Question

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 :

• A. 7 $\Omega$
• B. 9 $\Omega$
• C. 11 $\Omega$
• D. Zero

Answer 1

Answer: (C)

11 $\Omega$

Answer 2

$\left(20-I_{1}\right)R=200$
$R=\frac{200}{\left(20-I_{1}\right)}$
${R->}$Minimum
when ${20-I1->}$ maximum & $I_1$ cannot be zero
so $R\simeq11\,\Omega$