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A current of 2 A flows through a 2 Ω resistor when connected across a battery. The same battery supplies a current of 0.5 A when connected across a 9 Ω resistor. The internal resistance of the battery is

1. 0.5 Ω
2. 1/3 Ω
3. 1/4 Ω
4. 1 Ω

Correct answer: 0.5 Ω

Solution

Using Ohm's law, the total resistance in the first case is R + r = 2 Ω, and in the second case, R + r = 9 Ω. Solving these equations gives the internal resistance r = 0.5 Ω.

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