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Question
An electric bulb is marked 100 W – 200 V, what information does it convey? How many joules of energy are consumed by this bulb in two hours? How long will it take for the bulb to consume two kilowatt-hours?
Numerical
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Solution
100 W means, it will consume energy at a rate of 100 J per second.
200 V means, it will operate with optimum efficiency, when the p.d. at its ends is 200 V.
(a) Energy consumed by the bulb in two hours = P × t
\[ =100 \frac{\mathrm{~J}}{\mathrm{~s}} \times 7200 \mathrm{~s}\]
= 720000 J
(b) Energy = Power × time
2kWh = 100 W × time
2000 Wh = 100 W × time
∴ Time = 20 h
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