A cll balances against a length of 150 cm on a potentiometer wire when it is shunted by a resistance of `5 Omega`. But when it is shunted by a resistance of `10 Omega`, the balancing length increases by 25 cm. What is the balancing length when the cell is in an open circuit ?

Options

A. 200 cm
B. 210 cm
C. 225 cm
D. 250 cm

Answer

B

Explanation

Let l be the balancing length, when the cell is in an open circuit and r be the internal resistance of the cell then in the first case `r= 5((l-150)/(150))` and in the second case, balancing length
150+25 =175 cm
`therefore r = 10 ((l-175)/(175))`
`therefore 5((l-150)/(150)) = 10((l-175)/(175))`
`therefore (l-150)/(6)=(2(l-175))/(7)`
`therefore 7 l-1050= 12l -2100`
`therefore 5 l =1050 therefore l=210 cm`