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Two cells of emfs ε₁ and ε₂ with internal resistances r₁ and r₂ are connected in SERIES (positive of one to negative of other). The equivalent emf and internal resistance of the combination are
Aε_eq = (ε₁ + ε₂)/2, r_eq = r₁·r₂/(r₁+r₂)
Bε_eq = ε₁ + ε₂, r_eq = r₁·r₂/(r₁+r₂)
Cε_eq = ε₁ + ε₂, r_eq = r₁ + r₂
Dε_eq = ε₁ − ε₂, r_eq = r₁ + r₂
Answer & Solution
Correct answer: C. ε_eq = ε₁ + ε₂, r_eq = r₁ + r₂
Series cells (head-to-tail) add both the emf and the internal resistance. Parallel cells would invert and combine reciprocally.
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