Question

The resistance of hot tungeston filment is about $10times$ cold resistance. The resistance $100W$ and $200Vlamp$, when not in use is

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

Answer 1

The correct option is A $40\Omega$

$R_{hot}=\frac{V^2}{P}=\frac{200\times 200}{100}=400\Omega$

Cold resistance $=\frac{R_{hot}}{10}=\frac{400}{10}=40\Omega$