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Two cells, having the same e.m.f., are connected in series through an external resistance R. Cells have internal resistances r₁ and r₂ (r₁ > r₂) respectively. When the circuit is closed, the potential difference across the first cell is zero. The value of R is

1. r₁ + r₂ / 2
2. r₁ - r₂ / 2
3. r₁ + r₂
4. r₁ - r₂

Correct answer: r₁ - r₂

Solution

The potential difference across the first cell is zero when the current through the circuit balances the internal resistance of the cells. Using KVL, R = r₁ - r₂.

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