Q. The resistance of hot tungsten filament is about 10 times the cold resistance. What will be the resistance of 100 W and 200 lamps, when not in use?
A) $40\ \Omega$
B) $20\ \Omega$
C) $400\ \Omega$
D) $200\ \Omega$
Explanation
(a) $\ \ \ \ \ \ \ \ \ P = \frac{V^{2}}{R}$
$\therefore R_{hot} = \frac{V^{2}}{P}$
$\ \ \ \ \ \ \ \ \ \ \ \ \ = \frac{200 \times 200}{1000} = 400\ \Omega$
$R_{cold} = \frac{400}{10} = 40\ \Omega$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.