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Magnetism
Magnetism
In
physics,
magnetism is a phenomenon by which
materials
exert an attractive or repulsive
force on other
materials. Magnetism arises whenever electrically charged particles are in
motion. Some
well known materials that exhibit magnetic properties are
iron, some
steels, and the
mineral lodestone.
All materials are influenced to one degree or another by the presence of a
magnetic field, although in some cases the influence is too small to detect
without special equipment.
Magnetic
forces are fundamental forces that arise due to the movement of electrically
charged particles.
Maxwell's equations describe the origin and behavior of the fields that
govern these forces (see also
Biot-Savart's Law).
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. If a loop is formed, such
that the charged particles are traveling in a circle, then all of the field
lines in the center of the loop are directed in the same direction. The result
is called a magnetic
dipole.
When placed in a magnetic field, a magnetic dipole will tend to align itself
with that field. For the case of a loop, if the fingers of the right hand are
directed in the direction of current flow, the thumb will point in the direction
corresponding to the North pole of the dipole.
-
volt
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tesla
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gauss
-
oersted
-
weber
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ampere
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maxwell
Magnetic
dipoles or
magnetic moments can often result on the atomic scale due to the movements of
electrons. Each electron has magnetic moments that originate from two sources.
The first is the orbital motion of the electron around the nucleus. In a sense
this motion can be considered as a current loop, resulting in a magnetic moment
along its axis of rotation. The second source of electronic magnetic moment is
due to a
quantum mechanical property called
spin.
In an atom the orbital magnetic moments of some electron
pairs cancel each other. The same is true for the spin magnetic moments. The
overall magnetic moment of the atom is thus the sum of all of the magnetic
moments of the individual electrons, accounting for moment cancellation between
properly paired electrons. For the case of a completely filled electron shell or
subshell, the magnetic moments completely cancel each other out. Thus only atoms
with partially filled electron shells have a magnetic moment. The magnetic
properties of materials are in large part determined by the nature and magnitude
of the atomic magnetic moments.
Several forms of magnetic behavior have been observed
including:
-
Diamagnetism
-
Paramagnetism
-
Ferromagnetism
-
Antiferromagnetism
-
Ferrimagnetism
-
Metamagnetism
-
Spin
glass
-
Superparamagnetism
Highly magnetic
stars called
magnetars
are also believed to exist.
Magnetic material may be modelled by a system of spins
located at positions in a
lattice,
where the interaction of neighboring spins contributes to the total energy of
the system and the states of the spins change according to some
non-deterministic (probabalistic) rule (the dynamics of the system). In
the
Ising model spins have only two possible states (up and down),
whereas in the
Potts model they may have more than two possible states.
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