The surface charge density on a solid is defined as the total amount of charge q per unit area A,

The surface charge on a surface S with surface charge density is therefore given by

In cgs, Gauss's law requires that across a boundary

where is the electric field in medium i and is the unit normal vector, which is equivalent to

where is the voltage in medium i. Similarly, in MKS,

where is the permittivity of free space, and

In a dielectric, the so-called bound surface charge density is given in MKS by

where P is the electric polarization.