A technician is investigating a solenoid connected to a battery. She notices that the solenoid attracts a nearby iron nail. She then makes changes to the solenoid to increase the strength of its magnetic field. Describe four factors that could increase the strength of the magnetic field produced by the solenoid, and explain why inserting an iron core into the solenoid makes it an electromagnet.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A solenoid is a wire wrapped into the shape of a coil. When current flows through it, it produces a strong and uniform magnetic field inside, similar in shape to the field of a bar magnet.
Model answer (5 marks)
1. Increase the current through the solenoid – a larger current produces a larger magnetic field.
2. Increase the number of turns in the coil – more turns give a stronger field.
3. Increase the cross‑sectional area of the coil – a larger area increases the field strength.
4. Use a longer solenoid – a longer coil gives a stronger field.
5. Inserting an iron core makes the solenoid an electromagnet because the iron becomes magnetised in the field, adding its own field and allowing magnetic lines to pass through more easily, thereby strengthening the overall field.
2. Increase the number of turns in the coil – more turns give a stronger field.
3. Increase the cross‑sectional area of the coil – a larger area increases the field strength.
4. Use a longer solenoid – a longer coil gives a stronger field.
5. Inserting an iron core makes the solenoid an electromagnet because the iron becomes magnetised in the field, adding its own field and allowing magnetic lines to pass through more easily, thereby strengthening the overall field.
Examiner tips
- Use the exact wording from the mark scheme – e.g. "increase the current", "number of turns", "cross‑sectional area", "length of the coil". Explain the effect of the iron core in two sentences: it becomes a magnetised core and increases permeability. Keep each factor as a separate bullet to show you have covered all required points.
Common mistakes
- Mixing up the effect of core material with the effect of coil geometry. Using vague terms like "more wire" instead of "more turns". Failing to mention that the iron core itself becomes magnetised and adds to the field.
Mark scheme (5 marks)
- Any one of: increasing the size of the current through the solenoid increases the magnetic field strength
- Any second distinct factor from: increasing the number of turns/coils, increasing the length of the coil, or increasing the cross-sectional area of the coil
- Any third distinct factor from the remaining options (number of turns, length of coil, cross-sectional area, or type of core used)
- Inserting an iron core means the solenoid becomes an electromagnet / the iron core is an induced magnet that strengthens the field
- Iron is easier for magnetic field lines to pass through than air, so the magnetic field is strengthened / iron becomes magnetised in the field, adding to the overall field
Key terms in this question
solenoid · electromagnet · magnetic field · iron core
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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