A student is building an electromagnet for a school project. They wind a coil of wire around an iron core and connect it to a power supply. Explain how the strength of the magnetic field produced by the electromagnet can be increased, and why using an iron core rather than leaving the centre empty makes the electromagnet stronger.

OCR A-Level Physics A (H556) — 4.4 Waves · Explain · 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. When current flows through the coil, a magnetic field is generated.

Model answer (5 marks)

Increasing the current through the coil increases the magnetic field, because B∝I. Increasing the number of turns of wire (N) also increases B, since B∝N. Using an iron core makes the field stronger because the core becomes an induced magnet – the domains in the iron align with the field, producing a large magnetising field that adds to the field of the coil. The induced magnetism disappears when the current is switched off, which is why a soft‑iron core (not a hard steel core) is used.

Examiner tips

  • Show the proportional relationships B∝I and B∝N; include the core effect; explain domain alignment; mention loss of magnetism when current stops

Common mistakes

  • Failing to mention that the core must be soft‑iron so the magnetism is lost when current stops
  • Confusing the core material with steel or ignoring the domain alignment explanation

Mark scheme (5 marks)

  1. Increasing the current increases the strength of the magnetic field
  2. Increasing the number of turns/coils on the solenoid increases the strength of the magnetic field
  3. The iron core increases the strength of the magnetic field
  4. The iron core becomes an induced magnet / the domains in the iron align with the field
  5. The induced magnetism in the iron core is lost when the current is switched off, which is why iron (not steel) is used

Key terms in this question

electromagnet · magnetic field · iron core

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