Two cells of emfs E1 and E2 are connected in series. Their internal resistances are r1 and r2 respectively. Compute the equivalent emf and equivalent internal resistance.

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When two cells of EMFs E₁ and E₂ are connected in series, their equivalent EMF is the algebraic sum of their EMFs. So, the equivalent EMF E will be:

E = E₁ + E₂

When two cells are connected in series, their internal resistances also add up to give the equivalent internal resistance r:

r = r₁ + r₂

Thus, the equivalent EMF is E₁ + E₂ and the equivalent internal resistance is r₁ + r₂.

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