A security system uses a solenoid to lock a door. When current flows through the solenoid, it produces a magnetic field that holds an iron bolt in place. A technician suggests that increasing the number of turns in the solenoid coil and inserting an iron core will make the lock stronger. Explain why these two changes each increase the strength of the magnetic field produced by the solenoid.

OCR A-Level Physics B: Advancing Physics (H557) — 4.4 Waves · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

A solenoid is used in a door-locking mechanism. The solenoid must produce a strong enough magnetic field to hold an iron bolt firmly in place. The technician proposes two modifications: increasing the number of turns in the coil, and inserting an iron core into the centre of the solenoid.

Model answer (5 marks)

Increasing the number of turns increases the magnetic field because the field from each turn adds together; the total field inside the solenoid is proportional to the number of turns per unit length (B∝nI).\n\nInserting an iron core increases the field because iron has a high magnetic permeability; the core becomes an induced magnet and allows the magnetic field lines to concentrate, effectively increasing the field strength inside the solenoid.\n\nA stronger magnetic field exerts a greater magnetic force on the iron bolt, so the lock becomes more secure.

Examiner tips

  • Use the formula B=μ₀nI to justify the effect of more turns; mention μ for iron core; link stronger B to greater force on bolt.
  • Show the chain: more turns → higher B → greater force; core → higher μ → higher B → greater force.

Common mistakes

  • Saying the field is stronger because the current is higher – the current is unchanged.
  • Confusing the magnetic field with the magnetic flux density; not mentioning permeability of iron.
  • Assuming the core reduces the field instead of increasing it.

Mark scheme (5 marks)

  1. Increasing the number of turns increases the strength of the magnetic field
  2. Each turn contributes to the overall field / the fields from each turn align and add together
  3. Inserting an iron core increases the strength of the magnetic field
  4. The iron core becomes an induced magnet / it is easier for magnetic field lines to pass through iron than air
  5. A stronger magnetic field exerts a greater force on the iron bolt, making the lock more secure

Key terms in this question

solenoid · magnetic field · iron core · number of turns

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