Microphones (HT only) — AQA GCSE Physics
Test yourself on Microphones (HT only) with AQA GCSE practice questions.
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Microphones (HT only) explained
A moving-coil microphone is a loudspeaker run backwards, and the physics is the generator effect rather than the motor effect.
Read the full explanation
Sound arriving at the microphone is a series of compressions and rarefactions, so the air pressure at the diaphragm rises and falls. The diaphragm is pushed in and out by those pressure changes, and a coil attached to it moves with it, in and out of the field of a permanent magnet. The coil is therefore a conductor moving across a magnetic field, so a potential difference is induced across it, and in a complete circuit that drives a current. A louder sound moves the diaphragm further and faster, giving a larger current; a higher frequency sound makes it vibrate more often, so the current varies more rapidly.
Students should be able to explain how a moving-coil microphone works.
Build your explanation of a moving-coil microphone around the parts visible in a cross-section diagram. The diaphragm is the thin sheet facing the sound, the coil is wound on a former fixed behind it, and the permanent magnet is fixed to the body. Explain the sequence: sound waves cause the diaphragm to vibrate, making the attached coil vibrate with it. As the moving coil cuts the magnetic field lines of the fixed magnet, a potential difference is induced across the coil. This induced potential difference drives a varying current around the circuit. A microphone uses the generator effect, whereas a loudspeaker uses the motor effect to transfer energy in the opposite direction.
Your focus
- Trace the chain from pressure variations in the air to a varying current in a microphone circuit.
- Explain which effect, motor or generator, is at work in a microphone, and justify the choice.
- Relate the loudness and the pitch of a sound to the amplitude and the frequency of the induced current.
Show all 6 objectives
- Label the diaphragm, coil and magnet on a cross-section of a moving-coil microphone.
- Sequence an explanation running from the arriving sound wave to the induced current in the circuit.
- Compare a microphone with a loudspeaker in terms of which effect each uses and the direction of energy transfer.
Microphones (HT only) exam tips
Marking Points
- one mark for the pressure variations of the sound wave making the diaphragm vibrate
- one mark for the coil attached to the diaphragm moving in the field of the permanent magnet
- one mark for a potential difference being induced because the coil moves relative to the field
- one mark for the induced current varying at the same frequency as the sound wave
- one mark for a louder sound giving a larger movement and so a larger induced current
- identifying that sound waves cause the diaphragm to vibrate
- stating that the coil is attached to the diaphragm and vibrates with it
- explaining that the moving coil cuts the magnetic field lines of the fixed permanent magnet
- stating that this induces a potential difference across the ends of the coil
- explaining that the induced potential difference drives a varying alternating current in the closed circuit
- comparing with a loudspeaker by stating the microphone uses the generator effect while the loudspeaker uses the motor effect
Examiner Tips
- 💡Start your answer at the air and finish at the circuit, because the examiner follows that direction.
- 💡Name compressions and rarefactions when you describe what reaches the diaphragm.
- 💡Write induced every time you mention the potential difference here; it separates this from the loudspeaker.
- 💡Annotate the diagram first, then write one sentence for each labelled part to build a logical sequence.
- 💡Ensure your explanation links cause and effect, from the arriving sound wave to the induced current.
Common Mistakes
- explaining the motor effect, which belongs to the loudspeaker, in an answer about a microphone
- saying that the sound wave itself carries a current into the circuit
- leaving out the permanent magnet, so there is no field for the coil to move relative to
- saying a louder sound raises the frequency of the current rather than its amplitude
- having the magnet move and the coil stay still, which contradicts standard microphone diagrams
- describing the current as being produced by the magnet rather than induced in the coil
- writing a sequence of steps with no cause linking them, such as jumping from sound arriving to current flowing without mentioning the coil cutting field lines
- claiming that a microphone and a loudspeaker are built from fundamentally different components