A loudspeaker contains a coil attached to a cone placed inside a magnetic field. Explain how the loudspeaker produces sound when an alternating current is supplied to the coil.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A loudspeaker is connected to an audio amplifier. The amplifier supplies an alternating current at a frequency of 440 Hz to the coil of the loudspeaker.
Model answer (4 marks)
An alternating current flows through the coil, producing a magnetic field that interacts with the permanent magnet’s field (motor effect). The Lorentz force on the coil is F=IL×B, so the coil is pushed in one direction. When the current reverses, the force reverses, pulling the coil back. The coil therefore oscillates, carrying the attached cone with it. The cone’s rapid back‑and‑forth motion creates pressure waves in the air – sound – at the same frequency as the current, 440 Hz.
Examiner tips
- Show the motor effect and the force equation,
- Explain the reversal of force with current direction,
- Link coil motion to cone vibration,
- State the sound frequency equals the current frequency.
Common mistakes
- Forgetting that the force reverses with the current,
- Not mentioning the cone’s role in producing sound,
- Using the wrong force formula or omitting the magnetic field term.
Mark scheme (4 marks)
- The alternating current causes a force on the coil (due to the motor effect / the interaction of the current with the magnetic field)
- As the current alternates (changes direction), the force on the coil reverses direction repeatedly
- The coil is pushed away and pulled back (oscillates / vibrates), causing the attached cone to vibrate
- The vibrating cone produces sound waves at the same frequency as the alternating current (440 Hz produces a 440 Hz sound)
Key terms in this question
loudspeaker · alternating current · magnetic field
Related
- All Pearson Edexcel International GCSE Physics (4PH1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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