Question
Two cells, having the same emf, 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
Solution
A.
Current in the circuit is given by Ohm's law.
Net resistance of the circuit = 
Net emf in series = E + E = 2E
Therefore, from Ohm's law, current in the circuit.
It is given that, as circuit is closed, potential difference across the first cell is zero. That is,
Equating Eqs. (i) and (ii), we get
