Question

A wire of length $100cm$ is connected to a cell of e.m.f. $2V$ and negligible internal resistance. The resistance of the wire is $3\Omega$. The additional resistance required to produce a potential drop of $1$ millivolt per cm is

• A. $60\Omega$
• B. $47\Omega$
• C. $57\Omega$
• D. $35\Omega$

Answer 1

The correct option is C $57\Omega$

Voltage drop across wire of $100cm$

$=1mv/cm$

$={10}^{-2}\times 100$

$=0.1v$

Let constant be $I$

$0.1=I\times 3=3\times \frac{2}{R+3}$

$\frac{6}{R+3}=0.1$

$6=0.1R+0.3$

$R=57\Omega$.