Magnetic forces are fundamental forces that arise due to the movement of electrical charge. Maxwell's equations 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 orbital motion (there is no orbital motion of electrons around the
nucleus like planets around the sun, but there is an 'effective electron velocity') and
spin of electrons, resulting in what are known as '
permanent magnets'.
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.
Gauss (G) and
tesla (T) are units to define the intensity of magnetic fields. One
tesla is equivalent to 10 000
gauss.
Typically, the
field strength of
MRI scanners is between 0.15 T and 3 T.
See also
Diamagnetism,
Paramagnetism,
Superparamagnetism, and
Ferromagnetism.
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