Question 197 of 380

A d.c. main supply of e.m.f. $220V$ is connected across a storage battery of e.m.f. $200V$ through a resistance of $1\Omega$. The battery terminals are connected to external resistance $R$. The minimum value of $R$, so that a current passes through the battery to charge it is:

A$9\Omega$B$7\Omega$C$11\Omega$D$0$

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Answer: (C) $11\Omega$

Given,
emf of cell E = 200 V
Resistance = $1\Omega$
D. C. main supply voltage V = 220V
External resistance R = ?
$r=\left(\dfrac{E-V}{V}\right)R$
$1=\left(\dfrac{20}{220}\right)\times R$
$∴R=11\Omega$