As shown in the figure, a potentiometer wire of resistance $20\,\Omega$ and length $300\,cm$ is connected with resistance box (R.B.) and a standard cell of emf $4\,V$. For a resistance ' $R$ ' of resistance box introduced into the circuit, the null point for a cell of $20\,mV$ is found to be $60\,cm$. The value of ' $R$ ' is $.....\Omega$
JEE MAIN 2022, Medium
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$E =\frac{ AC }{ AB }\left( V _{ A }- V _{ B }\right)$
$\therefore 20 \times 10^{-3}=\frac{60}{300} \times \frac{4 \times 20}{ R +20}$
$\therefore R =780\,\Lambda$
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