MAGNETISM
Basic Electrical Theory
Generally, different types of materials have different values of reluctance (Figure 25). Air gap
is the air space between two poles of a magnet. Since air has a very high reluctance, the size
of the air gap affects the value of reluctance: the shorter the air gap, the stronger the field in the
gap. Air is nonmagnetic and will not concentrate magnetic lines. The larger air gap only
provides space for the magnetic lines to spread out.
Summary
The important information contained in this chapter is summarized below.
Magnetism Summary
Magnetic flux - group of magnetic field lines that are emitted outward from the
north pole of a magnet
Magnetic flux density - amount of magnetic flux per unit area of a section,
perpendicular to the direction of the flux
Weber - measure of magnetic flux
Permeability - ability of a material to concentrate magnetic lines of flux
Ferromagnetic materials - iron, steel, nickel, cobalt, and commercial alloys with
relative permeability ranging from 50-200
Paramagnetic materials - aluminum, platinum, manganese, and chromium with
relative permeability of slightly more than one
Diamagnetic materials - bismuth, antimony, copper, zinc, mercury, gold, and
silver with relative permeability of less than one
Magnetomotive force (mmf) - strength of a magnetic field in a coil of wire
dependent on current flowing through coil
Ampere turns - current flowing through a coil times the number of turns in the
coil
Field intensity - identifies the magnetic flux density per unit length of a coil
Reluctance - opposition to the production of flux in a material
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