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Electrical dipole moment
From Wikipedia, the free encyclopedia
In physics, the electric dipole moment is a measure of the polarity of a system of electric charges.
The electric dipole moment for a distribution of discrete point charges is simply the vector sum of the products of the charge and the position vector of each charge.
This discrete definition can also be given in a continuous form using the charge density, ρ, in place of the charge, q.
The dipole moment is most commonly used for systems which have an overall neutral charge. For example, a pair of opposite charges, or a neutral conductor in a uniform electric field. For such systems, the value of the electric dipole moment is independent of the origin of the axis system. For non-neutral systems, a dependence on the choice of origin arises. In order retain the electric dipole moment's utility in calculating the torque on a dipole as well as other uses, the origin is often taken to be the center of charge, 
, for the system, which is defined like the center of mass and is, for some systems, the same.
The electric dipole moment for a pair of opposite charges of magnitude q is defined as the magnitude of the charge times the distance between them and the defined direction is toward the positive charge.
It is a useful concept in atoms and molecules where the effects of charge separation are measurable, but the distances between the charges are too small to be easily measurable. It is also a useful concept in dielectrics and other applications in solid and liquid materials.