# An engineer is designing a transformer for use in the National Grid. The transformer has a primary coil and a secondary coil wound around an iron core. Explain why the transformer only works with alternating current in the primary coil, and describe how increasing the number of turns on the secondary coil compared to the primary coil affects the output potential difference.

> OCR A-Level Physics B: Advancing Physics (H557) — 4.5 Quantum physics · Explain · 5 marks

> Electricity is transmitted across the National Grid at very high voltages to reduce energy loss. Transformers are used to change the magnitude of the alternating voltage at different stages of transmission.

## Mark scheme (5 marks)

1. Alternating current in the primary coil produces a changing magnetic field
2. The changing magnetic field cuts through / links with the secondary coil
3. A direct current would produce a constant magnetic field so nothing would be induced in the secondary coil
4. A potential difference (and alternating current) is induced in the secondary coil
5. Having more turns on the secondary coil than the primary coil produces a step-up transformer, increasing the output potential difference

## Key terms

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

## Related

- [Revision notes for OCR A-Level Physics B: Advancing Physics (H557)](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/an-engineer-is-designing-a-transformer-fa7bae6b) · Published by Druglandscape Ltd.