Biology Reference
In-Depth Information
a diamagnetic substance and negative for a paramagnetic substance.
In other words, diamagnetic substances obtain energy if a magnetic field
is applied, and to reduce this excess energy due to the presence of the mag-
netic field, they have a tendency to escape from a magnetic field. If the mag-
netic field is homogeneous, this tendency does not appear (intuitively, the
substance does not know where to move to reduce energy if there is no gra-
dient of the magnetic field). However, if the field is inhomogeneous, the dia-
magnetic substance tries to escape from the magnetic field into a direction
that is expressed by (
grad
H
), thereby reducing its free energy. The magni-
tude of this force is proportional to three parameters,
−
,
H
, and (grad
H
).
Therefore, to increase this force the product
H
(grad
H
) must be increased.
This force has been given several different names, including magnetic
force, magnetization force, and Kelvin body force. Here we will call it the
magnetic force.
χ
Technical Background
Readers who are not interested in the instruments for applying magnetic
fields can skip this section.
Generation of magnetic fields by various magnets
Superconducting magnets, electromagnets, and permanent magnets can
all be used for the purpose of applying a magnetic field.
A superconducting magnet can provide magnetic fields up to about
15 T. Though the superconducting wire must be kept at a liquid helium
temperature to pass or apply an electric current of tens of amperes, it is not
accompanied by heat generation since electric resistance is absent.
Usually, there is a cylindrical bore inside the coil that is kept at room tem-
perature. Even if the coil is kept at an extremely low temperature, heat
insulation allows the bore temperature to be little affected by the low tem-
perature. Since the superconducting current is stable, such magnets offer
static magnetic fields of a large magnitude without fluctuation. They are
also energy efficient, since the zero electrical resistance does not require
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