In my physics textbook there is an example of using capacitor switches in comput

Question

In my physics textbook there is an example of using capacitor switches in computer keyboard:

Pressing the key pushes two capacitor plates closer together, increasing their capacitance. A larger capacitor can hold more charge, so a momentary current carries charge from the battery (or power supply) to the capacitor. This current is sensed, and the keystroke is then recorded.

That makes perfect sense, and is kind of neat. What I am curious about, is what happens to that extra charge afterwards. Is there some sort of discharge mechanism? I suppose, that would be also necessary to differentiate between single keystrokes and continuous depression (register stroke current, then register the discharge). What would happen to the capacitor if there was no such discharge mechanism, but its capacitance was suddenly reduced?

If capacitance is reduced, and the charge stays the same, then, according to Q=C?VC, the difference of potentials on plates of capacitor should increase and exceed that of a power supply thus reversing the current. Is that what is happening, and the keystrokes are recorded by sensing not only the existence of the current, but also its direction?

Explanation / Answer

The capacitance is proportional to Ad the ratio of the plate area to the distance. When d is decreased, the capacitance rises and the voltage would fall. If the capacitor is connected to a fixed voltage, it will draw current to restore the voltage. Then when d is increased again, it feeds that current back into the power supply, again keeping the voltage constant.

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