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Dictionary Definition

magnet

Noun

1 (physics) a device that attracts iron and produces a magnetic field
2 a characteristic that provides pleasure and attracts; "flowers are an attractor for bees" [syn: attraction, attractor, attracter, attractive feature]

User Contributed Dictionary

see Magnet

English

Etymology

From the Greek μαγνήτης λίθος (magnítis líthos), magnesian stone.

Noun

  1. A piece of material that attracts some metals by magnetism.
  2. In the context of "informal|figurative|preceded by a noun": A person or thing that attracts what is denoted by the preceding noun.
    He always had a girl on his arm - he's a bit of a babe-magnet.
    • 2007, J. Michael Fay, Ivory Wars: Last Stand in Zakouma, National Geographic (March 2007), 47,
      ...I wanted to show Nick the largest of the water holes, Rigueik, that act as magnets to life in the dry season.

Translations

piece of material that attracts some metals by magnetism
  • Chinese: 磁鐵, 磁铁
  • Czech: magnet
  • Dutch: magneet
  • Finnish: magneetti
  • French: aimant
  • German: Magnet
  • Greek: μαγνήτης
  • Hebrew: מגנט
  • Hungarian: mágnes
  • Icelandic: segull
  • Italian: magnete, calamita
  • Japanese: 磁石
  • Korean: 자석
  • Kurdish:
  • Maori: autō
  • Polish: magnes
  • Portuguese: ímã
  • Russian: магнит
  • Spanish: imán
  • Swedish: magnet
  • Telugu: అయస్కాంతము (ayaskaamtamu)
  • Urdu: مقناطیس (maqnatees)

Czech

Noun

  1. magnet

Swedish

Noun

magnet (plural: magneter, definite singular magneten, definite plural magneterna)
  1. magnet (piece of material that attracts metal by magnetism)

Extensive Definition

A magnet is a material or object that produces a magnetic field. A low-tech means to detect a magnetic field is to scatter iron filings and observe their pattern, as in the accompanying figure. A "hard" or "permanent" magnet is one that stays magnetized, such as a magnet used to hold notes on a refrigerator door. Permanent magnets occur naturally in some rocks, particularly lodestone, but are now more commonly manufactured. A "soft" or "impermanent" magnet is one that loses its memory of previous magnetizations. "Soft" magnetic materials are often used in electromagnets to enhance (often hundreds or thousands of times) the magnetic field of a wire that carries an electrical current and is wrapped around the magnet; the field of the "soft" magnet increases with the current.
Two measures of a material's magnetic properties are its magnetic moment and its magnetization. A material without a permanent magnetic moment can, in the presence of magnetic fields, be attracted (paramagnetic), or repelled (diamagnetic). Liquid oxygen is paramagnetic; graphite is diamagnetic. Paramagnets tend to intensify the magnetic field in their vicinity, whereas diamagnets tend to weaken it. "Soft" magnets, which are strongly attracted to magnetic fields, can be thought of as strongly paramagnetic; superconductors, which are strongly repelled by magnetic fields, can be thought of as strongly diamagnetic.

Background on the physics of magnetism and magnets

The magnetic moment of atoms in a ferromagnetic material cause them to behave something like tiny permanent magnets. They stick together and align themselves into small regions of more or less uniform alignment called magnetic domains or Weiss domains. Magnetic domains can be observed with Magnetic force microscope to reveal magnetic domain boundaries that resemble white lines in the sketch.There are many scientific experiments that can physically show magnetic fields.
In an antiferromagnet, unlike a ferromagnet, there is a tendency for the intrinsic magnetic moments of neighboring valence electrons to point in opposite directions. When all atoms are arranged in a substance so that each neighbor is 'anti-aligned', the substance is antiferromagnetic. Antiferromagnets have a zero net magnetic moment, meaning no field is produced by them. Antiferromagnets are less common compared to the other types of behaviors, and are mostly observed at low temperatures. In varying temperatures, antiferromagnets can be seen to exhibit diamagnetic and ferrimagnetic properties.
In some materials, neighboring electrons want to point in opposite directions, but there is no geometrical arrangement in which each pair of neighbors is anti-aligned. This is called a spin glass, and is an example of geometrical frustration.

Physics of ferrimagnetism

Like ferromagnetism, ferrimagnets retain their magnetization in the absence of a field. However, like antiferromagnets, neighboring pairs of electron spins like to point in opposite directions. These two properties are not contradictory, due to the fact that in the optimal geometrical arrangement, there is more magnetic moment from the sublattice of electrons which point in one direction, than from the sublattice which points in the opposite direction.
The first discovered magnetic substance, magnetite, was originally believed to be a ferromagnet; Louis Néel disproved this, however, with the discovery of ferrimagnetism.

Other types of magnetism

There are various other types of magnetism, such as and spin glass (mentioned above), superparamagnetism, superdiamagnetism, and metamagnetism.

Common uses of magnets

  • Magnetic recording media: Common VHS tapes contain a reel of magnetic tape. The information that makes up the video and sound is encoded on the magnetic coating on the tape. Common audio cassettes also rely on magnetic tape. Similarly, in computers, floppy disks and hard disks record data on a thin magnetic coating.
  • Credit, debit, and ATM cards: All of these cards have a magnetic strip on one of their sides. This strip contains the necessary information to contact an individual's financial institution and connect with their account(s).
  • Speakers and Microphones: Most speakers employ a permanent magnet and a current-carrying coil to convert electric energy (the signal) into mechanical energy (movement which creates the sound). The coil is wrapped around a bobbin attached to the speaker cone, and carries the signal as changing current which interacts with the field of the permanent magnet. The voice coil feels a magnetic force and in response moves the cone and pressurizes the neighboring air, thus generating sound. Dynamic microphones employ the same concept, but in reverse. A microphone has a diaphragm or membrane attached to a coil of wire. The coil rests inside a specially shaped magnet. When sound vibrates the membrane, the coil is vibrated as well. As the coil moves through the magnetic field, a voltage is generated across the coil (see Lenz's Law). This voltage drives current in the wire that is characteristic of the original sound.
  • Electric motors and generators: Some electric motors (much like loudspeakers) rely upon a combination of an electromagnet and a permanent magnet, and much like loudspeakers, they convert electric energy into mechanical energy. A generator is the reverse: it converts mechanical energy into electric energy.
  • Transformers: Transformers are devices that transfer electric energy between two windings that are electrically isolated but are linked magnetically.
  • A compass (or mariner's compass) is a navigational instrument for finding directions on the Earth. It consists of a magnetized pointer free to align itself accurately with Earth's magnetic field, which is of great assistance in navigation. The cardinal points are north, south, east and west. A compass can be used in conjunction with a marine chronometer and a sextant to provide a very accurate navigation capability. This device greatly improved maritime trade by making travel safer and more efficient. An early form of the compass was invented in China in the 11th century. The familiar mariner's compass was invented in Europe around 1300, as was later the liquid compass and the gyrocompass which does not work with a magnetic field.
  • Magic: Naturally magnetic Lodestones as well as iron magnets are used in conjunction with fine iron grains (called "magnetic sand") in the practice of the African-American folk magic known as hoodoo. The stones are symbolically linked to people's names and ritually sprinkled with magnetic sand to reveal the magnetic field. One stone may be utilized to bring desired things to a person; a pair of stones may be manipulated to bring two people closer together in love.
  • Art: 1 mm or thicker vinyl magnet sheets may be attached to paintings, photographs, and other ornamental articles, allowing them to be stuck to refrigerators and other metal surfaces.
  • Science Projects: Many topic questions are often based on magnets. For example; how is the strength of a magnet affected by glass, plastic, and cardboard?
  • Toys: Due to their ability to counteract the force of gravity at very close range, magnets are often employed in children's toys such as the Magnet Space Wheel to amusing effect.
  • Magnets can be used to make jewelry. Necklaces and bracelets can have a magnetic clasp. Necklaces and bracelets can be made from small but strong, cylindrical magnets and slightly larger iron or steel balls connected in a pattern that is repeated until it is long enough to fit on the wrist or neck. These accessories may be fragile enough to accidentally come apart, but they also can be disassembled and reassembled with a different design. When connected as a necklace or a bracelet, magnets lose their attraction to other pieces of iron steel because they are already attached to their own iron and steel balls.
  • Magnets can pick up magnetic items (iron nails, staples, tacks, paper clips) that are either too small, too hard to reach, or too thin for fingers to hold.
  • Magnetic levitation transport, or maglev, is a form of transportation that suspends, guides and propels vehicles (especially trains) via electromagnetic force. This method can be faster than wheeled mass transit systems, potentially reaching velocities comparable to turboprop and jet aircraft (900 km/h, 559 mph). The maximum recorded speed of a maglev train is 581 km/h (361 mph), achieved in Japan in 2003.
  • A recently developed use of magnetism is to connect portable computer power cables. Such a connection will occasionally break by accidentally pushing against the cable, but the computer battery prevents interruption of service, and the easy disconnection protects the cable from serious jerks or from being stepped on.

Magnetization and demagnetization

Ferromagnetic materials can be magnetized in the following ways:
  • Placing the item in an external magnetic field will result in the item retaining some of the magnetism on removal. Vibration has been shown to increase the effect. Ferrous materials aligned with the earth's magnetic field and which are subject to vibration (e.g. frame of a conveyor) have been shown to acquire significant residual magnetism. A magnetic field much stronger than the earth's can be generated inside a solenoid by passing direct current through it.
  • Stroking - An existing magnet is moved from one end of the item to the other repeatedly in the same direction.
  • Placing a steel bar in a magnetic field, then heating it to a high temperature and then finally hammering it as it cools. This can be done by laying the magnet in a North-South direction in the Earth's magnetic field. In this case, the magnet is not very strong but the effect is permanent.
Permanent magnets can be demagnetized in the following ways:
  • Heating a magnet past its Curie point will destroy the long range ordering.
  • Contact through stroking one magnet with another in random fashion will demagnetize the magnet being stroked, in some cases; some materials have a very high coercive field and cannot be demagnetized with other permanent magnets.
  • Hammering or jarring will destroy the long range ordering within the magnet.
  • A magnet being placed in a solenoid which has an alternating current being passed through it will have its long range ordering disrupted, in much the same way that direct current can cause ordering.
In an electromagnet which uses a soft iron core, ceasing the flow of current will eliminate the magnetic field. However, a slight field may remain in the core material as a result of hysteresis.

Types of permanent magnets

Magnetic metallic elements

Many materials have unpaired electron spins, and the majority of these materials are paramagnetic. When the spins interact with each other in such a way that the spins align spontaneously, the materials are called ferromagnetic (what is often loosely termed as "magnetic"). Due to the way their regular crystalline atomic structure causes their spins to interact, some metals are (ferro)magnetic when found in their natural states, as ores. These include iron ore (magnetite or lodestone), cobalt and nickel, as well the rare earth metals gadolinium and dysprosium (when at a very low temperature). Such naturally occurring (ferro)magnets were used in the first experiments with magnetism. Technology has since expanded the availability of magnetic materials to include various manmade products, all based, however, on naturally magnetic elements.

Composites

Ceramic or ferrite

Ceramic, or ferrite, magnets are made of a sintered composite of powdered iron oxide and barium/strontium carbonate ceramic. Due to the low cost of the materials and manufacturing methods, inexpensive magnets (or nonmagnetized ferromagnetic cores, for use in electronic component such as radio antennas, for example) of various shapes can be easily mass produced. The resulting magnets are noncorroding, but brittle and must be treated like other ceramics.

Alnico

Alnico magnets are made by casting or sintering a combination of aluminium, nickel and cobalt with iron and small amounts of other elements added to enhance the properties of the magnet. Sintering offers superior mechanical characteristics, whereas casting delivers higher magnetic fields and allows for the design of intricate shapes. Alnico magnets resist corrosion and have physical properties more forgiving than ferrite, but not quite as desirable as a metal.

Ticonal

Ticonal magnets are an alloy of titanium, cobalt, nickel, and aluminum, with iron and small amounts of other elements. It was developed by Philips for loudspeakers.

Injection molded

Injection molded magnets are a composite of various types of resin and magnetic powders, allowing parts of complex shapes to be manufactured by injection molding. The physical and magnetic properties of the product depend on the raw materials, but are generally lower in magnetic strength and resemble plastics in their physical properties.

Flexible

Flexible magnets are similar to injection molded magnets, using a flexible resin or binder such as vinyl, and produced in flat strips or sheets. These magnets are lower in magnetic strength but can be very flexible, depending on the binder used.

Rare earth magnets

'Rare earth' (lanthanoid) elements have a partially occupied f electron shell (which can accommodate up to 14 electrons.) The spin of these electrons can be aligned, resulting in very strong magnetic fields, and therefore these elements are used in compact high-strength magnets where their higher price is not a concern. The most common types of rare earth magnets are samarium-cobalt and neodymium-iron-boron (NIB) magnets.

Single-molecule magnets (SMMs) and single-chain magnets (SCMs)

In the 1990s it was discovered that certain molecules containing paramagnetic metal ions are capable of storing a magnetic moment at very low temperatures. These are very different from conventional magnets that store information at a "domain" level and theoretically could provide a far denser storage medium than conventional magnets. In this direction research on monolayers of SMMs is currently under way. Very briefly, the two main attributes of an SMM are:
  1. a large ground state spin value (S), which is provided by ferromagnetic or ferrimagnetic coupling between the paramagnetic metal centres.
  2. a negative value of the anisotropy of the zero field splitting (D)
Most SMM's contain manganese, but can also be found with vanadium, iron, nickel and cobalt clusters. More recently it has been found that some chain systems can also display a magnetization which persists for long times at relatively higher temperatures. These systems have been called single-chain magnets.

Nano-structured magnets

Some nano-structured materials exhibit energy waves called magnons that coalesce into a common ground state in the manner of a Bose-Einstein condensate.

Costs

The current cheapest permanent magnets, allowing for field strengths, are neodymium-iron-boron (NIB) magnets. These magnets are more expensive than most other magnetic materials per kg, but due to their intense field are smaller and cheaper in many applications.

Temperature

Temperature sensitivity varies, but when a magnet is heated to a temperature known as the Curie point, it looses all of its magnetism, even after cooling below that temperature. The magnets can often be remagnetised however. Additionally some magnets are brittle and can fracture at high temperatures.

Electromagnets

An electromagnet in its simplest form, is a wire that has been coiled into one or more loops, known as a solenoid. When electric current flows through the wire, a magnetic field is generated. It is concentrated near (and especially inside) the coil, and its field lines are very similar to those for a magnet. The orientation of this effective magnet is determined via the right hand rule. The magnetic moment and the magnetic field of the electromagnet are proportional to the number of loops of wire, to the cross-section of each loop, and to the current passing through the wire.
If the coil of wire is wrapped around a material with no special magnetic properties (e.g., cardboard), it will tend to generate a very weak field. However, if it is wrapped around a "soft" ferromagnetic material, such as an iron nail, then the net field produced can result in a several hundred- to thousandfold increase of field strength.
Uses for electromagnets include particle accelerators, electric motors, junkyard cranes, and magnetic resonance imaging machines. Some applications involve configurations more than a simple magnetic dipole; for example, quadrupole magnets are used to focus particle beams.

Units and calculations in magnetism

How we write the laws of magnetism depends on which set of units we employ. For most engineering applications, MKS or SI (Système International) is common. Two other sets, Gaussian and CGS-emu, are the same for magnetic properties, and are commonly used in physics.
In all units it is convenient to employ two types of magnetic field, B and H, as well as the magnetization M, defined as the magnetic moment per unit volume.
  1. The magnetic induction field B is given in SI units of teslas (T). B is the true magnetic field, whose time-variation produces, by Faraday's Law, circulating electric fields (which the power companies sell). B also produces a deflection force on moving charged particles (as in TV tubes). The tesla is equivalent to the magnetic flux (in webers) per unit area (in meters squared), thus giving B the unit of a flux density. In CGS the unit of B is the gauss (G). One tesla equals 104 G.
  2. The magnetic field H is given in SI units of ampere-turns per meter (A-turn/m). The "turns" appears because when H is produced by a current-carrying wire, its value is proportional to the number of turns of that wire. In CGS the unit of H is the oersted (Oe). One A-turn/m equals 4\pi x 10-3 Oe.
  3. The magnetization M is given in SI units of amperes per meter (A/m). In CGS the unit of M is the emu, or electromagnetic unit. One A/m equals 10-3 emu. A good permanent magnet can have a magnetization as large as a million amperes per meter. Magnetic fields produced by current-carrying wires would require comparably huge currents per unit length, one reason we employ permanent magnets and electromagnets.
  4. In SI units, the relation B = μ0(H + M) holds, where μ0 is the permeability of space, which equals 4\pi x 10-7 tesla meters per ampere. In CGS it is written as B = H + 4πM. [The pole approach gives μ0H in SI units. A μ0M term in SI must then supplement this μ0H to give the correct field within B the magnet. It will agree with the field B calculated using Amperian currents.]
Materials that are not permanent magnets usually satisfy the relation M = χH in SI, where χ is the (dimensionless) magnetic susceptibility. Most non-magnetic materials have a relatively small χ (on the order of a millionth), but soft magnets can have χ on the order of hundreds or thousands. For materials satisfying M = χH, we can also write B = μ0(1 + χ)H = μ0μrH = μH, where μr = 1 + χ is the (dimensionless) relative permeability and \mu=\mu_0\mu_r is the magnetic permeability. Both hard and soft magnets have a more complex, history-dependent, behavior described by what are called hysteresis loops, which give either B vs H or M vs H. In CGS M = χH, but χSI = 4πχCGS, and \mu=\mu_r.
Caution: In part because there are not enough Roman and Greek symbols, there is no commonly agreed upon symbol for magnetic pole strength and magnetic moment. The symbol m has been used for both pole strength (unit = A·m, where here the upright m is for meter) and for magnetic moment (unit = A·m²). The symbol μ has been used in some texts for magnetic permeability and in other texts for magnetic moment. We will use μ for magnetic permeability and m for magnetic moment. For pole strength we will employ qm. For a bar magnet of cross-section A with uniform magnetization M along its axis, the pole strength is given by qm = MA, so that M can be thought of as a pole strength per unit area.

Fields of a magnet

Far away from a magnet, the magnetic field created by that magnet is almost always described (to a good approximation) by a dipole field characterized by its total magnetic moment. This is true regardless of the shape of the magnet, so long as the magnetic moment is nonzero. One characteristic of a dipole field is that the strength of the field falls off inversely with the cube of the distance from the magnet's center.
Closer to the magnet, the magnetic field becomes more complicated, and more dependent on the detailed shape and magnetization of the magnet. Formally, the field can be expressed as a multipole expansion: A dipole field, plus a quadrupole field, plus an octupole field, etc.
At close range, many different fields are possible. For example, for a long, skinny bar magnet with its north pole at one end and south pole at the other, the magnetic field near either end falls off inversely with the square of the distance from that pole.

Calculating the magnetic force

Calculating the attractive or repulsive force between two magnets is, in the general case, an extremely complex operation, as it depends on the shape, magnetization, orientation and separation of the magnets.

Force between two magnetic poles

The force between two magnetic poles is given by:
where
F is force (SI unit: newton)
qm1 and qm2 are the pole strengths (SI unit: ampere-meter)
μ is the permeability of the intervening medium (SI unit: tesla meter per ampere, henry per meter or newton per ampere squared)
r is the separation (SI unit: meter).
The pole description is useful to practicing magneticians who design real-world magnets, but real magnets have a pole distribution more complex than a single north and south. Therefore, implementation of the pole idea is not simple. In some cases, one of the more complex formulae given below will be more useful.

Force between two nearby attracting surfaces of area A and equal but opposite magnetizations M

F=\fracAM^2 http://instruct.tri-c.edu/fgram/web/Mdipole.htm
where
A is the area of each surface, in m²
M is their magnetization, in A/m.
\mu_0 is the permeability of space, which equals 4\pi x 10-7 tesla-meters per ampere

Force between two bar magnets

The force between two identical cylindrical bar magnets placed end-to-end is given by:
F=\left[\frac \right] \left[ + - \right] http://instruct.tri-c.edu/fgram/web/Mdipole.htm
where
B0 is the magnetic flux density very close to each pole, in T,
A is the area of each pole, in m2,
L is the length of each magnet, in m,
R is the radius of each magnet, in m, and
x is the separation between the two magnets, in m
B0 =\fracM relates the flux density at the pole to the magnetization of the magnet.

See also

Online references

Printed references

1. "positive pole n." The Concise Oxford English Dictionary. Ed. Catherine Soanes and Angus Stevenson. Oxford University Press, 2004. Oxford Reference Online. Oxford University Press.
2. Wayne M. Saslow, "Electricity, Magnetism, and Light", Academic (2002). ISBN 0-12-619455-6. Chapter 9 discusses magnets and their magnetic fields using the concept of magnetic poles, but it also gives evidence that magnetic poles don't really exist in ordinary matter. Chapters 10 and 11, following what appears to be a 19th century approach, use the pole concept to obtain the laws describing the magnetism of electric currents.
3. Edward P. Furlani, "Permanent Magnet and Electromechanical Devices: Materials, Analysis and Applications", Academic Press Series in Electromagnetism (2001). ISBN 0-12-269951-3.
magnet in Arabic: مغناطيس
magnet in Aymara: Achkatasiri
magnet in Bulgarian: Магнит
magnet in Catalan: Imant
magnet in Czech: Magnet
magnet in Danish: Magnet
magnet in German: Magnet
magnet in Spanish: Imán (física)
magnet in Esperanto: Magneto
magnet in Persian: آهنربا
magnet in French: Aimant
magnet in Galician: Imán
magnet in Korean: 자석
magnet in Croatian: Magnet
magnet in Indonesian: Magnet
magnet in Icelandic: Segull
magnet in Italian: Magnete
magnet in Hebrew: מגנט
magnet in Latin: Magnes
magnet in Lithuanian: Magnetas
magnet in Hungarian: Mágnes
magnet in Dutch: Permanente magneet
magnet in Japanese: 磁石
magnet in Norwegian: Magnet
magnet in Norwegian Nynorsk: Magnet
magnet in Polish: Magnes
magnet in Portuguese: Íman
magnet in Russian: Магнит
magnet in Simple English: Magnet
magnet in Sindhi: مقناطيس
magnet in Slovak: Magnet
magnet in Slovenian: Magnet
magnet in Serbian: Магнет
magnet in Sundanese: Magnét
magnet in Finnish: Magneetti
magnet in Swedish: Magnet
magnet in Vietnamese: Nam châm
magnet in Turkish: Mıknatıs
magnet in Ukrainian: Магніт
magnet in Yiddish: מאגנעט
magnet in Yoruba: Gbéringbérin
magnet in Chinese: 磁鐵

Synonyms, Antonyms and Related Words

ambition, artificial magnet, bar magnet, catch, center of attraction, center of consciousness, center of interest, cynosure, dearest wish, desideration, desideratum, desire, electromagnetic lifting magnet, field magnet, focal point, focus, focus of attention, forbidden fruit, glimmering goal, golden vision, hope, horseshoe magnet, lodestar, lodestone, magnetic needle, magnetite, paramagnet, plum, point of convergence, polestar, prime focus, prize, solenoid, temptation, trophy, wish
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