Find the equivalent internal resistance and potential of the given circuit.

CONCEPT:

• Grouping of Cells: In a series grouping of cell’s their emf’s are additive or subtractive while their internal resistances are always additive.
• Series grouping: In series grouping anode of one cell is connected to the cathode of other cells and so on. 

• If n identical cells are connected in series

1. Equivalent emf of the combination  E_eq = nE
2. Equivalent internal resistance r_eq = nr

CALCULATION:

Given- emf of three cells = E₁ = E₂ = E₃ = E and internal resistance = r₁ = r₂ = r₃ = r

Equivalent emf of the combination  is

⇒ E_eq = E₁ + E₂ + E₃ = E + E + E = 3E

Similarly,

Equivalent internal resistance  is

⇒ r_eq = r₁ + r₂ + r₃ = r + r + r = 3r

So option 3 is correct.