In a potentiometer experiment, the balancing point with a cell is at a length 240 cm. on shunting the cell with a resistance of `2 Omega ` the balancing length becomes 120 cm. The internal resistance of the cell is

Options

A. `1 Omega`
B. `4 Omega`
C. `0.5 Omega`
D. `2 Omega`

Answer

D

Explanation

(d): E = `kl_(1)` ...(i)
`E-ir=E-(Er)/(2+r) =kl_(2) rArr E = kl_(2) [(2+r)/(2)]` ...(ii)
From eqn. (i) and (ii), `(2+r)/(2) = (l_(1))/(l_(2)) = (240)/(120)`
`rArr r= 2Omega`