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A primary cell has an emf of 1.5 V. When its terminals are short-circuited it delivers a current of 3 A. What is the internal resistance of the cell?

1. 0.5 Ohm
2. 4.5 Ohm
3. 2 Ohm
4. 1/4.5 Ohm

Correct answer: 0.5 Ohm

Solution

r = emf / I_short = 1.5 / 3 = 0.5 Ohm.

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