Question

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

Answer 1

The correct option is C $40\Omega$

Given: The resistance of hot tungsten filament is about 10 times the cold resistance.

To find the resistance of $100W$ and $200V$ lamp when not in use

Solution:

The resistance of the hot filament is found by following formula,

$R_{hot}=\frac{V^2}{P}⟹R_{hot}+\frac{(200{)}^2}{100}=400\Omega$

But according to the given criteria,

$R_{hot}=10\times R_{cold}⟹R_{cold}=\frac{R_{hot}}{10}⟹R_{cold}=\frac{400}{10}=40\Omega$

Hence, the resistance of $100W$ and $200V$ lamp when not in use = $40\Omega$