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?
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a
400 Ω
b
200 Ω
c
40 Ω
d
20 Ω
answer is C.
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Detailed Solution
It is given RHot = 10 RCold. Also resistance, at rated temperature, R=V2P =200 × 200100 = 400 ΩSo resistance when lamp not in use,RCold = RHot10 = 40010=40 Ω