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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 1Ω. 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 :

11Ω

9Ω

Zero

7Ω

Solution:

Given, emf of cell E = 200 V
the internal resistance of cells r=1Ω
D.C. main supply voltage = 220 V
External resistance R = ?
r=(E-V)/V)R
1=20/20R
∴R=11Ω