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A 220 V d.c. source is joined to a storage battery of emf 200 V through a 1 Ω resistor. The battery is also connected to an external resistor R at its terminals. What is the least value of R for which current flows into the battery and it gets charged?

1. 7 Ω
2. 9 Ω
3. 11 Ω
4. 0 Ω

Correct answer: 11 Ω

Solution

For current to flow into the battery, the voltage across the external resistor R must be greater than the battery's emf. The effective voltage from the source is 220 V, and the voltage drop across the 1 Ω resistor must be considered. The minimum resistance R can be calculated using Ohm's law, leading to the conclusion that R must be at least 11 Ω to ensure that the current flows into the battery.

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