Question

The coil of an electric bulb takes $40W$ to start glowing. If more then $40W$ is applied, $60\%$ of the extra power is converted into light and rest into heat. Bulb is rated $100W220V$. Find the percentage drop in light intensity at a point if supply voltage is changed from $220$ V to $200$ V.

• A. 16%
• B. 22%
• C. 28%
• D. 34%

Answer 1

Answer: (C)

28%

$P=100w,V=220$ V

Case I : Excess power $=100–40=60$w

Power converted to light $=(60\times 60)/100=36$w

Case II : Power $=\left(220\right)2/484=82.64$w

Excess power $=82.64–40=42.64$w

Power converted to light $=42.64\times \left(60/100\right)=25.584$w

$\Delta P=36–25.584=10.416$

Required $\%=\left(10.416\right)/36\times 100=28.93\approx 29\%$