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
- r1 + r2
- r1 - r2
- r1 + r22
- r1 - r22
Answer
(2) r1 - r2
Solution
Current in the circuit: I = 2 R + r1 + r2
As per NCERT, terminal potential difference of the first cell: V1 = - Ir1 = 0
= 2 r1R + r1 + r2
R + r1 + r2 = 2r1
R = r1 - r2
Answer: (2)