past paper question on microphone and capacitors... Watch

Jaydude
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Finding it difficult to answer the question above;

Although the front plate can "vibrate," how does this very small change in distance cause a alternating signal?

So Q=CV, the small change in distance causes maybe the electrons to shift to the back plate making it negative???

And then for every gap in the sound waves, the back plate moves back into position, so the electrons move to the front plate making it negative now??? So the charge now flows in opposite direction, Q proportional to V, so alternating P.D? :confused:

I feel I am completely wrong, and I don't exactly know what the mark scheme is saying.

Thanks!
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Stonebridge
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When the plates move a little closer together (due to the sound pressure wave maximum) the capacitance of the plates increases slightly.
When they move apart (due to sound pressure wave minimum) the capacitance decreases slightly.

Increasing the capacitance means more charge on the capacitor. Decreasing the capacitance means less charge on the capacitor.
So as the sound wave hits the plates they move together then apart. The capacitance of the plates gets more then less. The charge on the plates flows on then off. This means there's an alternating current in the circuit as the charge flows on and off the plates. The current varies at the same frequency as the sound wave.
The pd across the resistor varies at the same frequency as the sound wave.
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Jaydude
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(Original post by Stonebridge)
When the plates move a little closer together (due to the sound pressure wave maximum) the capacitance of the plates increases slightly.
When they move apart (due to sound pressure wave minimum) the capacitance decreases slightly.

Increasing the capacitance means more charge on the capacitor. Decreasing the capacitance means less charge on the capacitor.
So as the sound wave hits the plates they move together then apart. The capacitance of the plates gets more then less. The charge on the plates flows on then off. This means there's an alternating current in the circuit as the charge flows on and off the plates. The current varies at the same frequency as the sound wave.
Very clear explanation. But why does the charge go "off?" Is it due to the regular frequency of the sound waves? So once it just fall to zero, the max wave arrives and causes charge to go back up again due to plates moving closer together.

Thanks!
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Stonebridge
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(Original post by Jaydude)
Very clear explanation. But why does the charge go "off?" Is it due to the regular frequency of the sound waves? So once it just fall to zero, the max wave arrives and causes charge to go back up again due to plates moving closer together.

Thanks!
There's more to this question than you posted here. It appeared a couple of weeks ago on here and there was discussion about it. What happens in detail depends on the time constant of the circuit. You'll find it if you do a search on a couple of key words and limit to the last month.
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Jaydude
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http://www.thestudentroom.co.uk/show...ght=capacitors

Found it, thanks.
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