Question

What is the resistance of a 240 V, 60 W electric bulb under typical operating conditions? If two such bulbs are connected in series over a 240 V mains supply, why does one bulb appear to be less bright than the other?

Answer 1

Given:

Voltage, V = 240 V

Power, P = 60 W

1. The resistance of the bulb is given by:

R = `V^2/P`

= `(240^2)/60`

= `57600/60`

= 960 Ω

Therefore, the resistance of the bulb is 960 Ω​.

2. If two identical 240 V, 60 W bulbs are connected in series across a 240 V mains supply, the supply voltage is shared equally between them. Each bulb receives only about 120 V instead of its rated 240 V.

Since the power consumed by each bulb decreases greatly at the lower voltage, both bulbs glow dimly. As the bulbs are identical, they have the same resistance and the same current flows through both, so neither bulb is less bright than the other. Both glow with equal brightness.