A $d.c.$ main supply of $e.m.f.\, 220\, V$ is connected across a storage battery of $e.m.f.\, 200\, V$ through a resistance of $1\,\Omega $. The battery terminals are connected to an external resistance $'R'$ . The minimum value of $'R'$, so that a current passes through the battery to charge it is ............... $\Omega$
JEE MAIN 2014, Diffcult
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Given, $emf$ of cell $\mathrm{E}=200 \,\mathrm{V}$
Internal resistance of cells $=1 \,\Omega$
D. C main supply voltage $V=220\, \mathrm{V}$
External resistance $\mathrm{R}=?$
$r=\left(\frac{E-V}{V}\right) R$
$\mathrm{1}=\left(\frac{20}{220}\right) \times \mathrm{R}$
$\mathrm{R}=11 \,\Omega$
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