18), it is clear that the dipole field at large
distances does not involve q and a separately; it depends on the product
qa This suggests the definition of dipole moment The dipole moment
vector p of an electric dipole is defined by
p  = q × 2a ˆp
(1 19)
that is, it is a vector whose magnitude is charge q times the separation
2a (between the pair of charges q, –q) and the direction is along the line
from –q to q