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MAGNET INFORMATION
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Dictionary: magnet
(n. mag'nit)
1. An object that is surrounded by a magnetic field and that has
the property, either natural or induced, of attracting iron or steel.
2. An electromagnet.
3. A person, a place, an object, or a situation that exerts
attraction.
[Middle English, from Old French magnete, from Latin magnes, magnet-, from
Greek Magnes (lithos), Magnesian (stone), magnet, from Magnesia, Magnesia, an
ancient city of Asia Minor.]
How Magnets are Made: How is a magnet made?
Background
A magnet is a material that can exert a noticeable force on other materials
without actually contacting them. This force is known as a magnetic force and may
either attract or repel. While all known materials exert some sort of magnetic force,
it is so small in most materials that it is not readily noticeable. With other materials,
the magnetic force is much larger, and these are referred to as magnets. The Earth
itself is a huge magnet.
Some magnets, known as permanent magnets, exert a force on objects
without any outside influence. The iron ore magnetite, also known as lodestone, is a
natural permanent magnet. Other permanent magnets can be made by subjecting
certain materials to a magnetic force. When the force is removed, these materials
retain their own magnetic properties. Although the magnetic properties may change
over time or at elevated temperatures, these materials are generally considered to
be permanently magnetized, hence the name.
Other magnets are known as electromagnets. They are made by surrounding
certain materials with a coil of wire. When an electric current is passed through the
coil, these materials exert a magnetic force. When the current is shut off, the
magnetic force of these materials drops to nearly zero. Electromagnet materials
retain little, if any, magnetic properties without a flow of electric current in the
coil.
All magnets have two points where the magnetic force is greatest. These two
points are known as the poles. For a rectangular or cylindrical bar magnet, these
poles would be at opposite ends. One pole is called the north-seeking pole, or north
pole, and the other pole is called the south-seeking, or south pole. This terminology
reflects one of the earliest uses of magnetic materials such as lodestone. When
suspended from a string, the north pole of these first crude compasses would
always "seek" or point towards the north. This aided sailors in judging the direction
to steer to reach distant lands and return home.
In our present technology, magnet applications include compasses, electric
motors, microwave ovens, coin-operated vending machines, light meters for
photography, automobile horns, televisions, loudspeakers, and tape recorders. A
simple refrigerator note holder and a complex medical magnetic resonance imaging
device both utilize magnets.
History
Naturally occurring magnetic lodestone was studied and used by the Greeks as
early as 500 B.C. Other civilizations may have known of it earlier than that. The
word magnet is derived from the Greek name magnetis lithos, the stone of
Magnesia, referring to the region on the Aegean coast in present-day Turkey where
these magnetic stones were found.
The first use of a lodestone as a compass is generally believed to have
occurred in Europe in about A.D. 1100 to A.D. 1200. The term lodestone comes from
the Anglo-Saxon meaning "leading stone," or literally, "the stone that leads." The
Icelandic word is leider-stein, and was used in writings of that period in reference to
the navigation of ships.
In 1600, English scientist William Gilbert confirmed earlier observations
regarding magnetic poles and concluded that the Earth was a magnet. In 1820, the
Dutch scientist Hans Christian Oersted discovered the relationship between
electricity and magnetism, and French physicist Andre Ampere further expanded
upon this discovery in 1821.
In the early 1900s, scientists began studying magnetic materials other than
those based on iron and steel. By the 1930s, researchers had produced the first
powerful Alnico alloy permanent magnets. Even more powerful ceramic magnets
using rare earth elements were successfully formulated in the 1970s with further
advances in this area in the 1980s.
Today, magnetic materials can be made to meet many different performance
requirements depending on the final application.
Raw Materials
When making magnets, the raw materials are often more important than the
manufacturing process. The materials used in permanent magnets (sometimes
known as hard materials, reflecting the early use of alloy steels for these magnets)
are different than the materials used in electromagnets (some-times known as soft
materials, reflecting the use of soft, malleable iron in this application).
Permanent Magnet Materials
Permanent magnet lodestones contain magnetite, a hard, crystalline iron
ferrite mineral that derives its magnetism from the effect the earth's magnetic field
has on it. Various steel alloys can also be magnetized. The first big step in
developing more effective permanent magnet materials came in the 1930s with the
development of Alnico alloy magnets. These magnets take their name from the
chemical symbols for the aluminum-nickel-cobalt elements used to make the alloy.
Once magnetized, Alnico magnets have between 5 and 17 times the magnetic force
of magnetite.
Ceramic permanent magnets are made from finely powdered barium ferrite or
strontium ferrite formed under heat and pressure. Their magnetic strength is
enhanced by aligning the powder particles with a strong magnetic field during
forming. Ceramic magnets are comparable to Alnico magnets in terms of magnetic
force and have the advantage of being able to be pressed into various shapes
without significant machining.
Flexible permanent magnets are made from powdered barium ferrite or
strontium ferrite mixed in a binding material like rubber or a flexible plastic like
polyvinyl chloride.
In the 1970s, researchers developed permanent magnets made from powdered
samarium cobalt fused under heat. These magnets take advantage of the fact that
the arrangement of the groups of atoms, called magnetic domains, in the hexagonal
crystals of this material tend to be magnetically aligned. Because of this natural
alignment, samarium-cobalt magnets can be made to produce magnetic forces 50
times stronger than magnetite. Headphones for small, personal stereo systems use
samarium-cobalt permanent magnets. Samarium-cobalt magnets also have the
advantage of being able to operate in higher temperatures than other permanent
magnets without losing their magnetic strength.
Similar permanent magnets were made in the 1980s using powdered
neodymium iron boron which produces magnetic forces almost 75 times stronger
than magnetite. These are the most powerful permanent magnets commercially
available today.
Electromagnet Materials
Pure iron and iron alloys are most commonly used in electromagnets. Silicon
iron and specially treated iron-cobalt alloys are used in low-frequency power
transformers.
A special iron oxide, called a gamma iron oxide, is often used in the
manufacture of magnetic tapes for sound and data recording. Other materials for
this application include cobalt-modified iron oxides and chromium dioxide. The
material is finely ground and coated on a thin polyester plastic film.
Other Magnetic Materials
Magnetic fluids can be made by encapsulating powdered barium ferrite
particles in a single layer of molecules of a long-chain polymer plastic. The particles
are then held in suspension in a liquid like water or oil. Because of the plastic
encapsulation, the magnetic particles slide over each other with almost no friction.
The particles are so small that normal thermal agitation in the liquid keeps the
particles from settling. Magnetic fluids are used in several applications as sealants,
lubricants, or vibration damping materials.
The Manufacturing
Process
Just as the materials are different for different kinds of magnets, the
manufacturing processes are also different. Many electromagnets are cast using
standard metal casting techniques. Flexible permanent magnets are formed in a
plastic extrusion process in which the materials are mixed, heated, and forced
through a shaped opening under pressure.
Some magnets are formed using a modified powdered metallurgy process in
which finely powdered metal is subjected to pressure, heat, and magnetic forces to
form the final magnet. Here is a typical powdered metallurgy process used to
produce powerful neodymium-iron-boron permanent magnets with cross-sectional
areas of about 3-10 square inches (20-65 sq cm):
Preparing the powdered metal
* The appropriate amounts of neodymium, iron, and
boron are heated to melting in a vacuum. The vacuum prevents any chemical
reaction between air and the melting materials that might contaminate the final
metal alloy.
* Once the metal has cooled and solidified, it is broken
up and crushed into small pieces. The small pieces are then ground into a fine
powder in a ball mill.
Pressing
* The powdered metal is placed in a mold, called a die,
that is the same length and width (or diameter, for round magnets) as the finished
magnet. A magnetic force is applied to the powdered material to line up the powder
particles. While the magnetic force is being applied, the powder is pressed from the
top and bottom with hydraulic or mechanical rams to compress it to within about
0.125 inches (0.32 cm) of its final intended thickness. Typical pressures are about
10,000 psi to 15,000 psi (70 MPa to 100 MPa). Some shapes are made by placing the
powdered material in a flexible, air-tight, evacuated container and pressing it into
shape with liquid or gas pressure. This is known as isostatic compaction.
Heating
* The compressed "slug" of powdered metal is removed
from the die and placed in an oven. The process of heating compressed powdered
metals to transform them into fused, solid metal pieces is called sintering. The
process usually consists of three stages. In the first stage, the compressed material
is heated at a low temperature to slowly drive off any moisture or other
contaminants that may have become entrapped during the pressing process. In the
second stage, the temperature is raised to about 70-90% of the melting point of the
metal alloy and held there for a period of several hours or several days to allow the
small particles to fuse together. Finally, the material is cooled down slowly in
controlled, step-by-step temperature increments.
Annealing
* The sintered material then undergoes a second
controlled heating and cooling process known as annealing. This process removes
any residual stresses within the material and strengthens it.
Finishing
* The annealed material is very close to the finished
shape and dimensions desired. This condition is known as "nearnet" shape. A final
machining process removes any excess material and produces a smooth surface
where needed. The material is then given a protective coating to seal the
surfaces.
Magnetizing
* Up to this point, the material is just a piece of
compressed and fused metal. Even though it was subjected to a magnetic force
during pressing, that force didn't magnetize the material, it simply lined up the
loose powder particles. To turn it into a magnet, the piece is placed between the
poles of a very powerful electromagnet and oriented in the desired direction of
magnetization. The electromagnet is then energized for a period of time. The
magnetic force aligns the groups of atoms, or magnetic domains, within the
material to make the piece into a strong permanent magnet.
Quality Control
Each step of the manufacturing process is monitored and controlled. The
sintering and annealing processes are especially critical to the final mechanical and
magnetic properties of the magnet, and the variables of time and temperature must
be closely controlled.
Hazardous Materials,
Byproducts, and
Recycling
Barium and the barium compounds used to make barium ferrite permanent
magnets are poisonous and are considered toxic materials. Companies making
barium ferrite magnets must take special precautions in the storage, handling, and
waste disposal of the barium products.
Electromagnets can usually be recycled by salvaging the component iron cores
and copper wiring in the coil. Partial recycling of permanent magnets may be
achieved by removing them from obsolete equipment and using them again in
similar new equipment. This is not always possible, however, and a more
comprehensive approach to recycling permanent magnets needs to be
developed.
The Future
Researchers continue to search for even more powerful magnets than those
available today. One of the applications of more powerful permanent magnets would
be the development of small, high-torque electric motors for battery-powered
industrial robots and laptop computer disk drives. More powerful electromagnets
could be used for the levitation and propulsion of high-speed trains using pulsed
magnetic fields. Such trains, sometimes called maglev trains, would be supported
and guided by a central, magnetic "rail." They would move without ever contacting
the rail, thus eliminating mechanical friction and noise. Pulsed magnetic fields could
also be used to launch satellites into space without relying on expensive and heavy
booster rockets.
More powerful magnets could also be used as research tools to develop other
new materials and processes. Intense, pulsed magnet fields are currently being used
in nuclear fusion research to contain the hot, reacting nuclear plasma that would
otherwise melt any solid material vessel. Magnetic fields can also be used in
materials research to study the behavior of semiconductors used in electronics to
determine the effects of making micro-sized integrated circuits.
Where To Learn More
Books
Brady, George S. and Henry R. Clauser. Materials Handbook, 12th Ed.
McGraw-Hill, 1986.
Braithwaite, Nicholas and Graham Weaver, eds. Electronic Materials.
Butterworths, 1990.
Campbell, Peter. Permanent Magnet Materials and Their Design. Cambridge
University Press, 1994.
Verschuur, Gerrit L. Hidden Attraction: The History and Mystery of Magnetism.
Oxford University Press, 1993.
Periodicals
Boebinger, Greg, Al Passner, and Joze Bevk. "Building World-Record Magnets."
Scientific American, June 1995, pp. 58-66.
Duplessis, John. "An Attractive Proposition." Machine Design, June 11, 1993, p.
46.
[Article by: Chris Cavette]
Sci-Tech Encyclopedia: Magnet
An object or device that produces a magnetic field. Magnets are essential for
the generation of electric power and are used in motors, generators, labor-saving
electromechanical devices, information storage, recording, and numerous
specialized applications, for example, seals of refrigerator doors. The magnetic
fields produced by magnets apply a force at a distance on other magnets, charged
particles, electric currents, and magnetic materials. See also Generator; Magnetic
recording; Motor.
Magnets may be classified as either permanent or excited. Permanent magnets
are composed of so-called hard magnetic material, which retains an alignment of
the magnetization in the presence of ambient fields. Excited magnets use
controllable energizing currents to generate magnetic fields in either
electromagnets or air-cored magnets. See also Electromagnet; Ferromagnetism;
Superconductivity.
The essential characteristic of permanent-magnet materials is an inherent
resistance to change in magnetization over a wide range of field strength.
Resistance to change in magnetization in this type of material is due to two factors:
(1) the material consists of particles smaller than the size of a domain, a
circumstance which prevents the gradual change in magnetization which would
otherwise take place through the movement of domain wall boundaries; and (2) the
particles exhibit a marked magnetocrystalline anisotropy. During manufacture the
particles are aligned in a magnetic field before being sintered or bonded in a soft
metal or polyester resin. Compounds of neodymium, iron, and boron are used. See
also Iron alloys.
Electromagnets rely on magnetically soft or permeable materials which are
well annealed and homogeneous so as to allow easy motion of domain wall
boundaries. Ideally the coercive force should be zero, permeability should be high,
and the flux density saturation level should be high. Coincidentally the hysteresis
energy loss represented by the area of the hysteresis curve is small. This property
and high electrical resistance (for the reduction of eddy currents) are required where
the magnetic field is to vary rapidly. This is accomplished by laminating the core
and using iron alloyed with a few percent silicon that increases the
resistivity.
Electromagnets usually have an energizing winding made of copper and a
permeable iron core. Applications include relays, motors, generators, magnetic
clutches, switches, scanning magnets for electron beams (for example, in television
receivers), lifting magnets for handling scrap, and magnetic recording heads. See
also Cathode-ray tube; Clutch; Electric switch; Relay.
Special iron-cored electromagnets designed with highly homogeneous fields
are used for special analytical applications in, for example, electron or nuclear
magnetic resonance, or as bending magnets for particle accelerators. See also
Magnetic resonance; Particle accelerator.
Air-cored electromagnets are usually employed above the saturation flux
density of iron (about 2 T); at lower fields, iron-cored magnets require much less
power because the excitation currents needed then are required only to generate a
small field to magnetize the iron. The air-cored magnets are usually in the form of a
solenoid with an axial hole allowing access to the high field in the center. The
conductor, usually copper or a copper alloy, must be cooled to dissipate the heat
generated by resistive losses. In addition, the conductor and supporting structure
must be sufficiently strong to support the forces generated in the magnet. See also
Solenoid (electricity).
In pulsed magnets, higher fields can be generated by limiting the excitation to
short pulses (usually furnished by the energy stored in a capacitor bank) and cooling
the magnet between pulses. The highest fields are generally achieved in small
volumes. A field of 75 T has been generated for 120 microseconds.
Large-volume or high-field magnets are often fabricated with superconducting
wire in order to avoid the large resistive power losses of normal conductors. The
two commercially available superconducting wire materials are (1) alloys of
niobium-titanium, a ductile material which is used for generating fields up to about
9 T; and (2) a brittle alloy of niobium and tin (Nb3Sn) for fields above 9 T. Practical
superconducting wires use complex structures of fine filaments of superconductor
that are twisted together and embedded in a copper matrix. The conductors are
supported against the electromagnetic forces and cooled by liquid helium at 4.2 K
(-452?F). A surrounding thermal insulating enclosure such as a dewar minimizes the
heat flow from the surroundings.
Superconducting magnets operating over 20 T have been made with
niobium-titanium outer sections and niobium-tin inner sections. Niobium-titanium
is used in whole-body nuclear magnetic resonance imaging magnets for medical
diagnostics. Other applications of superconducting magnets include their use in
nuclear magnetic resonance for chemical analysis, particle accelerators,
containment of plasma in fusion reactors, magnetic separation, and magnetic
levitation. See also Magnetic levitation; Magnetic separation methods; Medical
imaging; Nuclear fusion; Nuclear magnetic resonance (NMR); Superconducting
devices.
The highest continuous fields are generated by hybrid magnets. A
large-volume (lower-field) superconducting magnet that has no resistive power
losses surrounds a water-cooled inner magnet that operates at the highest field.
The fields of the two magnets add. Over 35 T has been generated
continuously.
Britannica Concise Encyclopedia: magnet
Any material capable of attracting iron and producing a magnetic field outside
itself. By the end of the 19th century, all known elements and many compounds had
been tested for magnetism, and all were found to have some magnetic property.
However, only three elements — iron, nickel, and cobalt — exhibit ferromagnetism.
See also compass, electromagnet.
For more information on magnet, visit Britannica.com.
Science Dictionary: magnet
An object that attracts iron and some other materials. Magnets are said to
generate a magnetic field around themselves. Every magnet has two poles, called
the north and south poles. Magnetic poles exert forces on each other in such a way
that like poles repel and unlike poles attract each other. A compass is a small
magnet that is affected by the magnetic field of the Earth in such a way that it
points to a magnetic pole of the Earth. (See magnetic field and magnetism.)
Veterinary Dictionary: magnet
An object having polarity and capable of attracting iron.
* oral dose m. — see reticular magnet.
* reticular m. — a magnet placed in the reticulum to
attract and isolate sharp metal and help to prevent traumatic reticuloperitonitis in
ruminants.
Electronics Dictionary: magnet
Body that can be used to attract or repel magnetic materials.
Devil's Dictionary: magnet
A cynical view of the world by Ambrose Bierce
n.
Something acted upon by magnetism.
Word Tutor: magnet
pronunciation
IN BRIEF: A piece of some material such as the mineral iron oxide that is able
to attract iron.
Memory is a magnet. It will pull to it and hold only material nature has
designed it to attract. — Jassamyn West, American novelist, Quaker, best known for
her novel, The Friendly.
Wikipedia: magnet
Iron filings in a magnetic field generated by a bar magnet
Enlarge
Iron filings in a magnetic field generated by a bar magnet
A magnet is a material or object that produces a magnetic field. A "hard" or
"permanent" magnet is one which stays magnetized for a long time, such as
magnets often used in refrigerator doors. Permanent magnets occur naturally in
some rocks, particularly lodestone, but are now more commonly manufactured. A
"soft" or "impermanent" magnet is one which 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.
Qualities
The magnetic field, magnetic moment, and magnetization are vectors,
meaning they have direction and magnitude. The magnetic moment and
magnetization are properties only of the magnet, while the magnetic field it
produces depends on the position relative to the magnet. The magnetic moment
points from its south pole to its north pole. Also, its north pole points towards the
Earth's geographic north pole, which is a magnetic south pole. A compass needle is
approximately a bar magnet.
A magnetic field can be measured using a good magnetic compass (this is a
small permanent magnet). The direction of the field at a point in space is the
direction in which the compass needle points when it passes through that point and
is in equilibrium. The magnitude (or strength, usually denoted by the symbol B) of a
magnetic field can also be measured using a compass, if the field is, like the Earth's,
nearly uniform over the volume occupied by the needle. The needle is rotated about
its center, and this makes it oscillate about its equilibrium position. The period t of
oscillation is measured. For small oscillation angles, the frequency of the oscillation,
1/t, is proportional to the square root of B. This is a result from the theory of
rotational motion and the theory of the torque on a magnet, and can be tested by
creating an electromagnet, which makes a magnetic field proportional to the electric
current that it carries. The common unit of magnetic field is the tesla, denoted "T",
equal to one N/(A?m) (Force/(Current?Distance)), or Wb/m? (magnetic flux per area),
and about 20,000 times the Earth's magnetic field. Technically, B should be called
the magnetic induction field, because changing B induces an electric field, by
Faraday's Law of electromagnetic induction.
Magnetic moment
The magnetic moment ? of a magnet is the magnetic strength of the field at a
distance r from the magnet. At large distances, the magnetic field B is proportional
to ? and inversely proportional to r?. So, ? can be obtained by measuring B at a
distance r. The common unit for magnetic moment is A?m?. A wire in the shape of a
circle with area A and carrying current I has a magnetic moment equal to IA.
Magnetization
Magnetization of an object is its magnetic moment per unit volume. It is
usually denoted M and has the units A/m. A good bar magnet may have a magnetic
moment of 0.1 A?m? and a volume of 1 cm?, or 0.000001 m?, and therefore a
magnetization of 100,000 A/m. Iron can have a magnetization of around a million
A/m.
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.
Effect of a magnet on the domains.
Enlarge
Effect of a magnet on the domains.
When a domain contains too many molecules, it becomes unstable and divides
into two domains aligned in opposite directions so that they stick together more
stably as shown at the right.
When exposed to a magnetic field, the domain boundaries move so that the
domains aligned with the magnetic field grow and dominate the structure as shown
at the left. When the magnetizing field is removed, the domains may not return to a
unmagnetized state. This results in the ferromagnetic material being magnetized,
forming a permanent magnet.
When magnetized strongly enough that the prevailing domain overruns all
others to result in only one single domain, the material is magnetically saturated.
When a magnetized ferromagnetic material is heated to the Curie point temperature,
the molecules are agitated to the point that the magnetic domains lose the
organization and the magnetic properties they cause cease. When the material is
cooled, this domain alignment structure spontaneously returns as the material
develops its crystalline structure.
Physical origin of magnetism
Magnetism, ultimately, is due to the motion of electric charge. For a
macroscopic object, like a wire loop, an electric current flowing through it has a
magnetic moment. Far from the loop there is a magnetic field proportional in
strength to its magnetic moment.
For a microscopic object, the physical picture is more complex. An electron
within an atom can have orbital angular momentum and a magnetic moment
proportional to that orbital angular momentum; the electron also has intrinsic
angular momentum, or spin, and a magnetic moment proportional to that spin
angular momentum. The orbital and spin angular momentum of an electron are
comparable in magnitude, as are their magnetic moments. Far from the electron
there is a magnetic field proportional in strength to its magnetic moment.
In addition, within the atomic nucleus are both neutrons and protons, and
these too have orbital and spin angular momentum, and associated magnetic
moments. However, the nuclear magnetic moment typically is much smaller than the
electron magnetic moment, because although the magnetic moment is proportional
to its angular momentum (comparable to that of the electron) it is also inversely
proportional to its mass. Nevertheless, it is the nucleus's relatively small nuclear
magnetic moment that is responsible for nuclear magnetic resonance (NMR), which
is the basis for magnetic resonance imaging (MRI).
Although most atoms and molecules have a net magnetic moment at
temperatures well below room temperature, at room temperature they typically have
no net magnetic moment. However, they can often be magnetized. If the orbital
magnetic properties dominate, the response typically will be diamagnetic; if the
intrinsic magnetic properties dominate, the response typically will be
paramagnetic.
Solids are collections of atoms and molecules. At room temperature most
solids are either diamagnetic or paramagnetic.
Although for many purposes it is convenient to think of a magnet as having
magnetic poles, it must be remembered that no isolated magnetic pole has ever
been observed. As indicated above, the proper description is ultimately one due to
electrical currents. For a magnet, these currents should be thought of as circulating
about its atoms, and flowing without any electrical resistance. This physical picture
is due to Andr?-Marie Amp?re, and these atomic currents are known as Amperian
currents. For a uniformly magnetized bar magnet in the shape of a cylinder, the net
effect of the atomic currents is to make the magnet behave as if there is a sheet of
current flowing around the cylinder, with local flow direction normal to the cylinder
axis. A right-hand-rule due to Amp?re tells us how the currents flow, for a given
magnetic moment. Align the thumb of your right hand along the magnetic moment,
and with that hand grasp the cylinder. Your fingers will then point along the
direction of current flow.
Permanent magnets
A few elements -- especially iron, cobalt, and nickel -- are ferromagnetic at
room temperature. When quantum mechanics and the Pauli Exclusion Principle are
accounted for, the electrical energy within these atoms is found to be lower if the
magnetic moments of the valence electrons are aligned. This makes them
ferromagnetic. Every ferromagnet has its own individual temperature, called the
Curie temperature, or Curie point, above which it loses its ferromagnetic properties.
This is because the thermal tendency to disorder overwhelms the energy lowering
due to ferromagnetic order. A perfectly aligned ferromagnet is said to have
long-range order because all of its atoms have their magnetic moments pointing in
the same direction. Real ferromagnets are not perfectly aligned, but rather contain
perfectly aligned regions, called magnetic domains, which have their own
magnetization directions.
A long bar magnet appears to have a north pole at one end and a south pole
at the other. Near either end the magnetic field falls off inversely with the square of
the distance from that pole.
For a magnet of any shape, at distances large compared to its size, the
strength of the magnetic field falls off inversely with the cube of the distance from
the magnet's center.
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 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 (i.e., 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.
Characteristics
Permanent magnets and dipoles
All magnets appear to have at least one north pole (reckoned positive) and at
least one south pole (reckoned negative), and the net pole strength of every magnet
is zero. Despite their apparent reality, as suggested by the image at the top of the
page, where iron filings concentrate in regions of large magnetic field, poles are not
physical objects on or in the magnet. They are simply a useful concept for
describing magnets. Rather than poles being the fundamental unit, it is the
magnetic dipole that is the fundamental unit. A magnetic dipole can be thought of
as a combination of a positive and a negative pole that are microscopically close to
one another and inseparable. This is not a bad description of the magnetic dipole of
an electron in a magnetic material.
The effect of aligning many dipoles and placing them head-to-tail in a line is
that there appears a north pole at one end and a south pole at the other, with all the
intermediate north and south poles canceling out. The net effect is a very long
dipole that appears to have poles only at its ends. Alternatively, aligning many
dipoles and placing them on a sheet producing an object whose magnetic field is
like that of a wire carrying current around the perimeter of the sheet.
A standard naming system for the poles of magnets is important. Historically,
the terms north and south reflect awareness of the relationship between magnets
and the earth's magnetic field. A freely suspended magnet will eventually orient
itself north-to-south, because of its attraction to the north and south magnetic
poles of the earth. The end of a magnet that points (approximately) toward the
Earth's geographic North Pole is labeled as the north pole of the magnet;
correspondingly, the end that points south is the south pole of the magnet. (The
actual geographic north pole is in a slightly different location than the
corresponding magnetic pole; see Magnetic North Pole.)
The Earth's present geographic north is thus actually its magnetic south.
Confounding the situation further, magnetized rocks on the ocean floor show that
the Earth's magnetic field has reversed itself in the past, so this system of naming is
likely to be incorrect at some time in the future.
Fortunately, by using an electromagnet and the right hand rule relating the
electromagnet's current and the magnetic field it produces, the orientation of the
field of a magnet can be defined without reference to the Earth's geomagnetic
field.
To avoid the confusion between geographic and magnetic north and south
poles, the terms positive and negative are sometimes used for the poles of a
magnet. The positive pole is that which seeks geographical north.
Common uses
Magnets have many uses in toys. M-tic uses magnetic rods connected to metal
spheres for construction
* 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).
* Common televisions and computer monitors: TV and
computer screens using vacuum tube technology employ an electromagnet to guide
electrons to the screen, in order to produce an image -- see the article on cathode
ray tubes. Plasma screens and LCDs use different technologies.
* Speakers and microphones: Most speakers employ a
permanent magnet and a current-carrying coil to convert electric energy (the signal)
into mechanical energy (the sound). The coil is wrapped around the speaker cone,
and carries the signal, producing a changing magnetic field that interacts with the
field of the permanent magnet. The low mass coil feels a magnetic force and in
response moves the cone and the neighboring air, thus generating sound. Standard
microphones employ the same concept, but in reverse. A microphone has a cone 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 in 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.
* Chucks: Chucks are used in the metalworking field to
hold objects. If these objects can be held securely with a magnet then a permanent
or electromagnetic chuck may be used. Magnets are also used in other types of
fastening devices, such as the magnetic base, the magnetic clamp and the
refrigerator magnet.
* 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. Magnetic
lip-rings and earrings are sometimes employed to avoid piercing.
* 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
(900km/h, 559 mph). The maximum recorded speed of a maglev train is 581km/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.
* Placing the item in a solenoid with a direct current
passing 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.
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.
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.
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 children's magnetic building toys. Even tiny neodymium magnets are very
powerful and have important safety considerations.[1]
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.
Magnetic behaviors
There are many forms of magnetic behavior, and all materials exhibit at least
one of these behaviors. Magnets vary in the permanency of their magnetization and
the strength of the magnetic field that is created.
Paramagnetism
Most popularly found in paper clips, paramagnetism is exhibited in substances
which do not emit fields by themselves, but when exposed to a magnetic field, its
electrons will begin to spin in such a manner that the substance emits a field of its
own. A good analogy for this behavior can be found in a bucket of nails - if you pick
up a single nail, you can expect that other nails will not follow. However, you can
apply an intense magnetic field to the bucket, pick up one nail, and find that many
will come with it.
Diamagnetism
Unscientifically referred to as 'non-magnetic,' diamagnets actually exhibit
some magnetic behavior - just to very small magnitudes. While paramagnetism is
affected more by the direction of the spin of electrons, diamagnetism is affected by
electrons' centripetal forces. Under the influence of a field, electrons of opposite
spin will see opposite effects to their centripetal force: one will increase and one
will decrease. This results in a very small magnetic force. All materials exhibit this
type of magnetism, however, when diamagnetism pairs with a stronger type of
magnetic behavior, the diamagnetic effect is severely overshadowed.
Ferromagnetism
This is the 'popular' perception of a magnet. Ferromagnetic materials have a
high retainment for magnetization, and a common example is a traditional
refrigerator magnet. By technicality, ferromagnetism exists when all of the atoms
contribute to the magnetic force emitted. The mechanical explanation of this is
similar to that of paramagnetism - the electrons' spins align such it creates a
magnetic force. However, unlike paramagnetic substances, a ferromagnet will retain
this spin alignment.
Ferrimagnetism
Like ferromagnetism, ferrimagnets retain their magnetization in the absence
of a field. However, they are arranged such that some of its atoms oppose the
magnetic moment. These atoms are said to be anti-aligned. The first discovered
magnetic substance, magnetite, was originally believed to be a ferromagnet; Louis
N?el disproved this, however, with the discovery of ferrimagnetism.
Antiferromagnetism
When all atoms are arranged in a substance so that they are anti-aligned, the
substance is antiferromagnetic. Antiferromagnets have a zero net magnetic
moment, meaning no field is emitted 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.
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 (Syst?me 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 T (tesla). 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 G (gauss). One T equals 104 G.
(2) The magnetic field H is given in SI units of ampere-turns/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 Oe (oersted). One A-turn/m equals 4p x 10-3 Oe.
(3) The magnetization M is given in SI units of ampere/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 A/m.
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=?0(H+M) holds, where ?0 is the permeability of
space, which equals 4p x 10-7 tesla?meter/ampere. In CGS it is written as
B=H+4pM.
Materials that are not permanent magnets usually satisfy the relation M=?H in
SI, where ? is the (dimensionless) magnetic susceptibility. Most non-magnetic
materials have a relatively small ? (on the order of a millionth), but soft magnets can
have ?'s on the order of hundreds or thousands. For materials satisfying M=?H, we
can also write B=?0(1+?)H=?0?rH=?H, where ?r=1+? is the (dimensionless) relative
permeability and ? = ?0?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=?H, but ?(SI) =
4p?(CGS), and ? = ?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 "m is for meter") and for magnetic moment (unit = A-m?). The symbol ? has
been used in some texts for magnetic permeability and in other texts for magnetic
moment. We will use ? 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.
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:
F={{\mu q_{m1} q_{m2}}\over{4\pi r^2}} [1]
where
F is force (SI unit: newton)
qm1 and qm2 are the pole strengths (SI unit:
ampere-meter)
? is the permeability of the intervening medium (SI unit:
tesla meter per ampere or henry per meter)
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=\frac{\mu_0}{2}AM^2 [2]
where
A is the area of each surface, in m2
M is their magnetization, in ampere/m.
?0 is the permeability of space, which equals 4p x 10-7
tesla?meter/ampere
Force between two bar magnets
The force between two identical cylindrical bar magnets placed end-to-end is
given by:
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] [3]
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=\frac{\mu_0}{2}M relates the flux density at the pole to the magnetization
of the magnet.
Online references
* HyperPhysics E/M, good complete tree diagram of
electromagnetic relationships with magnets
* Maxwell's Equations and some history...
* Detailed Theory on Designing a Solenoid or a Coil
Gun
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.
References
1. ^ Magnet Man, Magnet Basics -
2. ^ Nanomagnets Bend The Rules.
3. ^ Nanomagnets bend the rules.
Translations: Magnet
Dansk (Danish)
n. - magnet, legeme som virker ved magnetisme
Nederlands (Dutch)
magneet, trekpleister (figuurlijk), magnetiet
Fran?ais (French)
n. - aimant, p?le d'attraction
Deutsch (German)
n. - Magnet
Italiano (Italian)
magnete
Portugu?s (Portuguese)
n. - magneto (m)
Espa?ol (Spanish)
n. - im?n
Svenska (Swedish)
n. - magnet
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