Saturday, March 31, 2007

Magnetism: Ferro, Para, Dia

FERRO | Ferromagnetism is the "normal" form of magnetism which most people are familiar with, as exhibited in horseshoe magnets and refrigerator magnets, for instance. It is responsible for most of the magnetic behavior encountered in everyday life. The attraction between a magnet and ferromagnetic material is "the quality of magnetism first apparent to the ancient world, and to us today," according to a classic text on ferromagnetism. Ferromagnetism is defined as the phenomenon by which materials, such as iron, in an external magnetic field become magnetized and remain magnetized for a period after the material is no longer in the field. All permanent magnets are either ferromagnetic or ferrimagnetic, as are the metals that are noticeably attracted to them.

No interesting video clip to show. Just find your own magnet!

PARA | Paramagnetism is a form of magnetism which occurs only in the presence of an externally applied magnetic field. Paramagnetic materials are attracted to magnetic fields, hence have a relative magnetic permeability greater than unity (or, equivalently, a positive magnetic susceptibility). However, unlike ferromagnets which are also attracted to magnetic fields, paramagnets do not retain any magnetisation in the absence of an externally applied magnetic field.

Elements/compounds could be paramagnetic if they have unpaired electrons. The following are some examples of paramagnetic elements:

Aluminium Al [13] (metal)
Barium Ba [56] (metal)
Oxygen. O [8] (non-metal)
Platinum Pt [78] (metal)
Sodium Na [11] (metal)
Strontium Sr [38] (metal)
Uranium U [92] (metal)
Technetium Tc [43] (artificial)


Many salts of the d and f transitional metal group show paramagnetic behaviour. Examples are:

Copper sulphate
Dysprosium oxide
Ferric chloride
Ferric oxide
Holmium oxide
Manganese chloride

The below video clip shows a liquid oxygen bridge suspended by a strong u-shaped magnet. This demonstrates the fact that magnetic force > gravitational force.

DIA | "This is a live frog. An object does not need to be superconducting to levitate. Normal things, even humans, can do it as well, if placed in a strong magnetic field. Although the majority of ordinary materials, such as wood or plastic, seem to be non-magnetic, they, too, expel a very small portion (0.00001) of an applied magnetic field, i.e. exhibit very weak diamagnetism. The molecular magnetism is very weak (millions times weaker than ferromagnetism) and usually remains unnoticed in everyday life, thereby producing the wrong impression that materials around us are mainly nonmagnetic. But they are all magnetic. It is just that magnetic fields required to levitate all these "nonmagnetic" materials have to be approximately 100 times larger than for the case of, say, superconductors. This experiment was conducted at the Nijmegen High Field Magnet Laboratory."

A large black superconducting disk was cooled with liquid nitrogen. When the disk goes into the superconducting state it expels magnetic field. This is called perfect diamagnetism. If you place a magnet above the disk when it is superconducting then it will levitate. This is known as the Meissner effect.

[99% Wikipedia and YouTube.]

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