Question

A torch bulb when cold has `1 Omega` resistance. It draws a current of `0.3 A` when glowing from a source of `3 V`. Calculate the resistance of the bulb when glowing and explain the reason for the difference in resistance.

Answer 1

Here, potential difference, `V = 3` volt
current through the bulb when glowing, `I = 0.3 A`
resistance of the bulb when glowing, `R = (V)/(I) = (3 V)/(0.3 A) = 10 Omega`
The resistance of the filament of the bulb increase from `1 Omega` (when cold) to `10 Omega` (when it becomes hot and glows) on account of an increase in its temperature.