Question:
Two cells of same emf E but internal resistance r1 and r2 are connected in series to an external resistor R. What should be the value of R so that the potential difference across the terminals of the first cell becomes zero.
Solution:
Effective emf of two cells = E + E = 2E
Effective resistance = R + r1 + r2
Electric current is given as
I = 2E/R+r1+r2
Potential difference is given as
V1 – E – Ir1 = 0
Which f=gives R = r1 – r2