n identical cells are joined in series with two cells A and B with reversed polarities. EMF of each cell is E and internal resistance is r . The potential difference across the cell A or B is ( n > 4 )

Step-by-step solution

Total number of cells = n Total resistance = nr n - 2 cells provide a forward emf, 2 cells provide a reverse emf so: Total emf = n - 2 E - 2 E = ne - 4 E = E n - 4 Current, I   = total   emf total   resistance I = E n - 4 nr PD across A or B (voltage across internal resisitance) V = Ir + E = E n - 4 / nr r + E = E n - 4 /n + E = E n - 4 ) / n + 1 = E n - 4 + n / n = E 2 n - 4 / n = 2 E n - 2 / n = 2 E 1 - 2 / n