# Explain why alternating current (AC) must be used in the primary coil of a transformer, and describe how a step-up transformer differs from a step-down transformer in both its structure and its effect on voltage.

> Edexcel A-Level Physics (9PH0) — 13.1 Simple harmonic motion and damping · Explain · 5 marks

> Transformers are used throughout the National Grid to change the magnitude of an alternating voltage. They consist of an iron core with a primary coil and a secondary coil wound around opposite ends.

## Mark scheme (5 marks)

1. AC in the primary coil creates a changing magnetic field (in the iron core)
2. The changing magnetic field cuts through / links with the secondary coil, inducing a current (or potential difference) in it
3. DC would produce a constant (not changing) magnetic field, so no current/pd would be induced in the secondary coil
4. A step-up transformer has more turns on the secondary coil than the primary coil, resulting in a larger (higher) potential difference in the secondary coil
5. A step-down transformer has fewer turns on the secondary coil than the primary coil, resulting in a smaller (lower) potential difference in the secondary coil

## Key terms

- [alternating current (AC)](https://www.gradenine.co.uk/glossary/alternating-current-ac)
- [transformer](https://www.gradenine.co.uk/glossary/transformer)
- [step-up transformer](https://www.gradenine.co.uk/glossary/step-up-transformer)
- [step-down transformer](https://www.gradenine.co.uk/glossary/step-down-transformer)
- [primary coil](https://www.gradenine.co.uk/glossary/primary-coil)

## Related

- [Revision notes for Edexcel A-Level Physics (9PH0)](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)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-alternating-current-ac-must-852aa060) · Published by Druglandscape Ltd.