Q14 of 26 Page 1

(i) When an AC source is connected to an ideal capacitor, show that the average power supplied by the source over a complete cycle is zero.

(ii) A bulb is connected in series with a variable capacitor and an A.C. source as shown. What happens to the brightness of the bulb when the key is plugged in and capacitance of the capacitor is gradually reduced?



(i) The instantaneous power supplied to the capacitor is




∴ because average of sin2ωt is zero over the cycle.


(ii) As the capacitance is gradually reduced, so the capacitive reactance XC will increase. (∵)


∴ the overall resistance will increase leading to the decrease in current. So, the brightness of the bulb reduces.


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