Question

The balancing length for a cell is 560 cm in a potentiometer experiment. When an external resistance of 10 Ω is connected in parallel to the cell, the balancing length changes by 60cm. If the internal resistance of the cell is (N/10) Ω, where N is an integer then value of N is____ .

Answer 1

Let the emf of cell is ε internal resistance is 'r' and potential gradient is x.

Only cell connected :

ε = 560 x ...(i)

After connecting the resistor

((ε x 10)/(10 + r)) = 500 x ...(ii) 

from (1) and (2)

56 = 50 + 5r

r = 6/5 = 1.2 Ω

n = 12

Answer 2

r = R((x - x')/x') 

= 10 × (60/500)

= 12