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Five identical cells, each of emf E and internal resistance r, are joined in series. If one of them is mistakenly connected with reversed polarity, what are the net emf and net internal resistance of the combination?

1. 3E and 5r
2. 5E and 5r
3. 3E and 3r
4. 5E and 3r

Correct answer: 3E and 5r

Solution

Net emf = 5E - 2E = 3E (one reversed cell cancels its own +E and an equal +E from the rest), while all five internal resistances still add to 5r.

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