in electricity and magnetism, point where electric or magnetic force appears to be concentrated. A single electric charge
located at a point is sometimes referred to as an electric monopole. An electric dipole consists of two equal and opposite charges separated by a distance. Some molecules, although electrically neutral as a whole, do not have their charges distributed symmetrically, so that the separation of the centers of positive and negative charge constitutes an electric dipole; such molecules are called polar molecules. In calculating the electric potential
at a distance r
from an electric dipole, it is found that it varies principally as 1/r2
, while the potential around a single charge varies as 1/r.
More complex arrangements of charges may have potentials whose principal term contains a higher power of the distance r.
A charge configuration for which the principal term of the potential varies as 1/r3
is called an electric quadrupole; similarly, an octupole is characterized by a potential varying as 1/r4
, a 16-pole by 1/r5
, and so forth. In magnetism
, poles may be defined in an analogous way, so that an ordinary bar magnet with a north pole at one end and a south pole at the other constitutes a magnetic dipole. The potential energy associated with a given arrangement of magnets may be analyzed similarly to that of an array of charges. The analogy is not complete, however, since no isolated magnetic charges (magnetic monopoles) have been found in nature, though some scientists believe their existence possible.
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