Electric dipole

Two equal and opposite charges +q and -q, separated by a small distance 2a, together constitute an electric dipole. The strength of a dipole is quantified by its electric dipole moment, a vector p defined as p = q(2a), where the direction of p is taken, by convention, from the negative charge toward the positive charge along the line joining the two charges (the dipole axis). The SI unit of dipole moment is the coulomb-metre (C m). For a more general collection of point charges q1, q2, ..., qn located at position vectors r1, r2, ..., rn (measured from some fixed origin), the electric dipole moment of the whole system is defined generally as p = sum of qi ri -- for the simple two-charge case this general definition reduces exactly to p = q(2a) directed from the negative to the positive charge. Molecules whose positive and negative charge centres are permanently offset, such as the water molecule, possess a permanent dipole moment even in the absence of any external field; ot …