Question: Q. 15. (i) When an $a c$ source is connected to an ideal capacitor show that the average power supplied by the source over a complete cycle is zero.
(ii) A lamp is connected in series with a capacitor. Predict your observation when the system is connected first across a $d c$ and then an $a c$ source. What happens in each case if the capacitance of the capacitor is reduced?
A&E [Delhi Comptt. I, II, III 2013]
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Solution:
Ans. (i) When an ideal capacitor is connected with $a c$ source, the current flow continuously but due to dielectric in between the plates of capacitor, there is no current, i.e.,
$$ \begin{aligned} & I_{a v g}=0 \ & \therefore \quad P_{a v g}=V \times I_{a v g} \ & P_{a v g}=0 \end{aligned} $$
(ii) For $d c$, lamp will not shine as capacitor blocks $d c$ even if we reduce the capacitance, the lamp will not shine.
$$ 1 $$
Lamp will shine, if $a c$ is used on reducing $C$, with increase of impedance.
Hence, lamp will shine less brightly.
1
