Q. The resistance of hot tungsten filament is about 10 times the cold resistance. What will be the resistance of 100 W and 200 V lamp, when not in use?
A) 40$\Omega$
B) 20$\Omega$
C) 400$\Omega$
D) 20$\Omega$
Explanation
(a) $P = \frac{V^{2}}{R}$
$\therefore R_{hot} =$ $\frac{V^{2}}{R}$
$= \frac{200 \times 200}{100}$
$= 400\Omega$
$R_{cold} = \frac{400}{100} = 40\Omega$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.