Basic Electrical Theory
MAGNETISM
Example 3:
The 20 cm coil used in Example 1 with the same current is now wound around
an iron core 40 cm in length. Find the field intensity.
Solution:
N = 80, I = 6A, and NI = 480 At
H
480 At
0.4 m
1200 At
m
Note that field intensity for Examples 2 and 3 is the same.
Figure 25 Different Physical Forms of Electromagnets
Reluctance
Opposition to the production of flux in a material is called reluctance, which corresponds to
resistance. The symbol for reluctance is R, and it has the units of ampere-turns per weber
(At/wb).
Reluctance is related to magnetomotive force, mmf, and flux, F, by the relationship shown in
equation (1-15).
(1-15)
R
mmf
F
Reluctance is inversely proportional to permeability (µ). Iron cores have high permeability and,
therefore, low reluctance. Air has a low permeability and, therefore, a high reluctance.
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