Question

In order to measure the internal resistance $r_1$ of a cell of emf $E$, a meter bridge of wire resistance $R_0=50\Omega$, a resistance $R_0/2$, another cell of emf $E/2$ (internal resistance $r$ ) and a galvanometer $G$ are used in a circuit, as shown in the figure. If the null point is found at $\ell =72cm$, then the value of $r_1=$ _____ $\Omega$

Answer 1

Answer: 3

$R_{AB}=50\Omega$
So, $R_{\Lambda P}=\frac{50}{100}\times 72=36\Omega$
$I=\frac{\epsilon }{r_1+50+25}$ ...(i)
$-36I-\frac{\epsilon }{2}-I{r}_1+\epsilon =0$
Solving equation (i) and (ii)
$r_1=3\Omega$