Permanent magnets
Centuries ago, it was discovered that certain types of mineral
rock possessed unusual properties of attraction to the metal iron.
One particular mineral, called lodestone, or magnetite,
is found mentioned in very old historical records (about 2500 years
ago in Europe, and much earlier in the Far East) as a subject of
curiosity. Later, it was employed in the aid of navigation, as it
was found that a piece of this unusual rock would tend to orient
itself in a north-south direction if left free to rotate (suspended
on a string or on a float in water). A scientific study undertaken
in 1269 by Peter Peregrinus revealed that steel could be similarly
"charged" with this unusual property after being rubbed against one
of the "poles" of a piece of lodestone.
Unlike electric charges (such as those observed when amber is
rubbed against cloth), magnetic objects possessed two poles of
opposite effect, denoted "north" and "south" after their
self-orientation to the earth. As Peregrinus found, it was
impossible to isolate one of these poles by itself by cutting a
piece of lodestone in half: each resulting piece possessed its own
pair of poles:
Like electric charges, there were only two types of poles to be
found: north and south (by analogy, positive and negative). Just as
with electric charges, same poles repel one another, while opposite
poles attract. This force, like that caused by static electricity,
extended itself invisibly over space, and could even pass through
objects such as paper and wood with little effect upon strength.
The philosopher-scientist Rene Descartes noted that this
invisible "field" could be mapped by placing a magnet underneath a
flat piece of cloth or wood and sprinkling iron filings on top. The
filings will align themselves with the magnetic field, "mapping" its
shape. The result shows how the field continues unbroken from one
pole of a magnet to the other:
As with any kind of field (electric, magnetic, gravitational),
the total quantity, or effect, of the field is referred to as a
flux, while the "push" causing the flux to form in space is
called a force. Michael Faraday coined the term "tube" to
refer to a string of magnetic flux in space (the term "line" is more
commonly used now). Indeed, the measurement of magnetic field flux
is often defined in terms of the number of flux lines, although it
is doubtful that such fields exist in individual, discrete lines of
constant value.
Modern theories of magnetism maintain that a magnetic field is
produced by an electric charge in motion, and thus it is theorized
that the magnetic field of a so-called "permanent" magnets such as
lodestone is the result of electrons within the atoms of iron
spinning uniformly in the same direction. Whether or not the
electrons in a material's atoms are subject to this kind of uniform
spinning is dictated by the atomic structure of the material (not
unlike how electrical conductivity is dictated by the electron
binding in a material's atoms). Thus, only certain types of
substances react with magnetic fields, and even fewer have the
ability to permanently sustain a magnetic field.
Iron is one of those types of substances that readily magnetizes.
If a piece of iron is brought near a permanent magnet, the electrons
within the atoms in the iron orient their spins to match the
magnetic field force produced by the permanent magnet, and the iron
becomes "magnetized." The iron will magnetize in such a way as to
incorporate the magnetic flux lines into its shape, which attracts
it toward the permanent magnet, no matter which pole of the
permanent magnet is offered to the iron:
The previously unmagnetized iron becomes magnetized as it is
brought closer to the permanent magnet. No matter what pole of the
permanent magnet is extended toward the iron, the iron will
magnetize in such a way as to be attracted toward the magnet:
Referencing the natural magnetic properties of iron (Latin = "ferrum"),
a ferromagnetic material is one that readily magnetizes (its
constituent atoms easily orient their electron spins to conform to
an external magnetic field force). All materials are magnetic to
some degree, and those that are not considered ferromagnetic (easily
magnetized) are classified as either paramagnetic (slightly
magnetic) or diamagnetic (tend to exclude magnetic fields).
Of the two, diamagnetic materials are the strangest. In the presence
of an external magnetic field, they actually become slightly
magnetized in the opposite direction, so as to repel the external
field!
If a ferromagnetic material tends to retain its magnetization
after an external field is removed, it is said to have good
retentivity. This, of course, is a necessary quality for a
permanent magnet.
- REVIEW:
- Lodestone (also called Magnetite) is a
naturally-occurring "permanent" magnet mineral. By "permanent," it
is meant that the material maintains a magnetic field with no
external help. The characteristic of any magnetic material to do
so is called retentivity.
- Ferromagnetic materials are easily magnetized.
- Paramagnetic materials are magnetized with more
difficulty.
- Diamagnetic materials actually tend to repel external
magnetic fields by magnetizing in the opposite direction.
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