MAGNETISM
Basic Electrical Theory
Example:
Find the flux density in teslas, when the flux is 800 µWb and the area is 0.004 m2.
Given: F = 800 µWb = 8 x 10-4 Wb
A = 0.0004 m2 = 4 x 10-4 m2
B
F
A
8 x 104 Wb
4 x 104 m2
2 Wb/m2
Magnetic Materials
Magnetic materials are those materials that can be either attracted or repelled by a magnet and
can be magnetized themselves. The most commonly used magnetic materials are iron and steel.
A permanent magnet is made of a very hard magnetic material, such as cobalt steel, that retains
its magnetism for long periods of time when the magnetizing field is removed. A temporary
magnet is a material that will not retain its magnetism when the field is removed.
Permeability (µ) refers to the ability of a material to concentrate magnetic lines of flux. Those
materials that can be easily magnetized are considered to have a high permeability. Relative
permeability is the ratio of the permeability of a material to the permeability of a vacuum (µo).
The symbol for relative permeability is µR (mu).
µR =
where µo = 4p10-7H/m
(1-12)
µo
µo
Magnetic materials are classified as either magnetic or nonmagnetic based on the highly magnetic
properties of iron. Because even weak magnetic materials may serve a useful purpose in some
applications, classification includes the three groups described below.
Ferromagnetic Materials: Some of the ferromagnetic materials used are iron, steel, nickel,
cobalt, and the commercial alloys, alnico and peralloy. Ferrites are nonmagnetic, but have the
ferromagnetic properties of iron. Ferrites are made of ceramic material and have relative
permeabilities that range from 50 to 200. They are commonly used in the coils for RF (radio
frequency) transformers.
Paramagnetic Materials: These are materials such as aluminum, platinum, manganese, and
chromium. These materials have a relative permeability of slightly more than one.
Diamagnetic Materials: These are materials such as bismuth, antimony, copper, zinc, mercury,
gold, and silver. These materials have a relative permeability of less than one.
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