Dipole Moments:

A physical dipole is a particle, atom or molecular structure exhibiting two equal and opposite charges. The charges can be electric or magnetic. Dipoles can be characterized by their dipole moment, which is a vector quantity (field description). The moment occurs when the charge points from one pole toward the other, and the field can be measured by the simple formula of magnitude (strength of each charge) times the distance between the charges.

The electron displays a magnetic dipole moment as an intrinsic property of the particle. Any configuration of charges or currents has a dipole moment which describes the dipole in terms of the field which provides the best approximation of the given configuration at large distances.

Molecules may be polarized, thus exhibit dipole moments. This can be permanent as in molecules that bind oppositely charged atoms, or instantaneous when electrons become concentrated at one place in the molecule. Dipole moment can also be induced when a permanent dipolar molecule repels another molecule's electrons.

Even though the total charge on a molecule is zero, the nature of chemical bonds is such that the positive and negative charges do not completely overlap in most molecules. Such molecules are said to be polar because they possess a permanent dipole moment. A good example is the dipole moment of the water molecule. Molecules with mirror symmetry like oxygen, nitrogen, carbon dioxide, and carbon tetrachloride have no permanent dipole moments. Even if there is no permanent dipole moment, it is possible to induce a dipole moment by the application of an external electric field. This is called polarization and the magnitude of the dipole moment induced is a measure of the polarizability of the molecular species.

The dipolar interaction between water molecules represents a large amount of internal energy and is a factor in water's large specific heat. The dipole moment of water provides a "handle" for interaction with microwave electric fields in a microwave oven. Microwaves can add energy to the water molecules, whereas molecules with no dipole moment would be unaffected.