A student investigates the strength of a solenoid by testing how many small iron paperclips it can hold. The student wants to make the solenoid as strong as possible. Describe four factors the student could change to increase the strength of the magnetic field produced by the solenoid, and explain why adding an iron core increases the field strength.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student wraps insulated wire around a cardboard tube, connects it to a battery, and uses the solenoid to pick up small iron paperclips. The student notices that changing certain features of the solenoid affects how many paperclips it can hold.
Model answer (5 marks)
1. Increase the number of turns of wire.
2. Increase the current through the wire.
3. Decrease the length of the coil (or increase its cross‑sectional area) so that the turns are more concentrated.
4. Use an iron core.
Adding an iron core increases the field because iron is a ferromagnetic material that allows magnetic field lines to pass through it more easily than air, concentrating the field inside the solenoid. The iron core becomes magnetised by the solenoid’s field and produces its own magnetic field, which adds to the solenoid’s field, giving a stronger overall field.
2. Increase the current through the wire.
3. Decrease the length of the coil (or increase its cross‑sectional area) so that the turns are more concentrated.
4. Use an iron core.
Adding an iron core increases the field because iron is a ferromagnetic material that allows magnetic field lines to pass through it more easily than air, concentrating the field inside the solenoid. The iron core becomes magnetised by the solenoid’s field and produces its own magnetic field, which adds to the solenoid’s field, giving a stronger overall field.
Examiner tips
- Use the exact wording from the mark scheme (e.g. "increase the number of turns", "increase the current", "decrease the length of the coil"), list each factor separately, and give a brief but clear explanation for the iron core.
- Show the cause–effect relationship: more turns → higher B, higher current → higher B, shorter coil → higher B, iron core → higher B due to permeability and induced magnetisation.
Common mistakes
- Mixing up the effect of increasing cross‑sectional area (it actually reduces B for a fixed number of turns); students often say it increases B. Failing to mention that the iron core adds its own field rather than just concentrating the existing field. Using vague phrases like "more wire" or "more current" without specifying the factors explicitly.
Mark scheme (5 marks)
- Increase the number of turns (coils) of wire on the solenoid
- Increase the size (magnitude) of the current flowing through the solenoid
- Any one of: decrease the length of the coil (so turns are more concentrated) OR increase the cross-sectional area of the coil
- An iron core allows magnetic field lines to pass through it more easily than air / concentrates the field lines inside the solenoid
- The iron core becomes an induced magnet, adding its own magnetic field to that of the solenoid, increasing the overall field strength
Key terms in this question
solenoid · iron core · magnetic field
Related
- All OCR A-Level Physics B: Advancing Physics (H557) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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