Question

The plot of the variation of potential difference across a combination of three identical cells in series, versus current is as shown below. What is the emf of each cell?

Answer 1

Let ε  be emf and r the internal resistance of each cell. The equation of terminal potential difference

V = ε_eff - i r_int becomes

V = 3ε  - i r_int .......(1)

where r_int is effective (total) internal resistance.

From fig., when i = 0, V = 6 x 0V

∴ From (1),

6 = 3ε - 0 => ε = 6/3 =2V

i.e., emf of each cell, e = 2V