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When a current of 4 A is driven through a battery from its positive terminal to its negative terminal (charging), the potential difference across it is 12 V. When a current of 2 A flows through it from the negative terminal to the positive terminal (discharging), the potential difference across it is 9 V. Determine the internal resistance and the e.m.f. of the battery.

1. 0.5 Ohm, 10 V
2. 0.1 Ohm, 4 V
3. 0.2 Ohm, 5 V
4. 0.7 Ohm, 10 V

Correct answer: 0.5 Ohm, 10 V

Solution

Solving E + 4r = 12 and E - 2r = 9 gives r = 0.5 Ohm and E = 10 V.

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