Question

A bulb is rated 100 W – 250 V.

1. What information does the marking on the bulb convey?
2. What is the magnitude of the current drawn by the bulb when connected to 250 V mains?
3. What is the magnitude of the resistance of the bulb?
4. It is found that the actual resistance of the bulb is far more than the calculated value in (c). Give a reason.
5. If on the average bulb is used for 4 hours a day find the electric energy consumed by the bulb in 30 days in kWh.

Answer 1

(a) The marking means that the bulb is designed to work at 250 V and, when connected to a 250 V supply, it consumes 100 W of electrical power, i.e. 100 J of energy per second.

(b) `I = P/V`

`= (100 W)/(250 V)`

= 0.4 A

(c) R = `V/I`

`= (250 V)/(0.4 A)`

= 625 Ω

(d) The resistance of the tungsten filament increases with temperature. When the bulb is glowing, the filament is at a very high temperature, so its actual resistance is much higher than its resistance at ordinary temperature. This is why the actual resistance differs from the calculated value.

(e) Total time = `4 "hours"/"day" × 30  "days"` = 120 hours.

Energy = P × t

= 0.1 kW × 120 h

= 12 kWh