Happy dussehra...A resistance of 100cm long potentiometer wire

Question

Rasistance of 100cm long potentiometer  wire is 10ohm, it is connected to a battery 2v and a resistance R in series. A source 10mv gives null point at 40cm length, then external resistance R is   a) 490ohm b) 790ohm                                        c) 590ohm c) 990ohm

Answer 1

Here The voltage applied to potentiometric circuit is = 10V

Resistance connected in series with the potentiometer  is R

Rasistance of 100cm long potentiometer  wire is 10ohm

So current through potetiometric circuit I = 2/(R+10)

Length of potentiometer wire is 100 cm.

A source of FMF of 10mv gives null point at 40cm length,

Resistance of 40 cm wire will be r=(10/100)x 40 cm =4 ohm

Hence potential across 40cm wire will be same as the emf of the source

So rxI = 10x10^-3 V

=> 4x2/(R+10) =10^-2

=>R+10 =800

=>R = 790 ohm