EMF and internal resistance

When current flows round a circuit energy is transformed in both the external resistor but also in the cell itself. All cells have a resistance of their own and we call this the internal resistance (r) of the cell. The voltage produced by the cell is called the electromotive force or e.m.f for short and this produces a p.d across the cell and across the external resistor (R).

E = V + Ir = IR + Ir

The e.m.f (E) of the cell can be defined as the maximum p.d that the cell can produce across its terminals, or the open circuit p.d since when no current flows from the cell no electrical energy can be lost within it.

The quantity of useful electrical energy available outside the cell is IR and Ir is the energy transformed to other forms within the cell itself.