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The potential difference between the terminals of a cell in an open circuit is 2.2 V. When a resistor of 5Ω is connected across the terminals of the cell, the potential difference between the terminals of the cell is found to be 1.8 V. The internal resistance of the cell is

1. 7 / 12 Ω
2. 10 / 9 Ω
3. 9 / 10 Ω
4. 12 / 7 Ω

Correct answer: 10 / 9 Ω

Solution

The internal resistance (r) of the cell can be calculated using the formula: V = E - Ir, where E is the emf (2.2 V), V is the terminal voltage (1.8 V), and I is the current. The current I = V/R = 1.8/5 = 0.36 A. Substituting, 1.8 = 2.2 - 0.36r, solving gives r = 10/9 Ω.

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