Question

An electric bulb is rated '100 W, 250 V' . What is the resistance of bulb ?

• A. 2.5 ohm
• B. 625 ohm
• C. 25000 ohm
• D. 1500 ohm

Answer 1

Answer: (B)

625 ohm

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.
$SinceP=\frac{V^2}{R},R=\frac{V^2}{P}.$
For the bulb rated as 100 W - 250 V, the resistance of its filament
$R=\frac{V^2}{P}=\frac{{250}^2}{100}=625\Omega .$
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.