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 ?
20 Ω
40 Ω
200 Ω
400 Ω
We know thatP=V2R or R=V2P∴ R=(200)2100=400 When in use Now, the resistance when it is not in use=R10=40010=40Ω