Question

a torch bulb when cold has 1 ohm resistance. it draws a current of 0.3A when glowing from a source of 3V . calculate the resistance of bulb when glowing and explain the reason for the difference in resistance​

Answer 1

Answer:

Hence, resistance of the bulb when hot = 9 ohm

Explanation :

Given,

Current drawn by torch, I = 0.3 A

Potential Difference of torch, V = 3 V

Resistance of the bulb when cool, R' = 1 ohm

Then, let the total resistance of the bulb be R. Then,

We know that,

Resistance, R = Potential Difference, V/Current, I

R = V/I = 3 / 0.3 = 10 ohm

Then, resistance of the bulb when hot, R" = Total resistance - Resistance of the bulb when cool = R - R' = 10 - 1 = 9 ohm

Hence, resistance of the bulb when hot = 9 ohm

Answer 2

Given that, a torch bulb when cold has 1 ohm resistance. It draws a current of 0.3A when glowing from a source of 3V .

From the above data we have, I = 0.3A and V = 3V

Ohm's law: The potential difference i.e. V across the ends of the wire is directly proportional to the square of current i.e. I flowing through that wire provides the temperature remains constant.

V ∝ I

V = IR (here, R is constant)

From ohm's law we can say that,

V = IR (where R is resistance)

Substitute the known in the above formula,

→ 3 = 0.3R

→ 3/0.3 = R

→ 10 = R

Therefore, the resistance of the bulb is 10 ohm when it glows. But also given that, the resistance of bulb is 1 ohm when it is cold.

Difference in resistance = Resistance of bulb when it is (glows - cold)

= (10 - 1) ohm = 9 ohm