A potentiometer wire of length 1000 cm has a resistance of 10 ohms. It is connected in series with a resistance and a cell of e.m.f 2 volts and of negligible internal resistance. A source of e.m.f 10 millivolts is balanced against a length of 40 cm of the potentiometer wire. What is the value of the external resistance?

Options

A. 1.5V
B. 3.0V
C. 0.67V
D. 1.33V

Answer

2

Explanation

`because E=((V)/(L))xxlrArrE=((IR_(W))/(LL))xxl`
`rArr E=(E_(0))/(R+r+R_(W))xx(R_(W))/(L)xxl`
`E=(10)/(4+1+5)xx(5)/(3)xx3=3` volt