• # question_answer 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                                      [AIEEE 2005] A)            $400\,\Omega$                B)            $200\,\Omega$ C)            $40\text{ }\Omega$       D)            $20\,\Omega$

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. ${{R}_{Cold}}=\text{ }\frac{{{R}_{Hot}}}{10}=\frac{400}{10}=40\,\Omega$