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Question

An electric bulb is rated '100 W, 250 V' . What is the resistance of bulb ?
  1. 2.5 ohm
  2. 25000 ohm
  3. 1500 ohm
  4. 625 ohm

A
2.5 ohm
B
25000 ohm
C
625 ohm
D
1500 ohm
Solution
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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=V2R,R=V2P.
For the bulb rated as 100 W - 250 V, the resistance of its filament
R=V2P=2502100=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.

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