© examsnet.com
Question : 19
Total: 29
(a) 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.
(b) A lamp is connected in series with a capacitor. Predict your observations 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?
(b) A lamp is connected in series with a capacitor. Predict your observations 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?
Solution:
(a) AC source connected to a ideal capacitor:
V 0 s i n ω t =
Or, q = C V 0 s i n ω t
Or,I =
= C V 0 ω cos ω t
∴ I = I 0 cos ω t = I 0 s i n ( ω t +
)
Average Power=
[ V 0 s i n ω t × I 0 s i n ( ω t +
) ] dt
=
[
V 0 I 0 s i n 2 ω t ] dt
= 0
(b) Reactance of a capacitor= X C =
π f C
For DC,f = 0 , X C = ∝
ForAC , f ≠ 0 , X C ≠ ∝
Capacitance is reduced.
For DC,f = 0 , X C = ∝
ForAC C X C
Using KVL,
Or,
Average Power
(b) Reactance of a capacitor
For DC,
For
Capacitance is reduced.
For DC,
For
© examsnet.com
Go to Question: