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A cell of fixed EMF produces equal heat in the same time when resistors R1 and R2 are separately connected across its terminals. Find the internal resistance r of the cell.

1. sqrt(R1*R2)
2. (R1 + R2)/2
3. R1*R2/(R1+R2)
4. R1 + R2

Correct answer: sqrt(R1*R2)

Solution

Equating the two heat expressions leads to r² = R1 R2, so r = sqrt(R1 R2).

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