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
• E. 6 V

Answer 1

Answer: (B)

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 $