Question

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

• A. $40\Omega$
• B. $20\Omega$
• C. $400\Omega$
• D. $2\Omega$

Answer 1

The correct option is A $40\Omega$

Resistance of the tungsten filament when hot, $R_0=\frac{V^2}{P}=\frac{{200}^2}{100}=400\Omega$

Given, Resistance of the tungsten filament when cold, $R_c=\frac{R_0}{10}$

Thus, $R_c=\frac{400}{10}=40\Omega$

The resistance of the tungsten filament when not in use is $40\Omega$