In a meter bridge experiment, null point is obtained at `20 cm` from one end of the wire when resistance `X` is balanced against another resistance `Y`. If `X lt Y`, then the new position of the null point from the same end, if one decides to balance a resistance of `4 X` against `Y` will be at.

Options

A. `50 cm`
B. `80 cm`
C. `40 cm`
D. `70 cm`

Answer

A

Explanation

(a) In balancing condition, `(R_(1))/(R_(2)) = (l_(1))/(l_(2)) = (l_(1))/(100 - l_(1))`
`(X)/(Y) = (20)/(80) = (1)/(4)`
and `(4 X)/(Y) = (20)/(40) = (1)/(4)`
`implies (4)/(4) = (l)/(100 - 1) implies l = 50 cm`