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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
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r1 + r2 2 -
r1 - r2 2 - r1 + r2
- r1 - r2
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Correct Option: D
Current in the circuit
| = | = | ||
| r1 + r2 + R | r1 + r2 + R |
P.D. across first cell = E – ir1
| = E - | = 0 | |
| (r1 + r2) + R |
| Now, E = | = 0 | |
| (r1 + r2) + R |
| ⇒ E = | ⇒ 2r1 = r1 + r2 + R | |
| (r1 + r2) + R |
⇒ R = r1 - r2
