# An engineer designs a junkyard crane that uses an electromagnet to pick up scrap iron. The electromagnet is a solenoid with an iron core. The engineer wants to make the electromagnet as strong as possible. Describe four factors that affect the strength of the magnetic field produced by the electromagnet, and explain why replacing the iron core with an air core would make the electromagnet weaker.

> OCR A-Level Physics A (H556) — 4.2 Energy, power and resistance · Describe · 5 marks

> An electromagnet is a solenoid with an iron core. The strength of its magnetic field depends on several factors including the current, the number of turns, and the core material.

## Mark scheme (5 marks)

1. Any one from: increasing the current increases the field strength / increasing the number of turns increases the field strength / increasing the length of the coil increases the field strength / increasing the cross-sectional area increases the field strength
2. A second distinct factor from the list above that also increases field strength
3. A third distinct factor from the list above that also increases field strength
4. A fourth distinct factor — the type of core used affects field strength / iron core gives a stronger field than an air core
5. Explanation that it is easier for magnetic field lines to pass through iron than air, so removing the iron core reduces the strength of the field / iron is magnetised (becomes an induced magnet) and adds to the overall magnetic field, so removing it weakens the field

## Key terms

- [electromagnet](https://www.gradenine.co.uk/glossary/electromagnet)
- [solenoid](https://www.gradenine.co.uk/glossary/solenoid)
- [iron core](https://www.gradenine.co.uk/glossary/iron-core)
- [magnetic field](https://www.gradenine.co.uk/glossary/magnetic-field)

## Related

- [Revision notes for OCR A-Level Physics A (H556)](https://www.gradenine.co.uk/learn)
- [How to answer "Describe" 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-designs-a-junkyard-crane-359cd953) · Published by Druglandscape Ltd.