First slide

Electrical power

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?

Easy

A

$400 Ω$
B

$200 Ω$
C

$40 Ω$
D

$20 Ω$

Solution

It is given R_Hot = 10 R_Cold. Also resistance, at rated temperature, $R = \frac{V^{2}}{P} = \frac{200 \times 200}{100} = 400 Ω$
So resistance when lamp not in use,
$R_{Cold} = \frac{R_{Hot}}{10} = \frac{400}{10} = 40 Ω$

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