An electric power source consists in principle of a conductive device which is connected to the outside by two metallic contacts. In addition, an essential property has to exist. A power source can apply a force on the internal electrons to move them from one external contact towards the other. The kind of force is different for different kinds of power sources. Within a battery chemical forces are active, within a generator electromagnetic forces can be applied. The forces are of different nature than Coulomb forces and are called Electro-Motive-Forces (short: EMK), as forces who can set electrically charged particles in motion.
The action of these forces is always the same: At one of the external contacts an excess of electrons will occur. These electrons are missing at the other contact and will show up there as a positive charge.
Fig. 1: Battery as power source
with surface charges at the metallic contacts
A basic law comes into play here: Additional electrons can never exist inside of a metallic conductor but only at its surface.
Why additional electrons do not leave such a surface but can be collected there is not easy to explain. Factors like the temperature of the conductor, the geometrical property of its surface and the electric property of the surrounding medium play an important role.
Any further explanation is not necessary for an understanding of the following. It is sufficient to accept as an experimentally proven fact that additional electrons can exist at the surface of a metallic conductor and only at its surface.
The larger the density of the additional positive or negative charges at the surface of the metallic contacts, the more these charge carries repel each other. A certain limit will be reached, which is characteristic for the actual power source, where these repelling Coulomb forces will prevent any further accumulation of electrons. A state of equilibrium will be established between the internal force of the power source (the EMK) and the back driving Coulomb forces