Question

A torch bulb when cold has 1 ohm resistance it draws a current of 0.3 ampere when blowing from source of three volt calculate the resistance of the bulb when glowing and explain the reason for the difference in resistance

Answer 1

given:

R(when cold)=1ohm

I=0.3 A

V=3V

solution:

By ohm's law,

V=IR

=> R=V/I

R=3/0.3

R(when hot)=10 ohm

Therefore, the resistance of torch bulb when current is passed through it is 10 ohm.

There is a difference in resistance because resistance depends on temperature. It is directly proportional to temperature( i.e. When the temperature increases the resistance also increases ).

Answer 2

Given,

Voltage = 3volts, Current = 0.3 Amperes, Default resistance= 1 Ohm.

To find,

Resistance after the bulb glows.

Solution,

Voltage =Resistance × Current,

⇒ 3= Resistance × 0.3

⇒ Resistance = $\frac{3}{0.3}$,

⇒ Resistance= 10 ohms.

The resistance changes because as the current starts flowing through the bulb, the filament gets heated up. As the temperature of the filament increases, the resistance also increases.