Residence of 100 cm long potentiometer wire is `10 Omega`, it is connected to a battery (2 volt) and a resistance R in series. A source of 10 mV gives null point at 40 cm length, then external resistance R is

Options

A. `490 Omega`
B. `790 Omega`
C. `590 Omega`
D. `790 Omega`

Answer

B

Explanation

(b) `E = (e)/((R + R_h + r)) (R )/(L) xx l`
`10 xx 10^(-3) = (2)/((10 + R + 0)) xx (10)/(1) xx 0.4 implies R = 790 Omega`