Tiny magnet with subatomic dimensions, equivalent to the flow of electric charge around a loop. Examples include electrons circulating around atomic nuclei, rotating atomic nuclei, and single subatomic particles with spin. On a large scale, these effects may add together, as in iron atoms, to make magnetic compass needles and bar magnets, which are macroscopic magnetic dipoles. The strength of a magnetic dipole, its magnetic moment, is a measure of its ability to turn itself into alignment with a given external magnetic field. When free to rotate, dipoles align themselves so that their moments point predominantly in the direction of the magnetic field. The SI unit for dipole moment is the ampere-square metre.
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Pair of equal and opposite electric charges, the centres of which do not coincide. An atom in which the centre of the negative cloud of electrons has been shifted slightly away from the nucleus by an external electric field is an induced electric dipole. When the external field is removed, the atom loses its dipolarity. A water molecule, in which two hydrogen atoms are situated to one side of an oxygen atom, is a permanent electric dipole. The oxygen side is always slightly negative, the hydrogen side slightly positive.
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In physics, there are two kinds of dipoles (Hellènic: di(s)- = two- and pòla = pivot, hinge):
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 which 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—confusingly, the "north" and "south" convention for magnetic dipoles is the opposite of that used to describe the Earth's geographic and magnetic poles, so that the 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 which points north if used as a compass.)
A physical dipole consists of two equal and opposite point charges: literally, 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 , as compared to for the next (quadrupole) term and higher powers of for higher terms, or for the monopole term.
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 dipole moments are consequently measured in units named debye in his honor.
With respect to molecules there are three types of dipoles:
The definition of an induced dipole given in the previous sentence is too restrictive and misleading. An induced dipole of any polarizable charge distribution (remember that a molecule has a charge distribution) is caused by an electric field external to . This field may, for instance, originate from an ion or polar molecule in the vicinity of 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 .
Typical gas phase values of some chemical compounds in debye units:
These values can be obtained from measurement of the dielectric constant. When the symmetry of a molecule cancels out a net dipole moment, the value is set at 0. The highest dipole moments are in the range of 10 to 11. From the dipole moment information can be deduced about the molecular geometry of the molecule. For example the data illustrate that carbon dioxide is a linear molecule but ozone is not.
Conversion to cylindrical coordinates is achieved using
where ρ is the perpendicular distance from the z-axis. Then,
with the same definitions as above.
This term appears as the second term in the multipole expansion of an arbitrary electrostatic potential Φ(r). If the source of Φ(r) is a dipole, as it is assumed here, this term is the only non-vanishing term in the multipole expansion of Φ(r). The electric field from a dipole can be found from the gradient of this potential:
where E is the electric field and is the 3-dimensional delta function. ( = 0 except at r = (0,0,0), so this term is ignored in multipole expansion.) Notice that this is formally identical to the magnetic field of a point magnetic dipole; only a few names have changed.
Since the direction of an electric field is defined as the direction of the force on a positive charge, electric field lines point away from a positive charge and toward a negative charge.
The resulting torque will tend to align the dipole with the applied field, which in the case of an electric dipole, yields a potential energy of
The energy of a magnetic dipole is similarly
In particular, a harmonically oscillating electric dipole is described by a dipole moment of the form where ω is the angular frequency. In vacuum, this produces fields:
Far away (for ), the fields approach the limiting form of a radiating spherical wave:
which produces a total time-average radiated power P given by
This power is not distributed isotropically, but is rather concentrated around the directions lying perpendicular to the dipole moment. Usually such equations are described by spherical harmonics, but they look very different. A circular polarized dipole is described as a superposition of two linear dipoles.
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