For a point charge q at position (i.e., an electric monopole), the electric field at a point r is defined in cgs to be

In MKS, the corresponding equation is

where is a fundamental constant called the permittivity of free space. The electric field is therefore defined such that for an electric monopole, E points away from a positive charge and towards a negative charge.

For an arbitrary collection of charge, the electric field can be determined experimentally as

where F is the force exerted by the field on a test particle of charge q.
For a static electric field E, the electric field is defined in terms of the electric potential (sometimes also denoted V and called the voltage) by

where is the gradient.In the presence of a time-varying magnetic field, the electric field is given in cgs by the differential form of one of the Maxwell equations,

with c is the speed of light and A is the magnetic vector potential. In MKS, the corresponding equation is

For a continuous distribution of charge , the electric field at a point r is given in cgs by

In MKS,

where is again the permittivity of free space.