NEET-XII-Physics
33: Thermal and Chemical Effects of Electric Current
- #11The coil of an electric bulb takes 40 watts to start glowing. If more than 40 W are supplied, 60% of the extra power is converted into light and the remaining into heat. The bulb consumes 100 W at 220 V. Find the percentage drop in the light intensity at a point if the supply voltage changes from 220 V to 200 V.Ans : Case-I
: When the supply voltage is 220 V.
Power consumed by the bulb = 100 W
Excess power = 100 - 40 = 60 W
Power converted to light = 60% of 60 W = 36 W
Case-II
: When the supply voltage is 200 V.
Power consumed = `` \frac{200}{220}\times 100`` = 82.64 W
Excess power = 82.64 - 40 = 42.64 W
Power converted to light = 60% of 42.64 W = 25.584 W
Percentage drop in light intensity,
`` p=\frac{36-25.584}{36}\times 100``
`` \Rightarrow p=28.93\approx 29\%``
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