Question

In a potentiometer experiment, the balancing length with a cell in the secondary circuit is found to be 480 cm. When a resistor of 8 ohm is connected in parallel to the cell, the balancing length is found to be 420cm. The internal resistance of the cell is :

• A. $1.14\Omega$
• B. $2\Omega$
• C. $4.12\Omega$
• D. $56\Omega$

Answer 1

The correct option is A $1.14\Omega$

Initially

$E_1=I\times r\times l_1[l_1=480cm]$

Case 2

$V=I\times r\times l_2[l_2=420cm]$

formula

$r=[\frac{E_1}{V}-1]R$

$\Rightarrow r=[\frac{l_1}{l_2}-1]\times 8$

$\Rightarrow r=[\frac{8}{7}-1]\times 8$

$r=8/7$

$r=1.14\Omega$