A magnet (from Greek μαγνήτις λίθος magnḗtis líthos, "Magnesia_ad_Sipylumstone") is a material or object that produces a Magnetic_field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other Ferromagneticmaterials, such as Iron, and attracts or repels other magnets.
A permanent magnet is an object made from a material that is Magnetizeand creates its own persistent magnetic field. An everyday example is a Refrigerator_magnetused to hold notes on a refrigerator door. Materials that can be magnetized, which are also the ones that are strongly attracted to a magnet, are called Ferromagnetism(or Ferrimagnetic). These include Iron, Nickel, Cobalt, some alloys of Rare_earth_element, and some naturally occurring minerals such as Lodestone. Although ferromagnetic (and ferrimagnetic) materials are the only ones attracted to a magnet strongly enough to be commonly considered magnetic, all other substances respond weakly to a magnetic field, by one of several other types of Magnetism.
Ferromagnetic materials can be divided into magnetically "soft" materials like Annealing_(metallurgy) Iron, which can be magnetized but do not tend to stay magnetized, and magnetically "hard" materials, which do. Permanent magnets are made from "hard" ferromagnetic materials such as Alnicoand Ferritethat are subjected to special processing in a powerful magnetic field during manufacture, to align their internal Crystallitestructure, making them very hard to demagnetize. To demagnetize a saturated magnet, a certain magnetic field must be applied, and this threshold depends on Coercivityof the respective material. "Hard" materials have high coercivity, whereas "soft" materials have low coercivity.
An Electromagnetis made from a coil of wire that acts as a magnet when an Electric_currentpasses through it but stops being a magnet when the current stops. Often, the coil is wrapped around a core of ferromagnetic material like steel, which enhances the magnetic field produced by the coil.
The overall strength of a magnet is measured by its Magnetic_momentor, alternatively, the total Magnetic_fluxit produces. The local strength of magnetism in a material is measured by its Magnetization.
Contents[Answers.com]See also: Magnetism
Ancient people learned about magnetism from Lodestone, naturally magnetized pieces of iron ore. They are naturally created magnets, which attract pieces of iron. The word magnet in Greek meant "stone from Magnesia_(peripheral_unit)", a part of ancient Greece where lodestones were found. Lodestones suspended so they could turn were the first Magnetic_compass. The earliest known surviving descriptions of magnets and their properties are from Greece, India, and China around 2500 years ago.Answers.comAnswers.comAnswers.comThe properties of Lodestoneand their affinity for iron were written of by Pliny_the_Elderin his encyclopedia Naturalis_Historia.Answers.com
By the 12th to 13th centuries AD, magnetic Compasswere used in navigation in China, Europe, and elsewhere.Answers.com
Background on the physics of magnetism and magnetsFile:The_Effects_of_Magnetism.JPGFile:The_Effects_of_Magnetism.JPGAn ovoid-shaped Rare_earth_magnethanging from another
Magnetic fieldMain article: Magnetic_fieldThe Magnetic_field(also called magnetic B field or just magnetic field, usually denoted B) is a Vector_field. The magnetic B field Euclidean_vectorat a given point in space is specified by two properties:
In SIunits, the strength of the magnetic B field is given in Tesla_(unit).Answers.com
Magnetic momentMain article: Magnetic_momentA magnet's magnetic moment (also called magnetic dipole moment and usually denoted μ) is a Vector_(geometry) that characterizes the magnet's overall magnetic properties. For a bar magnet, the direction of the magnetic moment points from the magnet's south pole to its north pole,Answers.comand the magnitude relates to how strong and how far apart these poles are. In SIunits, the magnetic moment is specified in terms of A·m2.
A magnet both produces its own magnetic field and responds to magnetic fields. The strength of the magnetic field it produces is at any given point proportional to the magnitude of its magnetic moment. In addition, when the magnet is put into an external magnetic field, produced by a different source, it is subject to a Torquetending to orient the magnetic moment parallel to the field.Answers.comThe amount of this torque is proportional both to the magnetic moment and the external field. A magnet may also be subject to a force driving it in one direction or another, according to the positions and orientations of the magnet and source. If the field is uniform in space, the magnet is subject to no net force, although it is subject to a torque.Answers.com
A wire in the shape of a circle with area A and carrying Electric_currentI is a magnet, with a magnetic moment of magnitude equal to IA.
MagnetizationMain article: MagnetizationThe magnetization of a magnetized material is the local value of its magnetic moment per unit volume, usually denoted M, with units Meter.Answers.comIt is a Vector_field, rather than just a vector (like the magnetic moment), because different areas in a magnet can be magnetized with different directions and strengths (for example, because of domains, see below). A good bar magnet may have a magnetic moment of magnitude 0.1 A·m2 and a volume of 1 cm3, or 1×10−6 m3, and therefore an average magnetization magnitude is 100,000 A/m. Iron can have a magnetization of around a million amperes per meter. Such a large value explains why iron magnets are so effective at producing magnetic fields.
Two models for magnets: magnetic poles and atomic currentsSee also: Magnetic_momentMagnetic polesFile:VFPt_cylindrical_magnet_thumb.svgFile:VFPt_cylindrical_magnet_thumb.svgField of a cylindrical bar magnet calculated with Ampère's model
Although for many purposes it is convenient to think of a magnet as having distinct north and south magnetic poles, the concept of poles should not be taken literally: it is merely a way of referring to the two different ends of a magnet. The magnet does not have distinct north or south particles on opposing sides. If a bar magnet is broken into two pieces, in an attempt to separate the north and south poles, the result will be two bar magnets, each of which has both a north and south pole.
However, a version of the magnetic-pole approach is used by professional magneticians to design permanent magnets. In this approach, the divergence of the magnetization ∇•M inside a magnet and the surface normal component M•n are treated as a distribution of Magnetic_monopoles. This is a mathematical convenience and does not imply that there are actually monopoles in the magnet. If the magnetic-pole distribution is known, then the pole model gives the magnetic field H (see also Demagnetizing_field). Outside the magnet, the field B is proportional to H, while inside the magnetization must be added to H (see Answers.com). An extension of this method that allows for internal magnetic charges is used in theories of ferromagnetism (see Micromagnetics).
Ampère modelAnother model is the André-Marie_Ampère model, where all magnetization is due to the effect of microscopic, or atomic, circular Bound_current, also called Ampèrian currents, throughout the material. For a uniformly magnetized cylindrical bar magnet, the net effect of the microscopic bound currents is to make the magnet behave as if there is a macroscopic sheet of Electric_currentflowing around the surface, with local flow direction normal to the cylinder axis. (Since scraping off the outer layer of a magnet will not destroy its magnetic field, it can be seen that this is just a model, and the tiny currents are actually distributed throughout the material). The Right-hand_ruletells which direction the current flows. It is usually difficult to calculate the Ampèrian currents on the surface of a magnet, whereas it is often easier to find the effective poles for the same magnet.
Pole naming conventionsThe north pole of a magnet is the pole that, when the magnet is freely suspended, points towards the Earth's North_Magnetic_Polewhich is located in northern Canada. Since opposite poles (north and south) attract, the Earth's "North Magnetic Pole" is thus actually the south pole of the Earth's magnetic field.Answers.comAnswers.comAnswers.comAnswers.comAs a practical matter, in order to tell which Poleof a magnet is north and which is south, it is not necessary to use the Earth's magnetic field at all. For example, one method would be to compare it to an Electromagnet, whose poles can be identified by the Right-hand_rule. The magnetic field lines of a magnet are considered by convention to emerge from the magnet's north pole and reenter at the south pole.Answers.comMagnetic materialsMain article: MagnetismThe term magnet is typically reserved for objects that produce their own persistent magnetic field even in the absence of an applied magnetic field. Only certain classes of materials can do this. Most materials, however, produce a magnetic field in response to an applied magnetic field; a phenomenon known as magnetism. There are several types of magnetism, and all materials exhibit at least one of them.
The overall magnetic behavior of a material can vary widely, depending on the structure of the material, particularly on its Electron_configuration. Several forms of magnetic behavior have been observed in different materials, including:
There are various other types of magnetism, such as Spin_glass, Superparamagnetism, Superdiamagnetism, and Metamagnetism.
Common uses of magnetsFile:Hard_disk.jpgFile:Hard_disk.jpgHard_disksrecord data on a thin magnetic coating.
Magnetic hand separator for heavy minerals
Magnets have many uses in Toys. M-tic uses magnetic rods connected to metal spheres for Construction. Note the geodesic pyramid.
If a ferromagnetic foreign body is present in human tissue, an external magnetic field interacting with it can pose a serious safety risk.Answers.com
A different type of indirect magnetic health risk exists involving pacemakers. If a Pacemakerhas been embedded in a patient's chest (usually for the purpose of monitoring and regulating the heart for steady electrically induced Cardiac_cycle), care should be taken to keep it away from magnetic fields. It is for this reason that a patient with the device installed cannot be tested with the use of an MRI, which is a magnetic imaging device.
Children sometimes swallow small magnets from toys, and this can be hazardous if two or more magnets are swallowed, as the magnets can pinch or puncture internal tissues; one death has been reported.Answers.com
Magnetizing ferromagnetsSee also: RemanenceFerromagneticmaterials can be magnetized in the following ways:
A stack of Ferrite_magnets
Magnetic metallic elementsMany 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). Because of the way their regular CrystallineAtomic_structurecauses their spins to interact, some Metalare (ferro)magnetic when found in their natural states, as Ore. These include Iron_ore(Magnetiteor Lodestone), Cobaltand Nickel, as well as the rare earth metals Gadoliniumand 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 man-made products, all based, however, on naturally magnetic elements. CompositesCeramic or ferriteMain article: Ferrite_(magnet)Ceramic, or ferrite, magnets are made of a SinteredAlloyof powdered iron oxide and barium/strontium carbonate Ceramic. Given the low cost of the materials and manufacturing methods, inexpensive magnets (or non-magnetized ferromagnetic cores, for use in Electronic_componentsuch as Radio_antennas, for example) of various shapes can be easily mass-produced. The resulting magnets are non-corroding but Brittleand must be treated like other ceramics.
AlnicoMain article: AlnicoAlnico magnets are made by Casting_(metalworking) or Sinteringa combination of Aluminium, Nickeland Cobaltwith Ironand 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. Trade names for alloys in this family include: Alni, Alcomax, Hycomax, Columax, and Ticonal.Answers.com
Injection-moldedInjection_moldingmagnets are a Mixtureof various types of Resinand 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 Plasticin their physical properties.
FlexibleFlexible magnets are similar to injection-molded magnets, using a flexible resin or binder such as Vinyl, and produced in flat strips, shapes or sheets. These magnets are lower in magnetic strength but can be very flexible, depending on the binder used. Flexible magnets can be used in industrial printers.
Rare-earth magnetsMain article: Rare-earth_magnetRare 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_magnetand Neodymium_magnetmagnets.
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 magnetic 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:Most SMMs 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 that persists for long times at higher temperatures. These systems have been called single-chain magnets.
Nano-structured magnetsSome nano-structured materials exhibit energy Wave, called Magnon, that coalesce into a common ground state in the manner of a Bose-Einstein_condensate.Answers.comAnswers.comCostsThe current[update] cheapest permanent magnets, allowing for field strengths, are flexible and ceramic magnets, but these are also among the weakest types. The ferrite magnets are mainly low-cost magnets since they are made from cheap raw materials- iron oxide and Ba- or Sr-carbonate. However, a new low cost magnet- Mn-Al alloy has been developed and is now dominating the low-cost magnets field. It has a higher saturation magnetization than the ferrite magnets. It also has more favorable temperature coefficients, although it can be thermally unstable. Neodymium_magnetmagnets are among the strongest. These cost more per kilogram than most other magnetic materials but, owing to their intense field, are smaller and cheaper in many applications.Answers.comTemperatureTemperature sensitivity varies, but when a magnet is heated to a temperature known as the Curie_point, it loses all of its magnetism, even after cooling below that temperature. The magnets can often be remagnetized, however.Additionally, some magnets are brittle and can fracture at high temperatures.
The maximum usable temperature is highest for alnico magnets at over 540 °C (1,000 °F), around 300 °C (570 °F) for ferrite and SmCo, about 140 °C (280 °F) for NIB and lower for flexible ceramics, but the exact numbers depend on the grade of material.
ElectromagnetsMain article: ElectromagnetAn 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 of a magnet. The orientation of this effective magnet is determined by 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.Answers.com
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_accelerator, Electric_motor, junkyard cranes, and Magnetic_resonance_imagingmachines. Some applications involve configurations more than a simple magnetic dipole; for example, Quadrupole_magnetand Sextupole_magnetare used to Strong_focusingParticle_beam.
Units and calculationsMain article: MagnetostaticsFor most engineering applications, MKS (rationalized) or SI(Système International) units are commonly used. Two other sets of units, Gaussian_unitsand CGS, 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 MagnetizationM, defined as the magnetic moment per unit volume.
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 μ =μ0μr is the magnetic permeability. Both hard and soft magnets have a more complex, history-dependent, behavior described by what are called Hysteresis, which give either B vs. H or M vs. H. In CGS, M = χH, but χSI = 4πχCGS, and μ = μr.
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 Mcan be thought of as a pole strength per unit area.
Fields of a magnetFar away from a magnet, the magnetic field created by that magnet is almost always described (to a good approximation) by a Dipolecharacterized by its total magnetic moment. This is true regardless of the shape of the magnet, so long as the magnetic moment is non-zero. 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, 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 forceMain article: Force_between_magnetsForce between two magnetic polesFurther information: Magnetic_momentClassical_mechanics, the force between two magnetic poles is given by:Answers.com
whereF is force (SI unit: Newton_(unit))qm1 and qm2 are the magnitudes of magnetic poles (SI unit: Ampere-meter)μis the Permeability_(electromagnetism) of the intervening medium (SI unit: Tesla_(unit) Meterper Ampere, henry per meter or newton per ampere squared)r is the separation (SI unit: meter).
The pole description is useful to the engineers designing 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 magnetized surfaces of area AThe mechanical force between two nearby magnetized surfaces can be calculated with the following equation. The equation is valid only for cases in which the effect of http://wiki.answers.com/w/index.php?title=Fringing&action=edit&redlink=1is negligible and the volume of the air gap is much smaller than that of the magnetized material:Answers.comAnswers.com
where:A is the area of each surface, in m2H is their magnetizing field, in A/mμ0 is the permeability of space, which equals 4π×10−7 T·m/AB is the flux density, in T.
Force between two bar magnetsThe force between two identical cylindrical bar magnets placed end to end is given by:Answers.com
whereB0 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, andx is the separation between the two magnets, in m. relates the flux density at the pole to the magnetization of the magnet.
Note that all these formulations are based on Gilbert's model, which is usable in relatively great distances. In other models (e.g., Ampère's model), a more complicated formulation is used that sometimes cannot be solved analytically. In these cases, Numerical_methodsmust be used.
Force between two cylindrical magnetsFor two cylindrical magnets with radius R and height t, with their magnetic dipole aligned, the force can be well approximated (even at distances of the order of t) by,Answers.com
where M is the magnetization of the magnets and xis the distance between them. In disagreement to the statement in the previous section, a measurement of the magnetic flux density very close to the magnet B0 is related to M by the formulaB0 = μ0M
The effective magnetic dipole can be written asm = MV
Where V is the volume of the magnet. For a cylinder, this is V = πR2t.
When t < < x, the point dipole approximation is obtained,
which matches the expression of the force between two magnetic dipoles.