Question

The resistance of a hot tungsten filament is about $10$ times the cold resistance. What will be the resistance of $100W,$$200V$ lamp when not in use?

• A. $20\Omega$
• B. $40\Omega$
• C. $200\Omega$
• D. $400\Omega$

Answer 1

The correct option is B $40\Omega$

When the lamp is in use, it is hot.

Using Joule's Law of heating,

$P=\frac{V^2}{R}$

$R=\frac{{200}^2}{100}=400\Omega$ when operating

It is given resistance of hot lamp is 10 times the cold lamp.

Hence, $R_{cold}=\frac{400}{10}=40\Omega$