Permanent magnet is a material that has their own magnetic
field. It is an object that creates its own persistent magnetic field when it
is magnetized. Permanent magnet exists in nature in the form of lodestone,
which are used as magnetic compass. The most common magnets in use are the
refrigerator magnet, speaker magnet, magnetic mariners compass magnet, ones
used in magnetic motors etc. Permanent magnets can also be made artificially by
passing strong direct current through an insulated coil of wire in which the
material to be magnetized is placed. The materials that can be magnetized which
are also strongly attracted to a magnet are called as ferromagnetic materials.
These include iron, steel, nickel, cobalt, alloys of rare earth metals,
etc. They are also used for making permanent magnets. Steel is slow to
magnetize but retains magnetism for long as it has low susceptibility to
magnetism but has high retentiveness.
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Ferromagnetic materials can be divided into:
1. Magnetically soft materials such as annealed iron, that
can be magnetized but do not tend to stay magnetized.
2. Magnetically hard materials that tend to stay magnetized
always.
Permanent magnets are made from hard ferromagnetic
materials such as alnico and ferrite when they are subjected to a special
processing in a powerful magnetic field during manufacture for aligning their
internal micro crystalline structure, which makes them hard to demagnetize.
Permanent Magnetism
Magnetic materials posses two poles of opposite effect
denoted as North and South because of their self orientation to the earth. The
poles of the magnet sustains even when the magnet is cut into two.
The two poles exist just like the electric charges, the positive charge and the
negative charge. Similarly the same poles repel each other and opposite poles
attract each other. The magnetic flux just like the static electric charge is
able to invisibly extend over space and pass through objects such as paper and
wood with little effect upon their strength.
Magnetic flux density also called as magnetic B field, is a
vector field. The magnetic B field vector at a given point has 2 properties.
1. The direction along the orientation of a compass needle.
2. The magnitude or strength which is proportional to the
how strongly the compass needle orients along that direction.
The strength of the magnetic field is given in tesla.
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The magnet's magnetic moment also called as dipole moment,
is a vector that denotes the magnet's overall magnetic properties. In a bar
magnet the direction of the magnetic moment points from the magnet's South Pole
to its North Pole. The magnitude relates to how strong and how farther away
these poles are. A magnet produces both its own magnetic field and also
responds to magnetic fields. The strength of the magnetic field it produces at
any given point is proportional to the magnitude of its magnetic moment. When a
magnet is subjected to an external magnetic field produced by another source,
it is subjected to a torque tending to orient the magnetic moment parallel to
the field. The amount of this torque is proportional to both the magnetic
moment and the external field. A magnet may also be subjected to a force
driving it in one direction or another according to the positions and
orientations of the magnet and the source.
The Models of Magnetism
Two different models for magnet exist, magnetic pole model
and atomic current model.
1. In the magnetic pole model, the magnet is being referred
to as having two distinct poles North and South. 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 at the opposing sides. When a bar magnet is
broken into two pieces, the result will be two bar magnets each of which has
both a North and a South pole.
2. In the atomic model, all the magnetization is due to the
effect of microscopic, atomic, circular bound currents or Ampere currents
throughout the material. For a uniformly cylindrical bar magnet, the net effect
of the microscopic bound current is to make the magnet behave as there is a
macroscopic sheet of electric current flowing around the surface with local
flow direction parallel to the cylinder axis.
The magnetism of a permanent magnetic field is produced by
an electric charge in motion. The magnetic field of the permanent magnet
is the result of the electrons within the atoms of iron spinning uniformly in
the same direction. This kind of uniform spinning of the electrons in the
atom is dictated by the electron binding in that material's atoms. So depending
upon the atomic structure of the material, only certain types of materials can
react to the magnetic fields and only fewer have the ability to permanently
sustain a magnetic field.
The overall magnetic behavior of a material can vary
depending upon the structure of the material, particularly its electron
configuration.
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Different Magnetic Materials
Different types of behaviors have been observed in different
materials:
Ferromagnetic and ferrimagnetic materials are attracted to a
magnet strongly. These materials can retain magnetization and become magnets.
Ferrimagnetic materials include ferrites, magnetite, lodestone etc.
Paramagnetic substances such as platinum, aluminum, and
oxygen are weakly attracted to either pole of a magnet. This attraction is
hundreds or thousand times weaker than that of the ferromagnetic materials.
Diamagnetic materials those which are repelled by the poles.
Diamagnetic materials include carbon, copper, water, and plastic are weakly
repelled by a magnet. All substances not possessing one of the other types of magnetism
are diamagnetic.
Common Uses of Magnet
Magnetic recording media. Recording media such as VHS tapes
has a reel of magnetic tape. The information that makes up the video and sound
is encoded on the magnetic coating on the tape.
Credit, Debit, and ATM Cards. All these cards have a
magnetic strip on one side.
Television and computer monitors. Tube type TV and computer
screens contain an electromagnet to guide electrons to the screen.
Speakers and microphones. Most speakers employ permanent
magnets and current carrying coil to convert electric energy to mechanical
energy.
Electric motors and generators. Some electric motors work
depending on a combination of an electromagnet and a permanent magnet. It
converts electrical energy into mechanical energy.
Electric guitars. These guitars use pickups to transduce the
vibration of guitar strings into electrical current that can be amplified.
Magnetic compass. A magnetic compass uses a permanent magnet
to align itself with a magnetic field, the earth’s magnetic field.
Demagnetizing A Permanent Magnet
For demagnetizing a permanent magnet, a certain required
magnetic field must be applied and the threshold to demagnetize depends upon
the coercivity of the material. Hard materials have high coercivity value
whereas the soft materials have low coercivity.
Magnetized ferromagnetic materials can be demagnetized by
Heating a magnet past its curie temperature, the molecular
motion destroys the alignment of the magnetic domains.
Placing the magnet in an alternating magnetic field with
intensity above the material’s coercivity and slowly drawing the magnet out and
decreasing the magnetic field.
Hammering or jarring, the mechanical disturbance will
randomize the magnetic domains.
