Dipole

The magnetic field of a sphere with a north magnetic pole at the top and a south magnetic pole at the bottom. By comparison, Earth has a south magnetic pole near its north geographic pole and a north magnetic pole near its south pole.

In electromagnetism, 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 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 points from the negative charge towards the positive charge, and has a magnitude equal to the strength of each charge times the separation between the charges. (To be precise: for the definition of the dipole moment, one should always consider the "dipole limit", where, for example, the distance of the generating charges should converge to 0 while simultaneously, the charge strength should diverge to infinity in such a way that the product remains a positive constant.)

For the current loop, the magnetic dipole moment points 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, has a magnetic dipole moment. That is because it generates a magnetic field that is identical to that generated by a very small current loop. However, the electron's magnetic moment is not due to a current loop, but is instead an intrinsic property of the electron.[3] It is also possible that the electron has an electric dipole moment although it has not yet been observed (see electron electric dipole moment for more information).

Contour plot of the electrostatic potential of a horizontally oriented electrical dipole of finite size. Strong colors indicate highest and lowest potential (where the opposing charges of the dipole are located).

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 may be labeled "north" and "south". In terms of the Earth's magnetic field, they are respectively "north-seeking" and "south-seeking" poles: if the magnet were freely suspended in the Earth's magnetic field, the north-seeking pole would point towards the north and the south-seeking pole would point towards the south. The dipole moment of the bar magnet points from its magnetic south to its magnetic north pole. The north pole of a bar magnet in a compass points north. However, that means that Earth's geomagnetic north pole is the south pole (south-seeking pole) of its dipole moment and vice versa.

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 δίς (dis), "twice"[4] and πόλος (polos), "axis".[5][6]

Classification

Electric field lines of two opposing charges separated by a finite distance.
Magnetic field lines of a ring current of finite diameter.
Field lines of a point dipole of any type, electric, magnetic, acoustic, etc.

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 exactly the 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 total 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.

Other Languages
العربية: ثنائي قطب
беларуская: Электрычны дыполь
беларуская (тарашкевіца)‎: Электрычны дыполь
български: Дипол
català: Dipol
dansk: Dipol
Deutsch: Dipol
eesti: Dipool
Esperanto: Dupoluso
Gaeilge: Déphol
հայերեն: Դիպոլ
हिन्दी: द्विध्रुव
hrvatski: Dipol
עברית: דיפול
ქართული: დიპოლი
қазақша: Диполь
Nederlands: Dipool
日本語: 双極子
norsk: Dipol
norsk nynorsk: Dipol
Piemontèis: Dipòlo elétrich
polski: Dipol
português: Dipolo
română: Dipol
Scots: Dipole
Simple English: Dipole
slovenčina: Elektrický dipól
slovenščina: Električni dipol
српски / srpski: Дипол
srpskohrvatski / српскохрватски: Dipolni moment
Türkçe: Dipol
українська: Диполь
中文: 偶極子