20)
At a point on the equatorial plane
E
= − 4p
πεor3
(r >> a)
(1 21)
FIGURE 1 17 Electric field of a dipole
at (a) a point on the axis, (b) a point
on the equatorial plane of the dipole p is the dipole moment vector of
magnitude p = q × 2a  and
directed from –q to q