Question

A small bulb has a resistance of 2ohm when cold. It takes up a current of 0.4 A from

a source of 4V and then starts glowing. Calculate (i) the resistance of bulb when it is glowing

and (ii) Elaborate on the reason for the difference in resistance?​

Answer 1

(i) Resistance of the bulb when it is glowing = 4/0.4 ohm = 10 ohm

ii) As the bulb starts glowing, the temperature of the bulb will increase (as electric energy is getting converted to light energy which generates heat) and with increase in temperature, resistance of an electric bulb increases. Similarly, in this case the resistance of the bulb increases from 2ohm to 4ohm. [Remember : Initially, the bulb's temperature was nornal and it was cool]