Rating Of Electrical Appliances

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 J 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.


For the bulb rated as 100 W - 250 V, the resistance of its filament

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.