Question

An electric bulb is rated at `200 V-100 W`. What is its resistance ? Five such bulbs burn for four hours. What is the electrical energy consumed ? Calculate the cost if the rate is `50` paise per unit.

Answer 1

Here, pd across the bulb, `V = 200 V`
power of the bulb, `P = 100 W`
As `P = (V^2)/(R ), R = (V^2)/(P) = ((200 V)^2)/(100 W) = (4 xx 10^4)/(100) Omega = 400 Omega`
Electric energy consumed by 1 bulb in 4 hours, i.e.,
`W = P t = 100 W xx 4 h = 400 Wh`
Electric energy consumed by 5 bulbs in 4 hours
`= 5 xx 400 Wh = 2000 Wh = 2 k Wh = 2` unit
Cost of `2` units of electric energy `= 2 xx 50` paise = Re. 1.