An electric bulb is marked `250 W - 200 V`. What information does it convey ? How many Joules of energy is consumed by this bulb in one hour ? How long will it take for the bulb to consume `1 kWh` ?

Answer

900000 J, 4 h

Explanation

A `250 W - 200 V` bulb consumes an electric power of `250 W` when working on `200 V` and the resistance of such a bulb is `(200 V)^2//250 W = 160 Omega (as P = V^2//R or R = V^2//P)`.
`W = Pt = (250 W)(60 xx 60 s) = 900000 J`
As `W = Pt, t = (W)/(P) = (1 kWh)/(250 W) = (1000 Wh)/(250 W) = 4 h`.