The length of a potentiometer wire is $1200\; \mathrm{cm}$ and it carries a current of $60 \;\mathrm{mA}$. For a cell of $emf\;5\; \mathrm{V}$ and intemal resistance of $20\; \Omega,$ the null point on it is found to be a $1000\; \mathrm{cm} .$ The resistance of whole wire is .............. $\Omega$
JEE MAIN 2020, Medium
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$5=\lambda \ell$
where $\lambda$ is potential gradient and $L$ is total length of wire.
$5=\frac{\Delta V}{L} \ell$
$\Delta \mathrm{V}=\frac{5 \times \mathrm{L}}{\ell}=5 \times \frac{12}{10}=6 \mathrm{V}=60 \;\mathrm{mA} \times \mathrm{R}$
$\mathrm{R}=100 \;\Omega$
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