Q 12-03-295JEE MainJEE Main 2018 (8 Apr)Easy
In a potentiometer experiment, it is found that no current passes through the galvanometer when the terminals of the cell are connected across $52$ cm of the potentiometer wire. If the cell is shunted by a resistance of $5\ \Omega$, a balance is found when the cell is connected across $40$ cm of the wire. Find the internal resistance of the cell.
Answer: (C) $1.5\ \Omega$
$E\propto l_1$ and the terminal voltage with the shunt $V\propto l_2$. Since $\dfrac EV = \dfrac{R + r}{R}$:
$$r = R\left(\frac{l_1}{l_2} - 1\right) = 5\left(\frac{52}{40} - 1\right) = 1.5\ \Omega$$
Solution by Sreeraj P, M.Sc Physics