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A dry cell has an EMF of approximately 1.5 V. When an ammeter of negligible resistance is connected directly across its terminals, it reads a short-circuit current of about 30 A. What is the approximate internal resistance of the cell?

1. 0.01 ohm
2. 0.02 ohm
3. 0.05 ohm
4. 0.1 ohm

Correct answer: 0.05 ohm

Solution

When the cell is short-circuited (ammeter of negligible resistance), the terminal voltage is zero and the full EMF drives the current through the internal resistance r only. So EMF = I * r, giving r = 1.5/30 = 0.05 ohm.

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