A student wraps a wire into a solenoid and connects it to a variable power supply. She places a soft iron core inside the solenoid to create an electromagnet. Explain how changing the design of this electromagnet can increase the strength of its magnetic field. Give four different ways.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
1. Increase the current through the coil – a higher I gives a stronger B.
2. Add more turns of wire – more N gives a stronger B.
3. Use a soft‑iron core instead of air – the core is magnetised by induction and adds to the field.
4. Reduce the length of the solenoid (or increase the turn density) – a shorter l or higher N/l gives a stronger B.
5. Increase the cross‑sectional area of the core – a larger A allows a larger magnetic flux and a stronger field.
2. Add more turns of wire – more N gives a stronger B.
3. Use a soft‑iron core instead of air – the core is magnetised by induction and adds to the field.
4. Reduce the length of the solenoid (or increase the turn density) – a shorter l or higher N/l gives a stronger B.
5. Increase the cross‑sectional area of the core – a larger A allows a larger magnetic flux and a stronger field.
Examiner tips
- Use the formula B=μ₀NI/l to justify each change; mention μ for iron. Show the effect of each variable clearly. Keep the answer concise and use correct terminology (current, turns, core, length, area).
Common mistakes
- Confusing current with voltage. Forgetting that μ changes when a core is added. Assuming increasing area always increases field strength without noting the effect on flux density.
Mark scheme (5 marks)
- Increasing the current increases the magnetic field strength
- Increasing the number of turns / coils of wire increases the magnetic field strength
- Using a soft iron core (rather than air) increases the magnetic field strength because field lines pass through iron more easily than air / iron is magnetised by induction and adds to the field
- Decreasing the length of the solenoid (while keeping the number of turns the same) increases field strength / increasing the density of turns per unit length increases the field
- Increasing the cross-sectional area of the solenoid increases the strength / size of the magnetic field produced
Key terms in this question
solenoid · electromagnet · soft iron core
Related
- All Edexcel A-Level Physics (9PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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