# The 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. - Physics

Sum

The 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.

#### Solution

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,

$p = \frac{36 - 25 . 584}{36} \times 100$

$\Rightarrow p = 28 . 93 \approx 29 %$

Concept: Electrical Energy and Power
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#### APPEARS IN

HC Verma Class 11, Class 12 Concepts of Physics Vol. 2
Chapter 11 Thermal and Chemical Effects of Current
Q 11 | Page 219