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30. Find the resistance of the filament of a 100 W-220 V
bulb when glowing. The length and radius of the
filament are 0.1 m and 2.5 x 10 m. Find the specific
resistance of the material of the filament.
Ans. 484 12,9.5 x 10-62-m.
Ω-m.
Answers
Answer:
Generally electrical appliances such as an electric bulb, geyser, heater, etc. are rated with power and voltage. If a bulb is rated 100W - 250V, it means that when a bulb is lit on a 250V supply, it consumes 100 W of electric power. This means that, 100 Joules of electrical energy gets converted into heat and light energy in 1 second.
From this rating we can calculate the resistance of the filament of the appliance while it is glowing and also the current which flows through it.
SinceP=RV2,R=PV2.
For the bulb rated as 100 W - 250 V, the resistance of its filament
R=PV2=1002502=625Ω.
The resistance of the filament of a bulb is much less than this value when it is not lit because the resistance of a filament increases with the increasing temperature