromagnetic induction · Explain · 5 marks

> Transformers are essential components of the National Grid. They contain two coils of wire wound around an iron core.

## Mark scheme (5 marks)

1. Alternating current continuously changes direction (is always changing)
2. This produces a continuously changing magnetic field in the iron core
3. The changing magnetic field induces a voltage (e.m.f.) in the secondary coil
4. Direct current produces a constant/steady magnetic field
5. A constant magnetic field does not induce a voltage, so no output is produced with DC

## Key terms

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

## Related

- [Revision notes for Eduqas A-Level Physics](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-transformer-is-used-in-the-003a7bb9) · Published by Druglandscape Ltd.