A loudspeaker is connected to the output of an audio amplifier. Explain how the loudspeaker produces sound waves from the electrical signal it receives.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A loudspeaker contains a fixed permanent magnet and a coil of wire attached to a cone. When an alternating current from an amplifier passes through the coil, sound is produced.
Model answer (5 marks)
The alternating current from the amplifier flows through the coil that is in the magnetic field of the permanent magnet. The current produces a magnetic field that interacts with the magnet’s field, giving the coil a force. Because the current alternates, the force changes direction, so the coil (and the attached cone) vibrate back and forth. The moving cone compresses and rarefies the surrounding air, creating pressure variations that propagate as sound waves.
Examiner tips
- Use the word ‘alternating’ to show the current changes direction.
- Explain the force on the coil as a result of the magnetic field interaction.
- Show the chain: current → force → vibration → pressure wave → sound.
- Mention the cone as the element that moves the air.
Common mistakes
- Failing to state that the current is alternating.
- Confusing the coil’s motion with the magnet’s motion.
- Omitting the role of the cone in moving the air.
Mark scheme (5 marks)
- The alternating current flows through the coil (in the magnetic field of the permanent magnet)
- The magnetic field from the current interacts with the magnetic field of the permanent magnet
- This causes the coil to move / experience a force
- Because the current is alternating, the coil (and cone) vibrate / move back and forth repeatedly
- The vibrating cone creates pressure variations in the surrounding air, producing sound waves
Key terms in this question
Related
- All Edexcel A-Level Physics (9PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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