A loudspeaker contains a coil of wire attached to a paper cone. The coil sits inside a permanent magnet. When an alternating current passes through the coil, the cone vibrates and produces sound. Explain why the cone moves backwards and forwards when an alternating current is used, rather than moving in only one direction.

Eduqas GCSE Physics — 8.2 Magnetic effects of currents and 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 uses the motor effect to convert electrical signals into sound. The coil of wire carrying a current sits within the magnetic field of a permanent magnet.

Model answer (4 marks)

A current‑carrying conductor in a magnetic field experiences a force (motor effect). The force is given by
F = I\,\mathbf{L}\times\mathbf{B}.
The direction of the force depends on the direction of the current. In an alternating current the current continuously reverses direction. Therefore the force on the coil, and hence on the attached cone, repeatedly reverses, causing the cone to vibrate back and forth.

Examiner tips

  • Show the motor effect equation or describe the force direction change with current reversal.
  • Explain that the alternating current causes the force to change sign, leading to vibration.
  • Use the word ‘alternating’ to justify the back‑and‑forth motion.
  • Mention the coil’s attachment to the cone so the force moves the cone.

Common mistakes

  • Forgetting that the force reverses with the current, so the cone moves in only one direction.
  • Not linking the alternating current to the changing direction of the magnetic force.
  • Using the wrong equation (e.g. F = BIL without the cross product) or omitting the magnetic field vector.

Mark scheme (4 marks)

  1. A current-carrying conductor in a magnetic field experiences a force (motor effect)
  2. The direction of the force depends on the direction of the current
  3. Alternating current continuously reverses direction
  4. So the force on the coil/cone repeatedly reverses, causing it to vibrate / move back and forth

Key terms in this question

alternating current

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