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Magnetism

2007 Schools Wikipedia Selection. Related subjects: Electricity and
Electronics

   Electromagnetism
   Electricity · Magnetism
           Electrostatics
   Electric charge
   Coulomb's law
   Electric field
   Gauss's law
   Electric potential
   Electric dipole moment
           Magnetostatics
   Ampère's law
   Magnetic field
   Magnetic dipole moment
          Electrodynamics
   Electric current
   Lorentz force law
   Electromotive force
   (EM) Electromagnetic induction
   Faraday-Lenz law
   Displacement current
   Maxwell's equations
   (EMF) Electromagnetic field
   (EM) Electromagnetic radiation
         Electrical Network
   Electrical conduction
   Electrical resistance
   Capacitance
   Inductance
   Impedance
   Resonant cavities
   Waveguides
   Magnetic lines of force of a bar magnet shown by iron filings on paper
   Magnetic lines of force of a bar magnet shown by iron filings on paper

   In physics, magnetism is one of the phenomena by which materials exert
   an attractive or repulsive force on other materials. Some well known
   materials that exhibit easily detectable magnetic properties are
   nickel, iron, some steels, and the mineral magnetite; however, all
   materials are influenced to greater or lesser degree by the presence of
   a magnetic field.

History

   In China, the earliest literary reference to magnetism lies in a 4th
   century BC book called Book of the Devil Valley Master (鬼谷子): "The
   lodestone makes iron come or it attracts it." The earliest mention of
   the attraction of a needle appears in a work composed between 20 and
   100 AD (Louen-heng): "A lodestone attracts a needle." By the 12th
   century the Chinese were known to use the lodestone compass for
   navigation. Far earlier Magnetotactic bacteria had evolved to build
   miniature magnets inside themselves and use them to establish their
   orientation relative to the Earth's magnetic field .

Physics of magnetism

   Magnetic forces are forces that arise from the movement of electrical
   charge. Maxwell's equations and the Biot-Savart law describe the origin
   and behaviour of the fields that govern these forces. Thus, magnetism
   is seen whenever electrically charged particles are in motion. This can
   arise either from movement of electrons in an electric current,
   resulting in " electromagnetism", or from the quantum-mechanical spin
   and orbital motion of electrons, resulting in what are known as "
   permanent magnets". Electron spin is the dominant effect within atoms.
   The so-called 'orbital motion' of electrons around the nucleus is a
   secondary effect that slightly modifies the magnetic field created by
   spin.

   The magnetic force is actually due to the finite speed (the speed of
   light) of a disturbance of the electric field which gives rise to
   forces that appear to be acting along a line at right angles to the
   charges. In effect, the magnetic force is the portion of the electric
   force directed to where the charge used to be. For this reason
   magnetism can be considered to be basically an electric force that is a
   direct consequence of relativity.

Charged particle in a magnetic field

   When a charged particle moves through a magnetic field B, it feels a
   force F given by the cross product:

          \vec{F} = q \vec{v} \times \vec{B}

   where q\, is the electric charge of the particle, \vec{v} \, is the
   velocity vector of the particle, and \vec{B} \, is the magnetic field.

   Because this is a cross product, the force is perpendicular to both the
   motion of the particle and the magnetic field. It follows that the
   magnetic force does no work on the particle; it may change the
   direction of the particle's movement, but it cannot cause it to speed
   up or slow down.

   One tool for determining the direction of the velocity vector of a
   moving charge, the magnetic field, and the force exerted is labeling
   the index finger "V", the middle finger "B", and the thumb "F" with
   your right hand. When making a gun-like configuration (with the middle
   finger crossing under the index finger), the fingers represent the
   velocity vector, magnetic field vector, and force vector, respectively.
   See also right hand rule.

Magnetic dipoles

   Normally, magnetic fields are seen as dipoles, having a " South pole"
   and a " North pole"; terms dating back to the use of magnets as
   compasses, interacting with the Earth's magnetic field to indicate
   North and South on the globe. Since opposite ends of magnets are
   attracted, the 'north' magnetic pole of the earth must be magnetically
   'south'.

   A magnetic field contains energy, and physical systems stabilize into
   the configuration with the lowest energy. Therefore, when placed in a
   magnetic field, a magnetic dipole tends to align itself in opposed
   polarity to that field, thereby canceling the net field strength as
   much as possible and lowering the energy stored in that field to a
   minimum. For instance, two identical bar magnets normally line up North
   to South resulting in no net magnetic field, and resist any attempts to
   reorient them to point in the same direction. The energy required to
   reorient them in that configuration is then stored in the resulting
   magnetic field, which is double the strength of the field of each
   individual magnet. (This is, of course, why a magnet used as a compass
   interacts with the Earth's magnetic field to indicate North and South).

Magnetic monopoles

   The modern understanding of magnetism posits that all magnetic effects
   are actually due to relativistic effects caused by relative motion
   between the observer and the charged particles. Since all magnetism is
   caused by moving charges, all magnets are in fact electromagnets.

   Even atoms have a tiny field. In the planetary model of an atom, the
   electrons orbit the nucleus and thus have a change in motion giving
   rise to a magnetic field. Permanent magnets have measurable magnetic
   fields because the atoms (and molecules) are arranged in a way that
   their individual tiny fields align and add up.

   In this model, the lack of a single pole makes intuitive sense; cutting
   a bar magnet in half does nothing to the arrangement of the molecules
   within, and you end up with two bars with the same arrangement, and
   thus the same field. This also explains how heating or simply hitting a
   magnet made from a soft material will degauss it, as the molecules
   within are moved about.

   Since all known forms of magnetic phenomena involve the motion of
   electrically charged particles, and since no theory suggests that
   "pole" is, in that context, a thing rather than a convenient fiction,
   it may well be that nothing that could be called a magnetic monopole
   exists or ever did or could.

   Contrary to normal experience, some theoretical physics models predict
   the existence of magnetic monopoles. Paul Dirac observed in 1931 that,
   because electricity and magnetism show a certain symmetry, just as
   quantum theory predicts that individual positive or negative electric
   charges can be observed without the opposing charge, isolated South or
   North magnetic poles should be observable. In practice, however,
   although charged particles like protons and electrons can be easily
   isolated as individual electrical charges, magnetic south and north
   poles have never been found in isolation. Using quantum theory Dirac
   showed that if magnetic monopoles exist, then one could explain why the
   observed elementary particles carry charges that are multiples of the
   charge of the electron.

   In modern elementary particle theory, the quantization of charge is
   realized in a spontaneous breakdown of a non- abelian gauge symmetry.
   Monopoles predicted in certain grand unified theories differ from the
   one originally thought of by Dirac. These monopoles, unlike elementary
   particles, are solitons, which are localized energy packets. If they
   exist at all, they contradict cosmological observations. A solution to
   this monopole problem in cosmology gave rise to the
   currently-interesting idea of inflation.

Atomic magnetic dipoles

   The physical cause of the magnetism of objects, as distinct from
   electrical currents, is the atomic magnetic dipole. Magnetic dipoles,
   or magnetic moments, result on the atomic scale from the two kinds of
   movement of electrons. The first is the orbital motion of the electron
   around the nucleus; this motion can be considered as a current loop,
   resulting in an orbital dipole magnetic moment along the axis of the
   nucleus. The second, much stronger, source of electronic magnetic
   moment is due to a quantum mechanical property called the spin dipole
   magnetic moment (although current quantum mechanical theory states that
   electrons neither physically spin, nor orbit the nucleus).
   Dipole moment of a bar magnet.
   Dipole moment of a bar magnet.

   The overall magnetic moment of the atom is the net sum of all of the
   magnetic moments of the individual electrons. Because of the tendency
   of magnetic dipoles to oppose each other to reduce the net energy, in
   an atom the opposing magnetic moments of some pairs of electrons cancel
   each other, both in orbital motion and in spin magnetic moments. Thus,
   in the case of an atom with a completely filled electron shell or
   subshell, the magnetic moments normally completely cancel each other
   out and only atoms with partially-filled electron shells have a
   magnetic moment, whose strength depends on the number of unpaired
   electrons.

   The differences in configuration of the electrons in various elements
   thus determine the nature and magnitude of the atomic magnetic moments,
   which in turn determine the differing magnetic properties of various
   materials. Several forms of magnetic behaviour have been observed in
   different materials, including:
     * Diamagnetism
     * Paramagnetism
          + Molecular magnet
     * Ferromagnetism
          + Antiferromagnetism
          + Ferrimagnetism
          + Metamagnetism
     * Spin glass
     * Superparamagnetism

   Magnetars, stars with extremely powerful magnetic fields, are also
   known to exist.

Types of magnets

Electromagnets

   Electromagnets are useful in cases where a magnet must be switched on
   or off; for instance, large cranes to lift junked automobiles.

   For the case of electric current moving through a wire, the resulting
   field is directed according to the "right hand rule." If the right hand
   is used as a model, and the thumb of the right hand points along the
   wire from positive towards the negative side ("conventional current",
   the reverse of the direction of actual movement of electrons), then the
   magnetic field will wrap around the wire in the direction indicated by
   the fingers of the right hand. As can be seen geometrically, if a loop
   or helix of wire is formed such that the current is traveling in a
   circle, then all of the field lines in the centre of the loop are
   directed in the same direction, resulting in a magnetic dipole whose
   strength depends on the current around the loop, or the current in the
   helix multiplied by the number of turns of wire. In the case of such a
   loop, if the fingers of the right hand are directed in the direction of
   conventional current flow (i.e., positive to negative, the opposite
   direction to the actual flow of electrons), the thumb will point in the
   direction corresponding to the North pole of the dipole.

Permanent and temporary magnets

   Permanent and temporary magnets are alike in that they do not require
   another influence to create their magnetic field, they rely on magnetic
   poles. There are always two poles, a north and a south. Even by cutting
   a magnet in numerous pieces you will not get a magnetic monopole, you
   will get many abated magnets. A helpful way to think of it is to think
   of a line of pencils, all facing the same way. each has a sharp end and
   an eraser end, or a north and south end. If you diverse that line into
   two lines each line will still have a sharp side and eraser side. A
   permanent magnet differs from a temporary magnet in that a temporary
   magnet is simply temporary. Stroking a metal onto a magnetized material
   such as magnetite (a naturally magnetized mineral) would turn the metal
   into a ferromagnetic material. Permanent magnets that contain other
   materials such as those in strong magnets are difficult to magnetize,
   but tend to keep its magnetism for a greater period. Permanent magnets
   may be metals such as steel, and iron, natural minerals such as
   magnetite, or even plastic magnets. Although they are called permanent
   they are not comprehensively permanent, if dropped, heated, or struck
   against a hard object at a fast speed the magnetic domains within the
   magnet may shift out of alignment causing the magnet to become
   debilitated. There are two types of rare-earth magnets:
     * Neodymium magnets - made from sintered neodymium, iron and small
       amounts of boron; the most powerful and affordable.

     * Samarium-cobalt magnets (SmCo5) are less common than neodymium
       magnets, are not as strong, and are more expensive, but they have a
       higher curie point, making them more applicable for situations when
       they will be under intense heat.

Magnetic metallic elements

   Many materials have unpaired electron spins, but 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, zinc 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.

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 factor.

Samarium-cobalt

   Samarium-cobalt magnets are highly resistant to oxidation, with higher
   magnetic strength and temperature resistance than alnico or ceramic
   materials. Sintered samarium-cobalt magnets are brittle and prone to
   chipping and cracking and may fracture when subjected to thermal shock.

Neodymium-iron-boron (NIB)

   Neodymium magnets, more formally referred to as neodymium-iron-boron
   (NdFeB) magnets, have the highest magnetic field strength, but are
   inferior to samarium cobalt in resistance to oxidation and temperature.
   This type of magnet has traditionally been expensive, due to both the
   cost of raw materials and licensing of the patents involved. This high
   cost limited their use to applications where such high strengths from a
   compact magnet are critical. Use of protective surface treatments such
   as gold, nickel, zinc and tin plating and epoxy resin coating can
   provide corrosion protection where required. Beginning in the 1980s,
   NIB magnets have increasingly become less expensive and more popular in
   other applications such as controversial children's magnetic building
   toys. Even tiny neodymium magnets are very powerful and have important
   safety considerations.

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.

   See results from NIST published April 2005, or

Units of electromagnetism

SI units related to magnetism

   SI electromagnetism units
   Symbol Name of Quantity Derived Units Unit Base Units
   I Magnitude of current ampere ( SI base unit) A A = W/V = C/s
   q Electric charge, Quantity of electricity coulomb C A·s
   V Potential difference or Electromotive force volt V J/C =
   kg·m^2·s^−3·A^−1
   R, Z, X Resistance, Impedance, Reactance ohm Ω V/A = kg·m^2·s^−3·A^−2
   ρ Resistivity ohm metre Ω·m kg·m^3·s^−3·A^−2
   P Power, Electrical watt W V·A = kg·m^2·s^−3
   C Capacitance farad F C/V = kg^−1·m^−2·A^2·s^4
   Elastance reciprocal farad F^−1 V/C = kg·m^2·A^−2·s^−4
   ε Permittivity farad per metre F/m kg^−1·m^−3·A^2·s^4
   χ[e] Electric susceptibility (dimensionless) - -
   G, Y, B Conductance, Admittance, Susceptance siemens S Ω^−1 =
   kg^−1·m^−2·s^3·A^2
   σ Conductivity siemens per metre S/m kg^−1·m^−3·s^3·A^2
   H Magnetic field, magnetic field intensity ampere per metre A/m A·m^−1
   Φ[m] Magnetic flux weber Wb V·s = kg·m^2·s^−2·A^−1
   B Magnetic flux density, magnetic induction, magnetic field strength
   tesla T Wb/m^2 = kg·s^−2·A^−1 = N·A^−1·m^−1
   Reluctance ampere-turn per weber A/Wb kg^−1·m^−2·s^2·A^2
   L Inductance henry H Wb/A = V·s/A = kg·m^2·s^−2·A^−2
   μ Permeability henry per metre H/m kg·m·s^−2·A^−2
   χ[m] Magnetic susceptibility (dimensionless)
   Π and Π * Electric and Magnetic hertzian vector potentials n/a n/a

Other units

     * gauss-The gauss, abbreviated as G, is the cgs unit of magnetic flux
       density or magnetic induction (B).
     * oersted-The oersted is the CGS unit of magnetic field strength.
     * maxwell-is the CGS unit for the magnetic flux.
     * μ[o] -common symbol for the permeability of free space (4πx10^-7
       N/(ampere-turn)^2).

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