n identical cells are joined in series with its two cells A and B in the loop with reversed polarities. Emf of each cell is E and internal resistance r. The potential difference across cell A or B is (Here n > 4)
2E(1−2n)
Equivalent emf of the circuit, Eeq=(n−2)E−2E=(n−4)E
Equivalent resistance, req=nr
So, current in the circuit, I=Eeqreq=(n−4)Enr
Potential difference across A or B, V=E+Ir=E+(n−4)Enr×r=2E(1−2n)