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 is use ?

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20 $Ω$

40 $Ω$

200 $Ω$

400 $Ω$

answer is B.

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Detailed Solution

We know that
$P = \frac{V^{2}}{R} or R = \frac{V^{2}}{P}$
$∴ R = \frac{(200)^{2}}{100} = 400$ When in use
Now, the resistance when it is not in use
$= \frac{R}{10} = \frac{400}{10} = 40 Ω$

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