According to the tenets of modern physics, there is always an electric field present around the electric charges. This electrically charged field is called an electrostatic field. An electromagnetic force acts on the charges in the electrostatic field. The ratio of the force that acts on the charge in a given field is called the electric field intensity or simply the electric field. The intensity of the electric field has a quantity of a vector. The direction of the vector E coincides with the direction of the force that acts on the positive charge. The magnitude of the electric field caused by a point charge q at a distance r from the charge is equal to
, where ε0 = 8.85∙10-12∙(C/(Nm2)) is an electric constant. This field is called Coulomb's field. An electric field caused by a set of charges satisfies the superposition principle. In this model, electric field lines are used for the graphic representation of an electric field. The direction of the vector of electric field E is tangent to the electric field line at any given point. The spacing of electric field lines is inversely proportional to the magnitude of the electric field. An energetic characteristic of a given field is referred to as the potential. The potential is a scalar quantity. At any point, the potential equals to the work of electric forces that is required to move a unit positive charge from the given point to infinity. The potential of Coulomb's field caused by a point charge q is equal to
An equipotential surface is defined as a surface where the potential is the same at every point. Note that field lines are always perpendicular to equipotential surfaces.