Magnet

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A "horseshoe magnet" made of alnico, an iron alloy. The magnet is made in the shape of a horseshoe to bring the two magnetic poles close to each other, to create a strong magnetic field there that can pick up heavy pieces of iron
Magnetic field lines of a solenoid electromagnet, which are similar to a bar magnet as illustrated above with the iron filings

A magnet (from Greek μαγνήτις λίθος magnḗtis líthos, "Magnesian stone") 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 ferromagnetic materials, such as iron, and attracts or repels other magnets.

A permanent magnet is an object made from a material that is magnetized and creates its own persistent magnetic field. An everyday example is a refrigerator magnet used 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 ferromagnetic (or ferrimagnetic). These include iron, nickel, cobalt, some alloys of rare earth metals, 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 annealed 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 alnico and ferrite that are subjected to special processing in a powerful magnetic field during manufacture, to align their internal microcrystalline structure, making them very hard to demagnetize. To demagnetize a saturated magnet, a certain magnetic field must be applied, and this threshold depends on coercivity of the respective material. "Hard" materials have high coercivity, whereas "soft" materials have low coercivity.

An electromagnet is made from a coil of wire that acts as a magnet when an electric current passes through it but stops being a magnet when the current stops. Often, the coil is wrapped around a core of "soft" ferromagnetic material such as steel, which greatly enhances the magnetic field produced by the coil.

The overall strength of a magnet is measured by its magnetic moment or, alternatively, the total magnetic flux it produces. The local strength of magnetism in a material is measured by its magnetization.

Discovery and development

Ancient people learned about magnetism from lodestones, 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",[1] a part of ancient Greece where lodestones were found. Lodestones suspended so they could turn were the first magnetic compasses. The earliest known surviving descriptions of magnets and their properties are from Greece, India, and China around 2500 years ago.[2][3][4] The properties of lodestones and their affinity for iron were written of by Pliny the Elder in his encyclopedia Naturalis Historia.[5]

By the 12th to 13th centuries AD, magnetic compasses were used in navigation in China, Europe, and elsewhere.[6]

Physics

Magnetic field

Iron filings that have oriented in the magnetic field produced by a bar magnet

The magnetic flux density (also called magnetic B field or just magnetic field, usually denoted B) is a vector field. The magnetic B field vector at a given point in space is specified by two properties:

  1. Its direction, which is along the orientation of a compass needle.
  2. Its magnitude (also called strength), which is proportional to how strongly the compass needle orients along that direction.

In SI units, the strength of the magnetic B field is given in teslas.[7]

Magnetic moment

A magnet's magnetic moment (also called magnetic dipole moment and usually denoted μ) is a vector 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,[8] and the magnitude relates to how strong and how far apart these poles are. In SI units, the magnetic moment is specified in terms of A•m2 (amperes times meters squared).

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 torque tending to orient the magnetic moment parallel to the field.[9] The 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.[10]

A wire in the shape of a circle with area A and carrying current I is a magnet, with a magnetic moment of magnitude equal to IA.

Magnetization

The magnetization of a magnetized material is the local value of its magnetic moment per unit volume, usually denoted M, with units A/m.[11] It 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.

Modelling magnets

Field of a cylindrical bar magnet calculated with Ampère's model

Two different models exist for magnets: magnetic poles and atomic currents.

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.[citation needed]

In this approach, the divergence of the magnetization ∇•M inside a magnet and the surface normal component Mn 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. Outside the magnet, the field B is proportional to H, while inside the magnetization must be added to H. An extension of this method that allows for internal magnetic charges is used in theories of ferromagnetism.

Another model is the Ampère model, where all magnetization is due to the effect of microscopic, or atomic, circular bound currents, 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 current flowing around the surface, with local flow direction normal to the cylinder axis.[12] Microscopic currents in atoms inside the material are generally canceled by currents in neighboring atoms, so only the surface makes a net contribution; shaving off the outer layer of a magnet will not destroy its magnetic field, but will leave a new surface of uncancelled currents from the circular currents throughout the material.[13] The right-hand rule tells which direction the current flows.

Pole naming conventions

The north pole of a magnet is the pole that, when the magnet is freely suspended, points towards the Earth's North Magnetic Pole in the Arctic. Since opposite poles (north and south) attract, the North Magnetic Pole is actually the south pole of the Earth's magnetic field.[14][15][16][17] As a practical matter, to tell which pole of 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.[17]

Magnetic materials

The 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

Hard disk drives record 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

Medical issues and safety

Because human tissues have a very low level of susceptibility to static magnetic fields, there is little mainstream scientific evidence showing a health hazard associated with exposure to static fields. Dynamic magnetic fields may be a different issue, however; correlations between electromagnetic radiation and cancer rates have been postulated due to demographic correlations (see Electromagnetic radiation and health).

If a ferromagnetic foreign body is present in human tissue, an external magnetic field interacting with it can pose a serious safety risk.[23]

A different type of indirect magnetic health risk exists involving pacemakers. If a pacemaker has been embedded in a patient's chest (usually for the purpose of monitoring and regulating the heart for steady electrically induced beats), 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.[24]

Magnetizing ferromagnets

Ferromagnetic materials can be magnetized in the following ways:

Demagnetizing ferromagnets

Magnetized ferromagnetic materials can be demagnetized (or degaussed) in the following ways:

Types of permanent magnets

A stack of ferrite 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). Because of the way their regular crystalline atomic structure causes their spins to interact, some metals are ferromagnetic when found in their natural states, as ores. These include iron ore (magnetite or lodestone), cobalt and nickel, as well as the rare earth metals gadolinium and dysprosium (when at a very low temperature). Such naturally occurring ferromagnets 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.

Composites

Ceramic, or ferrite, magnets are made of a sintered composite of 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 components such as radio antennas, for example) of various shapes can be easily mass-produced. The resulting magnets are non-corroding but brittle and must be treated like other ceramics.

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. Trade names for alloys in this family include: Alni, Alcomax, Hycomax, Columax, and Ticonal.[25]

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 magnets are composed of a high-coercivity ferromagnetic compound (usually ferric oxide) mixed with a plastic binder.[26] This is extruded as a sheet and passed over a line of powerful cylindrical permanent magnets. These magnets are arranged in a stack with alternating magnetic poles facing up (N, S, N, S...) on a rotating shaft. This impresses the plastic sheet with the magnetic poles in an alternating line format. No electromagnetism is used to generate the magnets. The pole-to-pole distance is on the order of 5mm, but varies with manufacturer. These magnets are lower in magnetic strength but can be very flexible, depending on the binder used.

Rare-earth magnets

An ovoid-shaped rare-earth magnet hanging from another

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 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:

  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 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 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.[27][28]

Costs

The current 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[citation needed] 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-iron-boron (NIB) magnets 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.[29]

Temperature

Temperature 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.

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 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.[30]

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 and sextupole magnets are used to focus particle beams.

Units and calculations

For most engineering applications, MKS (rationalized) or SI (Système International) units are commonly used. Two other sets of units, 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 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 4π×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 oersted (Oe). One A/m equals 10−3 emu/cm3. A good permanent magnet can have a magnetization as large as a million amperes per meter.
  4. In SI units, the relation B = μ0(H + M) holds, where μ0 is the permeability of space, which equals 4π×10−7 T•m/A. 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 Ampèrian 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 μ =μ0μr 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 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 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 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 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

Pull force of a single magnet

The strength of a given magnet is sometimes given in terms of its pull force— its ability to move (push/ pull) other objects. The pull force exerted by either an electromagnet or a permanent magnet at the "air gap" (i.e., the point in space where the magnet ends) is given by the Maxwell equation:[31]

F={{B^2 A}\over{2 \mu_{0}}},

where

F is force (SI unit: newton)
A is the cross section of the area of the pole in square meters
B is the magnetic induction exerted by the magnet

Therefore, if a magnet is acting vertically, it can lift a mass m in kilograms given by the simple equation:

m={{B^2 A}\over{2 \mu_{0} g_{n}}}.

Force between two magnetic poles

Classically, the force between two magnetic poles is given by:[32]

F={{\mu q_{m1} q_{m2}}\over{4\pi r^2}}

where

F is force (SI unit: newton)
qm1 and qm2 are the magnitudes of magnetic poles (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 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 A

The 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 fringing is negligible and the volume of the air gap is much smaller than that of the magnetized material:[33][34]

F=\frac{\mu_0 H^2 A}{2} = \frac{B^2 A}{2 \mu_0}

where:

A is the area of each surface, in m2
H is their magnetizing field, in A/m
μ0 is the permeability of space, which equals 4π×10−7 T•m/A
B is the flux density, in T.

Force between two bar magnets

The force between two identical cylindrical bar magnets placed end to end is given by:[33]

F=\left[\frac {B_0^2 A^2 \left( L^2+R^2 \right)} {\pi\mu_0L^2}\right] \left[{\frac 1 {x^2}} + {\frac 1 {(x+2L)^2}} - {\frac 2 {(x+L)^2}} \right]

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.
B_0 \,=\, \frac{\mu_0}{2}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 methods must be used.

Force between two cylindrical magnets

For 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,[35]

 F(x) = \frac{\pi\mu_0}{4} M^2 R^4 \left[\frac{1}{x^2} + \frac{1}{(x+2t)^2} - \frac{2}{(x + t)^2}\right]

where M is the magnetization of the magnets and x is the gap between the magnets. In disagreement to the statement in the previous section, a measurement of the magnetic flux density very close to the magnet  B_0 is related to M by the formula

 B_0 = \mu_0 M

The effective magnetic dipole can be written as

 m = M V

Where V is the volume of the magnet. For a cylinder, this is V = \pi R^2 t.

When  t << x , the point dipole approximation is obtained,

 F(x) = \frac{3\pi\mu_0}{2} M^2 R^4 t^2\frac{1}{x^4} = \frac{3\mu_0}{2\pi} M^2 V^2\frac{1}{x^4} = \frac{3\mu_0}{2\pi} m_1 m_2\frac{1}{x^4}

which matches the expression of the force between two magnetic dipoles.

See also

Notes

  1. ^ The location of Magnesia is debated; it could be the regional unit or Magnesia ad Sipylum. See, for example, "Magnet". Language Hat blog. 28 May 2005. Retrieved 22 March 2013. 
  2. ^ Fowler, Michael (1997). "Historical Beginnings of Theories of Electricity and Magnetism". Retrieved 2008-04-02. 
  3. ^ Vowles, Hugh P. (1932). "Early Evolution of Power Engineering". Isis 17 (2): 412–420 [419–20]. doi:10.1086/346662. 
  4. ^ Li Shu-hua (1954). "Origine de la Boussole II. Aimant et Boussole". Isis 45 (2): 175. JSTOR 227361. 
  5. ^ Pliny the Elder, The Natural History, BOOK XXXIV. THE NATURAL HISTORY OF METALS., CHAP. 42.—THE METAL CALLED LIVE IRON. Perseus.tufts.edu. Retrieved on 2011-05-17.
  6. ^ Schmidl, Petra G. (1996–1997). "Two Early Arabic Sources On The Magnetic Compass". Journal of Arabic and Islamic Studies 1: 81–132. 
  7. ^ Griffiths, David J. (1999). Introduction to Electrodynamics (3rd ed.). Prentice Hall. pp. 255–8. ISBN 0-13-805326-X. OCLC 40251748. 
  8. ^ Knight, Jones, & Field, "College Physics" (2007) p. 815
  9. ^ B. D. Cullity, C. D. Graham (2008). Introduction to Magnetic Materials (2 ed.). Wiley-IEEE Press. p. 103. ISBN 0-471-47741-9. 
  10. ^ Boyer, Timothy H. (1988). "The Force on a Magnetic Dipole". American Journal of Physics 56 (8): 688–692. Bibcode:1988AmJPh..56..688B. doi:10.1119/1.15501. 
  11. ^ "Units for Magnetic Properties". Lake Shore Cryotronics, Inc. Archived from the original on 2011-07-14. Retrieved 2012-11-05. 
  12. ^ Zachariah Allen (1852). Philosophy of the Mechanics of Nature, and the Source and Modes of Action of Natural Motive-Power. D. Appleton and Company. p. 252. 
  13. ^ Wayne M. Saslow (2002). Electricity, Magnetism, and Light (3rd ed.). Academic Press. p. 426. ISBN 978-0-12-619455-5. 
  14. ^ Serway, Raymond A.; Chris Vuille (2006). Essentials of college physics. USA: Cengage Learning. p. 493. ISBN 0-495-10619-4. 
  15. ^ Emiliani, Cesare (1992). Planet Earth: Cosmology, Geology, and the Evolution of Life and Environment. UK: Cambridge University Press. p. 228. ISBN 0-521-40949-7. 
  16. ^ Manners, Joy (2000). Static Fields and Potentials. USA: CRC Press. p. 148. ISBN 0-7503-0718-8. 
  17. ^ a b Nave, Carl R. (2010). "Bar Magnet". Hyperphysics. Dept. of Physics and Astronomy, Georgia State Univ. Retrieved 2011-04-10. 
  18. ^ Mice levitated in NASA lab. Livescience.com (2009-09-09). Retrieved on 2011-10-08.
  19. ^ Mallinson, John C. (1987). The foundations of magnetic recording (2nd ed.). Academic Press. ISBN 0-12-466626-4. 
  20. ^ "The stripe on a credit card". How Stuff Works. Retrieved July 2011. 
  21. ^ "Electromagnetic deflection in a cathode ray tube, I". National High Magnetic Field Laboratory. Retrieved July 2011. 
  22. ^ "Snacks about magnetism". The Exploratorium Science Snacks. Exploratorium. Retrieved 17 April 2013. 
  23. ^ Schenck JF (2000). "Safety of strong, static magnetic fields". J Magn Reson Imaging 12 (1): 2–19. doi:10.1002/1522-2586(200007)12:1<2::AID-JMRI2>3.0.CO;2-V. PMID 10931560. 
  24. ^ Oestreich AE (2008). "Worldwide survey of damage from swallowing multiple magnets". Pediatr Radiol 39 (2): 142. doi:10.1007/s00247-008-1059-7. PMID 19020871. 
  25. ^ Brady, George Stuart; Henry R. Clauser & John A. Vaccari (2002). Materials Handbook: An Encyclopedia for Managers. McGraw-Hill Professional. p. 577. ISBN 0-07-136076-X. 
  26. ^ "Fridge Magnet Transformed". Phys.Org. Retrieved March 11, 2011. 
  27. ^ "Nanomagnets Bend The Rules". Retrieved November 14, 2005. 
  28. ^ Della Torre, E.; Bennett, L.; Watson, R. (2005). "Extension of the Bloch T3/2 Law to Magnetic Nanostructures: Bose-Einstein Condensation". Physical Review Letters 94 (14): 147210. Bibcode:2005PhRvL..94n7210D. doi:10.1103/PhysRevLett.94.147210. 
  29. ^ Frequently Asked Questions. Magnet sales. Retrieved on 2011-10-08.
  30. ^ Ruskell‬, ‪Todd‬; Tipler‬, ‪Paul A. ‬; Mosca‬, ‪Gene‬ (2007‬). Physics for Scientists and Engineers‬ (6‬ ed.). Palgrave Macmillan. ISBN 1-4292-0410-9. 
  31. ^ Cardarelli, François (2008). Materials Handbook: A Concise Desktop Reference (Second ed.). Springer. p. 493. ISBN 9781846286681. 
  32. ^ "Basic Relationships". Geophysics.ou.edu. Retrieved 2009-10-19. 
  33. ^ a b "Magnetic Fields and Forces". Retrieved 2009-12-24. 
  34. ^ "The force produced by a magnetic field". Retrieved 2010-03-09. 
  35. ^ David Vokoun, Marco Beleggia, Ludek Heller, Petr Sittner (2009). "Magnetostatic interactions and forces between cylindrical permanent magnets". Journal of Magnetism and Magnetic Materials 321 (22): 3758–3763. Bibcode:2009JMMM..321.3758V. doi:10.1016/j.jmmm.2009.07.030. 

References

  • "The Early History of the Permanent Magnet". Edward Neville Da Costa Andrade, Endeavour, Volume 17, Number 65, January 1958. Contains an excellent description of early methods of producing permanent magnets.
  • "positive pole n". The Concise Oxford English Dictionary. Catherine Soanes and Angus Stevenson. Oxford University Press, 2004. Oxford Reference Online. Oxford University Press.
  • 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 do not 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.
  • Edward P. Furlani, Permanent Magnet and Electromechanical Devices:Materials, Analysis and Applications, Academic Press Series in Electromagnetism (2001). ISBN 0-12-269951-3.

External links