Read Variable Capacitors: Turn Motion into Current
With a battery attached, the voltage is fixed. A vibrating plate changes capacitance, so the circuit current comes from the changing charge Q = CV.
Electricity questions depend on identifying charge, a conducting path and the condition that controls current or magnetic effect.
Start with the condition
For small vibration amplitude, keep only the first-order change in C.
Build the governing relation
The current is largest where the capacitance changes fastest, not where the plate displacement is largest.
Finish with a physical check
Check the limiting case before selecting an option: remove the special feature in the diagram or make the relevant parameter very large or very small. The result should reduce to a familiar relation.
OlympIQ exam lens
- ✓State the physical condition before choosing a formula.
- ✓Keep direction, sign and constraint information explicit.
- ✓Test one limiting case before committing to an option.
Using i = C dV/dt gives zero here because the battery holds V constant; it misses the V dC/dt term.
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