where R is the distance from the centre of the dipole to the field point and the approximation is valid when r and R are almost equal.  In this case the dimension of the dipole (a) is small compared to the field point distance.

• p (=2aq) is called the electric dipole moment.  It's actually a vector pointing from the negative to the positive charge in the dipole so that,

• Many molecules have charge distributions which can be approximated as an electric dipole, water being one of the most common.