Question

The resistance of hot tungsten filament is about $10$ times the cold resistance. The resistance of $100W$ and $200V$ lamp, when not in use is:

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

Answer 1

Answer: (A)

$40\Omega$

Let the resistance be $R$.
$\Rightarrow P=\frac{V^2}{R}$
$\Rightarrow R=\frac{V^2}{P}=\frac{4\times {10}^4}{100}=400\Omega$
$R_{cold}=\frac{R_{hot}}{10}=\frac{400}{10}=40\Omega$