A student sets up an electromagnet by wrapping wire around an iron core and connecting it to a power supply. The student wants to use the electromagnet to pick up as many steel paperclips as possible. Explain how the student could increase the strength of the magnetic field produced by the electromagnet, and why the paperclips are attracted to the electromagnet.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
An electromagnet is made from a solenoid with an iron core. The solenoid is connected to a variable power supply so the current can be changed.
Model answer (5 marks)
1. Increase the current through the solenoid – a higher I gives a stronger field.
2. Add more turns of wire – more N gives a stronger field.
3. Use an iron core – iron has high permeability, so the field is concentrated.
4. The iron core also reduces the reluctance, allowing field lines to pass easily.
5. The paperclips become induced magnets in the field, producing an attractive force.
2. Add more turns of wire – more N gives a stronger field.
3. Use an iron core – iron has high permeability, so the field is concentrated.
4. The iron core also reduces the reluctance, allowing field lines to pass easily.
5. The paperclips become induced magnets in the field, producing an attractive force.
Examiner tips
- Use the exact phrases "increase the current", "increase the number of turns", "iron core", "induced magnetism", "attractive force".
- Show the logical link: stronger field → more attraction.
- Keep each point brief – 1 mark each.
- Remember to mention both current and turns as separate ways to increase B.
Common mistakes
- Writing "more wire" instead of "more turns". Forgetting to mention the iron core’s role in increasing permeability. Saying the paperclips are attracted because they are permanent magnets rather than induced magnets.
Mark scheme (5 marks)
- Increasing the current increases the strength of the magnetic field
- Increasing the number of turns/coils of wire increases the strength of the magnetic field
- The iron core increases the strength of the magnetic field / it is easier for field lines to pass through iron than air
- The paperclips are induced magnets / they become magnetised when placed in the magnetic field of the electromagnet
- Induced magnetism always produces an attractive force / the paperclips are attracted to the electromagnet
Key terms in this question
electromagnet · magnetic field
Related
- All OCR A-Level Physics A (H556) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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