The electric potential is the potential electric power per unit charge, measured in joules per coulomb (volts). The “electric potential” must be distinguished from the “electric potential” by noting that the “potential” is a “per unit of charge”. Like the potential mechanical energy, zero can be chosen at any point so that the potential difference, that is, the voltage, is the quantity that has physical importance.
The voltage between point A and point B is equal to the work that must be done in the charge unit against or by the electric field to move the charge from A to B. The voltage between the two ends of a path is the energy total required to move a small electric charge along that path divided by the size of the load. From a mathematical point of view, this is expressed as the integrated linear electric field and the magnitude of the magnetic field changes along that path. In general, both a static (non-changing) and a dynamic (electromagnetic) field (which varies with time) must be included in the voltage determination between two points.
From a historical point of view, this amount has also been called “tension” and “pressure”. The pressure is now outdated, but the voltage is still used, for example, within the expression of “high voltage” (HT), which is commonly used in thermowell-based electronics (vacuum tube).
The voltage is defined so that negatively charged objects are brought to higher voltages, while positively charged objects are brought to lower voltages. Therefore, the conventional current in a cable or resistance always comes from a higher voltage to a lower voltage. The current can flow from a lower voltage to a higher voltage, but only when there is a source of energy present to “push” it against the opposite electric field. This is the case in any electricity source. For example, within a battery, chemical reactions provide the energy required to flow the ion from the negative to the positive terminal.
The electric field is not the only factor that determines the rate of charge flow in a material, and different materials naturally develop differences in the electrical potential at equilibrium (galvanic potentials). The electrical potential of a material is not even a well-defined quantity, since it varies according to the subatomic scale. A more convenient definition of “tension” can be found instead of the Fermi level concept.
In this case, the tension between two bodies is the thermodynamic work required to move a load unit between them. This definition is practical because a real voltmeter really measures this work, not a difference in electrical potential.