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A resistor of 9.5 ohm across a battery gives a terminal voltage of 11.4 V, while a resistor of 11.5 ohm across the same battery gives a terminal voltage of 11.5 V. Which statement is correct?

The emf of the battery is 12.0 V
The internal resistance of the battery is 0.6 ohm
The emf of the battery is 11.45 V
The emf of the battery is 11.5 V

Correct answer: The emf of the battery is 12.0 V

Solution

The currents are 1.2 A and 1.0 A; solving E = V + I r for both cases gives r = 0.5 ohm and E = 12.0 V.

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