. E LECTRIC D IPOLE An electric dipole is a pair of equal and opposite point charges q and – q, separated by a distance a . The line connecting the two charges defines a direction in space. By convention, the direction from – q to q is said to be the direction of the dipole.
The mid-point of locations of – q and q is called the centre of the dipole. The total charge of the electric dipole is obviously zero. This does not mean that the field of the electric dipole is zero. Since the charge q and – q are separated by some distance, the electric fields due to them, when added, do not exactly cancel out.
However, at distances much larger than the separation of the two charges forming a dipole ( r >> a ), the fields due to q and – q nearly cancel out. The electric field due to a dipole therefore falls off, at large distance, faster than like / r (the dependence on r of the field due to a single charge q ). These qualitative ideas are borne out by the explicit calculation as follows: . .
The field of an electric dipole The electric field of the pair of charges (– q and q ) at any point in space can be found out from Coulomb’s law and the superposition principle. The results are simple for the following two cases: (i) when the point is on the dipole axis, and (ii) when it is in the equatorial plane of the dipole, i.e., on a plane perpendicular to the dipole axis through its centre. The electric field at any general point P is obtained by adding the electric fields E – q due to the charge – q and E + q due to the charge q, by the parallelogram law of vectors. (i) For points on the axis Let the point P be at distance r from the centre of the dipole on the side of the charge q, as shown in Fig.