A solenoid is used as an electromagnet to pick up iron components on a production line. An engineer wants to make the electromagnet stronger. Explain three factors the engineer could change to increase the strength of the magnetic field produced by the solenoid, and for each factor state how it should be changed.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A solenoid consists of a coil of wire wound around an iron core. When current flows through the wire, a strong magnetic field is produced inside the coil.
Model answer (5 marks)
1. Increase the number of turns of wire on the solenoid – more turns give a larger net magnetic field.
2. Increase the current flowing through the wire – a higher current produces a stronger field.
3. Use an iron core instead of an air core – iron concentrates the field lines, raising the field strength.
Explanation:
- More turns mean the magnetic fields produced by each loop add together, giving a larger overall field.
- A greater current increases the magnetic field around each loop (B∝I), so the combined field is stronger.
- Iron has a high magnetic permeability, so it guides the field lines and effectively increases the field inside the solenoid.
2. Increase the current flowing through the wire – a higher current produces a stronger field.
3. Use an iron core instead of an air core – iron concentrates the field lines, raising the field strength.
Explanation:
- More turns mean the magnetic fields produced by each loop add together, giving a larger overall field.
- A greater current increases the magnetic field around each loop (B∝I), so the combined field is stronger.
- Iron has a high magnetic permeability, so it guides the field lines and effectively increases the field inside the solenoid.
Examiner tips
- Use the exact terms 'number of turns', 'current', 'iron core'; explain at least two factors; keep answer concise and to the point.
- Mention the proportional relationships (B∝NI, B∝I) where relevant to show understanding.
Common mistakes
- Writing 'more wire' instead of 'more turns'; confusing current with voltage; not explaining why iron is better than air.
Mark scheme (5 marks)
- Increase the number of turns (coils) of wire on the solenoid — more turns means a stronger magnetic field
- Increase the current flowing through the wire — a greater current produces a stronger magnetic field
- Use an iron core (instead of air) — iron is easier for magnetic field lines to pass through than air, increasing field strength
- Any correct fourth factor clearly stated with correct directional change: increase the cross-sectional area of the solenoid OR increase the length of the coil — award 1 mark for a correct additional factor with correct change stated
- A clear and accurate explanation is given for at least two of the factors chosen — e.g. more turns causes the individual magnetic fields around each loop to combine/align, producing a larger overall field; or greater current produces a stronger field because the magnetic field strength around a wire depends on the magnitude of the current
Key terms in this question
solenoid · 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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