Encyclopedia
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
iron, some
steels, and the
mineral lodestone; however, all materials are influenced to greater or lesser degree by the presence of a
magnetic field.
Physics of magnetism
Magnetic forces are fundamental forces that arise from the movement of electrical charge. Maxwell's equations and the Biot-Savart law describe the origin and behavior 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.
When given a treatment with relativity in mind, depending on the frame of reference, electromagnetic forces acting on an object partition differently into magnetic and electric fields. In fact, for this reason, magnetism can be considered 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:
where
is the electric charge of the particle
is the velocity vector of the particle
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.
This might give you pause: Simple bar magnets seem to be entirely able to pick up small metal objects, which certainly seems to require that they do work on those objects. As David Griffiths points out in his
textbook Introduction to Electrodynamics, this law is absolute - the magnetic field doesn't do any work. However, quite like the
normal force of an
inclined plane, which also can't do work, the magnetic field can
redirect the efforts of existing forces, and then those forces can indeed do work in the relevant direction.
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". When making a gun-like configuration , 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.
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. .
Magnetic monopoles
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 not 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 localised
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 .
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 behavior have been observed in different materials, including:
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 , 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 center 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 , the thumb will point in the direction corresponding to the North pole of the dipole.
Permanent magnets
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 . Due to the way their regular
crystalline atomic structure causes their spins to interact, some
metals are magnetic when found in their natural states, as
ores. These include
iron ore ,
cobalt, and
nickel, as well the rare earth metals
gadolinium and dysprosium . Such naturally occurring magnets were used in the first experiments with magnetism. Technology has 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 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' elements have a partially occupied
f electron shell 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
Neodymium magnets, more formally referred to as neodymium iron boron magnets, have the highest magnetic field strength, but are inferior to samarium cobalt in resistance to oxidation and temperature. This type of magnet is expensive, due to both the cost of raw materials and licensing of the patents involved. This high cost limits 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.
Single-molecule magnets and single-chain magnets
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:
- a large ground state spin value , which is provided by ferromagnetic or ferrimagnetic coupling between the paramagnetic metal centres.
- a negative value of the anisotropy of the zero field splitting
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 magnetism units
Other magnetism units
See also
...
External links
- First scientific book on magnetism by the father of electrical engineering. Full English text, full text search.
- Info on electromagnetism and homemade projects & experiments.
- - Real images of magnetic flux.
- Theory of the Red Blood Cells .
- Resources from the fundamental theory of magnetism to advanced applications of magnetic materials.
References
Footnotes
