Question

In a potentiometer experiment, the balancing length of a cell is 560 cm. When an external resistance of $10\Omega$ is connected in parallel to the cell, the balancing length chages by 60 cm. The internal resistance of a cell is

• A. $1.4\Omega$
• B. $1.6\Omega$
• C. $0.12\Omega$
• D. $1.2\Omega$

Answer 1

Answer: (D)

Consider a cell of emf E and balancing length $l_1$
$E=k{l}_1$
The potential difference is balanced by length $l_2$ .
$V=k{l}_2$
The internal resistance of the cell is given by
$r=\left(\frac{E-V}{V}\right)R=\left(\frac{E}{V}-1\right)R=\left(\frac{l_1}{l_2}-1\right)R$
$=\left(\frac{560}{560-60}-1\right)10=\left(\frac{56}{50}-1\right)10$
$=\frac{6}{5}=1.2\Omega$