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

The correct option is C $11\Omega$

Given,

emf of cell E = 200 V

the internal resistance of cells r= $1\Omega$

D.C. main supply voltage = 220 V

External resistance R = ?

$r=\left(\frac{E-V}{V}\right)R$

$1=\frac{20}{220}R$

$\therefore R=11\Omega$