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Dipoles
In physics, there are two kinds of dipoles:
An electric dipole is a separation of positive and negative charges. The simplest example
of this is a pair of electric charges of equal magnitude but opposite sign, separated by
some, usually small, distance. A permanent electric dipole is called an electret.
A magnetic dipole is a closed circulation of electric current. A simple example of this is a
single loop of wire with some constant current flowing through it.[1][2]
Dipoles can be characterized by their dipole moment, a vector quantity. For the simple
electric dipole given above, the electric dipole moment would point from the negative
charge towards the positive charge, and have a magnitude equal to the strength of each
charge times the separation between the charges. For the current loop, the magnetic
dipole moment would point through the loop (according to the right hand grip rule), with
a magnitude equal to the current in the loop times the area of the loop.
In addition to current loops, the electron, among other fundamental particles, is said to
have a magnetic dipole moment. This is because it generates a magnetic field that is
identical to that generated by a very small current loop. However, to the best of our
knowledge, the electron's magnetic moment is not due to a current loop, but is instead an
intrinsic property of the electron. It is also possible that the electron has an electric dipole
moment, although this has not yet been observed (see electron electric dipole moment for
more information).
A permanent magnet, such as a bar magnet, owes its magnetism to the intrinsic magnetic
dipole moment of the electron. The two ends of a bar magnet are referred to as poles (not
to be confused with monopoles), and are labeled "north" and "south." The dipole moment
of the bar magnet points from its magnetic south to its magnetic north pole. What can be
confusing is that the "north" and "south" convention for magnetic dipoles is the opposite
of that used to describe Earth's geographic and magnetic poles, so that Earth's
geomagnetic north pole is the south pole of its dipole moment. (This should not be
difficult to remember; it simply means that the north pole of a bar magnet is the one that
points north if used as a compass.)
The only known mechanisms for the creation of magnetic dipoles are by current loops or
quantum-mechanical spin since the existence of magnetic monopoles has never been
experimentally demonstrated.
The term comes from the Greek di(s)- = "two" and pòla "pivot, hinge".
A physical dipole consists of two equal and opposite point charges: in the literal sense,
two poles. Its field at large distances (i.e., distances large in comparison to the separation
of the poles) depends almost entirely on the dipole moment as defined above. A point
(electric) dipole is the limit obtained by letting the separation tend to 0 while keeping the
dipole moment fixed. The field of a point dipole has a particularly simple form, and the
order-1 term in the multipole expansion is precisely the point dipole field.
Although there are no known magnetic monopoles in nature, there are magnetic dipoles
in the form of the quantum-mechanical spin associated with particles such as electrons
(although the accurate description of such effects falls outside of classical
electromagnetism). A theoretical magnetic point dipole has a magnetic field of the exact
same form as the electric field of an electric point dipole. A very small current-carrying
loop is approximately a magnetic point dipole; the magnetic dipole moment of such a
loop is the product of the current flowing in the loop and the (vector) area of the loop.
Any configuration of charges or currents has a 'dipole moment', which describes the
dipole whose field is the best approximation, at large distances, to that of the given
configuration. This is simply one term in the multipole expansion; when the charge
("monopole moment") is 0 — as it always is for the magnetic case, since there are no
magnetic monopoles. The dipole term is the dominant one at large distances: Its field
falls off in proportion to 1/r3, as compared to 1/r4 for the next (quadrupole) term and
higher powers of 1/r for higher terms, or 1/r2 for the monopole term.
Many molecules have such dipole moments due to non-uniform distributions of positive
and negative charges on the various atoms. Such is the case with polar compounds like
hydroxide (OH−), where electron density is shared unequally between atoms.
A molecule with a permanent dipole moment is called a polar molecule. A molecule is
polarized when it carries an induced dipole. The physical chemist Peter J. W. Debye was
the first scientist to study molecular dipoles extensively, and, as a consequence, dipole
moments are measured in units named debye in his honor.
With respect to molecules, there are three types of dipoles:
 Permanent dipoles: These occur when two atoms in a molecule have substantially
different electronegativity: One atom attracts electrons more than another,
becoming more negative, while the other atom becomes more positive. See
dipole-dipole attractions.
 Instantaneous dipoles: These occur due to chance when electrons happen to be
more concentrated in one place than another in a molecule, creating a temporary
dipole. See instantaneous dipole.
 Induced dipoles: These occur when one molecule with a permanent dipole repels
another molecule's electrons, "inducing" a dipole moment in that molecule. See
induced-dipole attraction.
The definition of an induced dipole given in the previous sentence is too restrictive and
misleading. An induced dipole of any polarizable charge distribution ρ (remember that a
molecule has a charge distribution) is caused by an electric field external to ρ. This field
may, for instance, originate from an ion or polar molecule in the vicinity of ρ or may be
macroscopic (e.g., a molecule between the plates of a charged capacitor). The size of the
induced dipole is equal to the product of the strength of the external field and the dipole
polarizability of ρ.