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An ideal cell of EMF 10 V and a non-ideal cell of EMF 15 V with internal resistance 1 ohm are connected in parallel between terminals A and B. What is the equivalent EMF between A and B?

1. 10 V
2. 15 V
3. 12.5 V
4. 25 V

Correct answer: 10 V

Solution

Because the 10 V cell is ideal (internal resistance = 0), it fixes the potential difference between A and B at exactly 10 V. The 15 V cell drives a current of (15-10)/1 = 5 A through its internal resistance but cannot alter the clamped voltage. Hence V_AB = 10 V.

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