When placed in an electric (E) or magnetic (B) field, equal but opposite forces arise on each side of the dipole creating a torque τ:
Strictly speaking a dipole contains only two point charges (or magnetic poles), however various arrangements of multiple charges or currents have dipole moments and may be treated as an effective dipole. For the case of magnetic dipoles (where single magnetic monopoles do not exist naturally), the simplest dipole is a single ring of current, which will make a dipole field. Other more complicated systems can be approximated as dipole systems mathematically, especially if the net charge is zero, but the positive and negative charges are not distributed symmetrically and the dipole field structure is the dominant one.
The magnetic or electric field near a dipole decreases with distance (r) as 1/r^{2} as opposed to the 1/r fall off of a monopole. The field which falls off proportionally to increasing powers of r are called the quadrapole component(1/r^{3}) of the field, the octopole component (1/r^{4}) of the field, and so on.
Many molecules have such dipole moments due to non-uniform distributions of positive and negative charges on the various atoms. For example:
(positive) H-Cl (negative)A molecule with a permanent dipole moment is called a polar molecule and is polarised. The physical chemist Peter J. W. Debye was the first scientist to study molecular dipoles extensively, and dipole moments are consequently measured in units named debyes in his honor.
With respect to molecules there are three types of dipoles:
The strength of a dipole magnetic field where:
From the point of view of the mathematics of distributions, a dipole can be taken to be the directional derivative of a Dirac delta function.