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Question
An electric bulb is rated 220 V and 100 W. When it is operated at 110 V, the power consumed will be
Options:
A .  100 W
B .  75 W
C .  50 W
D .  25 W
Answer: Option D
:
D
Power consumed by the electric bulb, P=V2R, where V is the potential difference and R is the electrical resistance.
Given for 1st case:
Voltage V1=220V
and power consumed by bulb P1=100W,
In the 2nd case:
Voltage of the bulb, V2=110V
Power consumed by the bulb = P2
Since the resistance is the same as it is the same bulb
P1P2=(V1)2(V2)2
100P2=(220)2(110)2
P2=100×(110)2(220)2=25W
Power consumed when the bulb is operated at 110V = 25W

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