A student places a magnetic compass near a current-carrying wire. When the current is switched on, the compass needle deflects. Explain why the needle deflects, and describe what happens to the deflection if the current in the wire is increased.

Eduqas GCSE Physics — 2.5 Magnetism and electromagnetism · Explain · 4 marks · View as Markdown

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

A straight wire is connected to a battery and a switch. A small magnetic compass is placed a few centimetres from the wire.

Model answer (4 marks)

A current flowing through the wire creates a magnetic field that circles the wire.
The field exerts a torque on the magnetic needle, so the needle deflects.
If the current is increased, the magnetic field becomes stronger.
A stronger field produces a larger torque and the needle deflects more.

Examiner tips

  • Use the phrase ‘creates a magnetic field that circles the wire’ – the examiners look for the right description of the field. Show the link between field strength and deflection – ‘stronger field → greater deflection’. Keep the answer concise – 4 marks allow one sentence per point.
  • Use the word ‘torque’ or ‘force’ to show understanding of the interaction.
  • Mention that the needle aligns with the resultant field, not just the wire’s field.

Mark scheme (4 marks)

  1. A current flowing through a wire produces a magnetic field around the wire
  2. The magnetic field of the wire exerts a force on the compass needle / interacts with the compass needle's own magnetic field
  3. Increasing the current increases the strength of the magnetic field produced by the wire
  4. A stronger magnetic field causes a greater deflection of the compass needle

Key terms in this question

current · deflection · compass needle

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