Question

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.

Answer 1

In the light intensity if the supply voltage changes at a point to 200 V from 220 V, then the percentage becomes 29.  

Explanation:

• Scenario 1: If the supply voltage is 220 V, then the bulb consumes power of 100 W. Then, the excess power is 100 − 40 = 60 W.
• The amount of power that is converted to light is 60% of 60W, which is 36W.  
• Scenario 2: If the supply voltage is 200 V, then the power consumed will be 25.584 W.
• So, the drop in the percentage of light intensity is p = 36 – 25.58 $\times$ 100 => p = 29.