Question

A torch bulb has a resistance of 1Ω when cold. It draws a current of 0.2 A from a source of 2 V and glows. Calculate
a)the resistance of the bulb when glowing and
b)explain the reason for the difference in resistance

Answer 1

I) When the bulb glows:
By Ohm's law V = IR
V = 2 volt, I = 0.2 amp
R = V/I = 2 / 0.2 = 10 ohm

(ii) Resistance of the filament of the bulb increases with increase in temperature. Hence when it glows its resistances is greater than when it is cold.

Answer 2

Hey!

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Resistance when the filament is cold = 1Ω

When the current flows through the filament of bulb, it gets heated and that is because of Joule's law of heating.

That is,

H = I²RT

That is,

Heat produced is directly proportional to

• Square of the current flowing
• Resistance of the metal wire
• Time for which the current flows

So, when the bulb is glowing, resistance will increase because of heat produced in wire

By Ohm's law,

V = IR

Given,
I = 0.2 A
V = 2 Volt
R = ?

V = IR
R = V/I
R = 2 / 0.2
R = 20/2
R = 10 ohm

Thus, the resistance now becomes 10 times.

(ii) As I already told, Resistance increases due to the heat produced in the wire.

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Hope it helps...!!!