Consider the circuit shown in Figure 1. The velocity of the electron is limited by the friction it experiences while traveling through the conductor, and on average will not change. In order to sustain the continuous flow of electrons around the circuit, the source must force the electrons from the terminal with the positive potential to the terminal with the negative potential. During this process, the source does work on the electrons, and increases their total mechanical energy. A steady current will flow through the circuit in Figure 1 if the increase of potential energy of the electrons in the source is equal to the change in the potential energy of the electrons along their path through the external circuit.
Batteries convert chemical energy into electric energy.
At this point the battery is discharged. The battery can be recharged by forcing a current through the battery in the reverse direction. Electric generators convert mechanical energy into electric energy. The principle of operation of electric generators will be discussed later. A fuel cell therefore burns fuel.