Cells, 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 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. the electromotive force () or e.m.f. is the energy provided by a cell or battery per coulomb of charge passing through it, it is measured in volts (V). It is equal to the potential difference across the terminals of the cell when no current is flowing.
ε =I(R + r)
ε = electromotive force in volts, V
I = current in amperes, A
R = resistance of the load in the circuit in Ω
r = internal resistance of the cell in Ω