Question

The balancing length of a potentiometer is at 120 cm. On shunting the cell with a resistance of 4 ohms, the balancing point shifts to a length of 60 cm. Then, find the internal resistance of the cell.

(a) 2 ohms

(b) 5 ohms

(c) 3 ohms

(d) 4 ohms

I got this question during an interview.

My query is from Current Electricity topic in portion Current Electricity of Physics – Class 12

Answer 1

Correct choice is (d) 4 ohms

To elaborate: l1   = 120 cm; l2 = 60 cm; R = 4 ohms

Internal resistance (r) = \([ \frac {(l2 – l1)}{I_2} ]\) × R

= \([\frac {(120 – 60)}{60} ]\) × 4

= \(\frac {60}{60}\) × 4

= 4 ohms

Therefore, the internal resistance is 4 ohms.