Question

A potentiometer wire of length $10\, m$ and resistance $20\,\Omega$ is connected in series with a $15\, V$ battery and an external resistance $40\,\Omega$ . A secondary cell of emf in the secondary circuit is balanced by $240 \,cm$ long potentiometer wire. The emf $E$ of the cell is

• A. 2.4 V
• B. 1.2 V
• C. 2.0 V
• D. 3 V

Answer 1

Answer: (B)

1.2 V

Answer 2

Total resistance or $R=20+40$
$R=60\Omega .$
Given: $V=15$ volt current $I=\frac{V}{R}=\frac{15}{60}$
$I=0.25\text{ }A$
Potential gradient $=\frac{V}{l}$
$\frac{20\times 0.25}{10}$
$=0.5V{{m}^{-1}}$
Potential difference across 240 cm
$E=0.5\times 2.4$ $E=1.2\,V$