A loudspeaker contains a coil of wire placed inside a magnetic field. When a current flows through the coil, the coil experiences a force and moves. Explain how reversing the direction of the current affects the force on the coil, and state two factors that would increase the size of this force.

AQA GCSE Physics (8463) — 4.7.2 The motor effect · Explain · 4 marks · View as Markdown

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

A loudspeaker works by passing a varying electrical current through a coil of wire situated inside a magnetic field. The interaction between the two magnetic fields causes the coil to move, pushing a cone back and forth to produce sound.

Model answer (4 marks)

Reversing the current reverses the direction of the force on the coil, because the force depends on the direction of the current (Fleming’s Left‑Hand Rule). Two factors that would increase the size of this force are: (1) increasing the magnitude of the current through the coil; (2) increasing the strength of the magnetic field in which the coil is placed.

Examiner tips

  • Use Fleming’s Left‑Hand Rule to explain the reversal of force; mention current direction explicitly.
  • State the two factors (current and magnetic field strength) in separate bullet points or sentences.

Mark scheme (4 marks)

  1. Reversing the current reverses the direction of the force on the coil
  2. This is because the direction of the force depends on the direction of the current (Fleming's Left-Hand Rule)
  3. Increasing the size of the current increases the force
  4. Increasing the strength of the magnetic field increases the force

Key terms in this question

magnetic field · current

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