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

Answer 1

Answer: (A)

$P=\frac{V^{2}}{R}$
$\therefore R_{hot}=\frac{V^{2}}{P}$
$=\frac{200\times200}{100}$
$=400\,\Omega$
$R_{cold}=\frac{400}{10}$
$=40\,\Omega$