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

$\begin {array} {1 1} (A)\;40\Omega & \quad (B)\;20\Omega \\ (C)\;400\Omega & \quad (D)\;200\Omega \end {array}$

$R_{hot} = \large\frac{V^2}{P}$ = $\large\frac{200 \times 200}{100} $$=400 \Omega Cold resistance = \large\frac{R_{hot}}{10} =\large\frac{400}{10}$$ = 40\Omega$
Ans : (A)

edited Sep 10, 2014