Page 492 - Basic Electrical Engineering

Where

l = length of the flux path
A = area of cross section of the flux path

μ = permeability of the magnetic material.

It can be observed that reluctance is inversely proportional to permeability
for a particular material. That is to say that a material with high permeability

allows more flux to be established for a given amount of magnetizing force.
Permeability is the ability of a magnetic material which allows the

establishment of flux through it. Thus, permeability is the reciprocal of
reluctance of a magnetic material. Permeability of iron is very high as
compared to air or any non-magnetic material. For free space, i.e., air,

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permeability μ is equal to 4μ × 10 H/m. The permeability of any magnetic
o
material is compared with the permeability of free space and is called relative
permeability μ . Relative permeability of iron is as high as 2000. This means
r
that iron is 2000 times more permeable than air. For the same amount of
ampere turns, an iron-core coil will produce about 2000 times more flux than

an air-core coil as shown in Fig. 5.14.
For the same amount of ampere turns, the flux produced by an iron-core

coil is much more than that produced by an air-core one. Actually, the
amount of flux produced in an iron-core coil is much more than what has

been shown.