A d.c. main supply of e.m.f. 220 V is connected across a storage… | JEE Main 2014 (9 Apr, Shift 1)

Q40JEE Main 2014 · 9 APR · SHIFT 1Medium

Cells: EMF, Internal Resistance & Combinations

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:

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Answer: (c)

Battery (200 V) and $R$ are in parallel, fed by 220 V through $1\ \Omega$ .
At the threshold, battery current is zero and its terminal voltage is 200 V: supply current $= \frac{220 - 200}{1} = 20\text{ A}$ , all through $R$ .
Charging needs $\frac{200}{R} < 20 \Rightarrow R > 10\ \Omega$
The smallest option above $10\ \Omega$ is $11\ \Omega$ . So the answer is (c).

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