Question

A cell can be balanced against 140 cm and 120 cm of uniform potentiometer wire, with and without being short circuited through a resistance of $7\Omega$ respectively. Find the internal resistance of the cell

• A. $\frac{6}{7}\Omega$
• B. $\frac{7}{6}\Omega$
• C. $\frac{7}{5}\Omega$
• D. $\frac{5}{7}\Omega$

Answer 1

The correct option is B $\frac{7}{6}\Omega$

We know internal resistance can be given as
$r=R\{\frac{l_1}{l_2}-1\}=7\{\frac{140}{120}-1\}=7\{\frac{7}{6}-1\}=\frac{7}{6}\Omega$