The fundamental equations of electrostatics are linear equations,

 (SI units),         (Gaussian units).

The principle of superposition holds.

The electrostatic force on a particle with charge q at position r  is F=qE(r).

. 

f is the electrostatic potential.

The field at r due to a point charge at r?:   

(Omit the factor to obtain the corresponding expressions in Gaussian units.)

(We consider volume, surface, and line charge distributions and point charges.)

The potential at r due to a point charge at r?:     

(SI units),    (Gaussian units).

In situations with enough symmetry, Gauss? law alone can be used to find E.

or, for a continuous charge distribution,

(SI units),      (Gaussian units).

The field of a dipole at the origin:

The potential of a dipole at the origin:

The torque on a dipole:

The energy of a dipole in an external field:

(SI units),         ,                                 ,                          f is continuous across the boundary.

(Gaussian units), ,                                 , f is continuous across the boundary.