A student builds a solenoid by wrapping insulated wire into a coil. When a current flows through the solenoid, a magnetic field is produced. Explain how the strength of the magnetic field produced by the solenoid could be increased, and explain why inserting an iron core into the centre of the solenoid also increases the field strength.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student notices that the solenoid they have built can only pick up a small number of steel paper clips. They want to make the solenoid more powerful without changing the battery or the wire material.
Model answer (5 marks)
1. Increase the number of turns of wire – more turns give a larger magnetic field.
2. Increase the current through the coil – a higher current produces a stronger field.
3. Make the coil longer or give it a larger cross‑sectional area – both change the field per unit length.
4. Insert an iron core – iron is a magnetic material, so the field lines are guided through it more easily than through air.
5. The iron core becomes an induced magnet, adding its own field to that of the solenoid, which increases the overall field strength.
2. Increase the current through the coil – a higher current produces a stronger field.
3. Make the coil longer or give it a larger cross‑sectional area – both change the field per unit length.
4. Insert an iron core – iron is a magnetic material, so the field lines are guided through it more easily than through air.
5. The iron core becomes an induced magnet, adding its own field to that of the solenoid, which increases the overall field strength.
Examiner tips
- Use the formula B=µ₀nI for a solenoid to justify each factor. Mention that µ increases with a magnetic core. Show the effect of each change on B directly. Keep the answer concise and use the exact terminology.
- common_mistakes
- :
- Confusing the effect of coil length with the number of turns. Forgetting that the iron core adds to the field rather than merely guiding it. Using vague terms like "more wire" without specifying turns or current.
Mark scheme (5 marks)
- Increasing the number of turns (coils) of wire increases the field strength
- Increasing the current through the solenoid increases the field strength
- Any other valid factor: increasing the length of the coil OR increasing the cross-sectional area
- Iron is a magnetic material, so inserting an iron core means the field lines pass through the core more easily than through air
- The iron core becomes an induced magnet, adding to the overall magnetic field of the solenoid
Key terms in this question
solenoid · magnetic field · iron core · current
Related
- All OCR A-Level Physics B: Advancing Physics (H557) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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