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 ............. $\Omega $
AIEEE 2005, Medium
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It is given ${R_{Hot}} = 10\,{R_{Cold}}$ also resistance at rated temperature
$R = \frac{{{V^2}}}{P} = \frac{{200 \times 200}}{{100}} = 400\,\Omega $.
So resistance when lamp not in use.
${R_{Cold}} = {\rm{ }}\frac{{{R_{Hot}}}}{{10}} = \frac{{400}}{{10}} = 40\,\Omega $
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