Electric generators are devices that transform other forms of energy into electrical energy.
On the generator terminals is maintained a potential difference, which is due to the process of converting different types of energies in electric potential. The main types are:
Despite the name, this quantity is just the work done per unit charge on a unit load transported within the generator. \(\mathscr{E}\) is the increment in the electric potential gained by the charges of the eletrical current that passes through the generator. (By convention, from the negative terminal to the positive one.) The unit of \(\mathscr{E}\) is the same as the potential difference; in the \(IS\) is the Volt, \([\mathscr{E}]= V = \frac{J}{C}\) , $$\mathscr{E} = \frac{w}{q}$$
The characteristic of a generator is its \(\mathscr{E}\) and its internal resistance \(r\). The internal resistance is inevitable, for a generator and any electrical appliance, since any material has some electrical resistance. Therefore, the \(\mathbb{V}\) between the terminals \(A\) and \(B\) of a generator is: $$\mathbb{V} = V_B - V_A = \mathbb{V}_{BA} = \mathscr{E} - ri,$$ where \(i\) is the current that the generator provides to the circuit. Each generator has a characteristic curve, which is the Cartesian graph of \(\mathbb{V}\) (at the generator terminals) as a function of the current \(i\) that goes through it.