A straight current-carrying wire is placed at right angles to a uniform magnetic field and experiences a force. Describe how the direction and magnitude of this force can be changed, and state the rule used to determine the direction of the force.

Pearson Edexcel International GCSE Physics (4PH1) — 6.2 Electromagnetism · Describe · 4 marks · View as Markdown

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

A horizontal wire carries a direct current from left to right and is positioned between the poles of a magnet so that the magnetic field acts vertically downward through the wire.

Model answer (4 marks)

1. Fleming’s left‑hand rule is used to find the force direction.
2. Reversing the direction of the current (or of the magnetic field) reverses the force direction.
3. Increasing the magnitude of the current increases the magnitude of the force.
4. Using a stronger magnetic field increases the magnitude of the force.

Examiner tips

  • Use the exact wording ‘Fleming’s left‑hand rule’ – examiners look for this phrase. Show the four ways to change the force (current, field, magnitude of each) in separate points to match the 4 marks.

Common mistakes

  • Confusing Fleming’s right‑hand rule (used for motors) with the left‑hand rule for force. Failing to mention that reversing either current or field reverses the force direction.

Mark scheme (4 marks)

  1. Fleming's left-hand rule is used to determine the direction of the force on the wire.
  2. Reversing the current OR reversing the magnetic field reverses the direction of the force.
  3. Increasing the magnitude of the current increases the magnitude of the force.
  4. Using a stronger magnetic field increases the magnitude of the force.

Key terms in this question

magnetic field · current

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