Two cells of same emf E but internal resistance r 1 and r 2 are connected in series to an external resistor R (Fig 3.7). What should be the value of R so that the potential difference across the terminals of the first cell becomes zero.

Two cells of same emf E but internal resistance r₁ and r₂ are connected in series to an external resistor R (Fig 3.7). What should be the value of R so that the potential difference across the terminals of the first cell becomes zero.

Effective emf

Current in the circuit =

Potential difference across first cell

Hence,

2