A student sets up an electromagnet by wrapping wire around an iron core and connecting it to a power supply. The student wants to make the electromagnet as strong as possible. Describe four factors the student could change to increase the strength of the magnetic field produced by the electromagnet, and for each factor state what change should be made.

OCR A-Level Physics B: Advancing Physics (H557) — 4.2 Energy, power and resistance · Describe · 5 marks · View as Markdown

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

An electromagnet consists of a solenoid with an iron core. The strength of the magnetic field it produces depends on several factors.

Model answer (5 marks)

1. Increase the magnitude of the current through the wire – use a larger supply voltage or lower resistance to raise I.
2. Increase the number of turns of wire around the core – wind more coils.
3. Use an iron core – replace an air core with iron to increase μ.
4. Increase the cross‑sectional area of the coil – use a wider coil or larger diameter wire.
5. Increase the length of the coil – extend the solenoid so more turns are distributed over a longer length.

Examiner tips

  • Use the exact wording from the mark scheme – e.g. ‘increase the magnitude of the current’, ‘increase the number of turns’, ‘use an iron core’, ‘increase the cross‑sectional area’, ‘increase the length of the coil’.
  • Show each factor as a separate point; the examiner expects five distinct items.
  • Mention the direction of change (increase) for each factor.
  • Keep the answer concise – no extra explanation beyond the required change.

Common mistakes

  • Confusing ‘increase the length of the coil’ with ‘decrease the length’; the mark scheme rewards increasing length. Failing to specify the direction of change (e.g. just saying ‘use iron core’ without noting it should be iron, not air). Listing a factor that is not in the mark scheme, such as changing wire gauge, and omitting one of the five required points.

Mark scheme (5 marks)

  1. Increase the size (magnitude) of the current flowing through the wire
  2. Increase the number of turns (coils) of wire around the core
  3. Use an iron core (rather than air or a non-magnetic material) / keep the iron core as it allows field lines to pass through more easily than air
  4. Increase the cross-sectional area of the coil / use a wider coil
  5. Increase the length of the coil (keeping other factors constant) to include more turns over a greater length

Key terms in this question

electromagnet · iron core · magnetic field

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