AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
Microphones use the generator effect to convert the pressure variations in…
Full specification wording
Microphones (HT only): Microphones use the generator effect to convert the pressure variations in sound waves into variations in current in electrical circuits.
A moving-coil microphone is a loudspeaker run backwards, and the physics is the generator effect rather than the motor effect.
Checked against the AQA 8463 specification (opens in a new tab) on .
- Point 1 of 2
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A moving-coil microphone is a loudspeaker run backwards, and the physics is the generator effect rather than the motor effect. 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.
What earns the marks
- The pressure variations of the sound wave making the diaphragm vibrate.
- The coil attached to the diaphragm moving in the field of the permanent magnet.
- A potential difference being induced because the coil moves relative to the field.
- The induced current varying at the same frequency as the sound wave.
- A louder sound giving a larger movement and so a larger induced current.
Common mistakes and how to put them right
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.
Exam 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.
Try the lesson
The start of the Microphones (HT only): Microphones use the generator effect to convert the pressure variations in sound waves into variations in current in electrical circuits. lesson, made from checked teaching notes for this specification point.
Have a go first
A moving-coil microphone changes the pressure variations of a sound wave into which quantity in the circuit?
Listen
A moving-coil microphone is a loudspeaker run backwards, and the physics is the generator effect rather than the motor effect. 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. A strong answer earns marks for these points. the pressure variations of the sound wave making the diaphragm vibrate. the coil attached to the diaphragm moving in the field of the permanent magnet. a potential difference being induced because the coil moves relative to the field. the induced current varying at the same frequency as the sound wave. Exam tip: Start your answer at the air and finish at the circuit, because the examiner follows that direction.
Key words
Say what each word means before you turn the card.
generator effect
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moving-coil microphone
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diaphragm
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pressure variations
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See every definition
- generator effect
- The effect used by microphones to convert the pressure variations in sound waves into variations in current in electrical circuits.
- moving-coil microphone
- A device that is a loudspeaker run backwards, in which the physics is the generator effect rather than the motor effect.
- diaphragm
- The part pushed in and out by the pressure changes, with a coil attached to it that moves with it in and out of the field of a permanent magnet.
- pressure variations
- A series of compressions and rarefactions in sound arriving at the microphone, so the air pressure at the diaphragm rises and falls.
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