Explain how a loudspeaker uses variations in current to produce sound waves.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A loudspeaker is connected to an audio amplifier. As music plays, the current supplied to the loudspeaker continually changes in size and direction.
Model answer (4 marks)
A loudspeaker contains a coil of wire that is placed in a magnetic field. When the audio amplifier supplies a varying electric current through the coil, the current produces a magnetic field that interacts with the permanent field, giving a force on the coil (Lorentz force). As the current changes in magnitude and direction, the force on the coil changes accordingly, causing the coil to move back and forth. The coil is attached to a diaphragm; the coil’s motion drives the diaphragm to vibrate. The diaphragm’s vibration compresses and rarefies the surrounding air, creating sound waves that we hear as music.
Examiner tips
- Use the term ‘Lorentz force’ or ‘force on the coil in a magnetic field’ to show understanding of the physics.
- Explain that the current’s change in size and direction causes the force to change, leading to back‑and‑forth motion of the coil.
- Mention that the coil is attached to the diaphragm, so its motion makes the diaphragm vibrate.
- Show the chain: current → force → coil motion → diaphragm vibration → sound waves.
Common mistakes
- Forgetting to mention the magnetic field or the coil’s attachment to the diaphragm.
- Using vague phrases like ‘the speaker makes sound’ without linking current to force and motion.
- Mixing up the direction of current with the direction of sound waves.
Mark scheme (4 marks)
- The varying current flows through a coil (in a magnetic field), producing a force on the coil
- As the current varies (in size and direction), the force on the coil varies, causing the coil to move back and forth
- The moving coil causes the diaphragm to vibrate
- The vibrating diaphragm produces corresponding sound waves (in the surrounding air)
Key terms in this question
loudspeaker · current · sound waves
Related
- All WJEC GCSE Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →