MAGNETISM
Basic Electrical Theory
Example:
Calculate the ampere-turns for a coil with 1000 turns and a 5 mA current.
N = 1000 turns and I = 5 mA
substitute
N = 1000 turns and I = 5 x 10-3
NI = 1000 (5 x 10-3) = 5 At
Field Intensity
When a coil with a certain number of ampere-turns is stretched to twice its length, the magnetic
field intensity, or the concentration of its magnetic lines of force, will be half as great. Therefore,
field intensity depends on the length of the coil.
Equation (1-14) is the mathematical
representation for field intensity, which is related to magnetomotive force as shown.
(1-14)
H
FM
L
NI
L
where
H
=
field intensity,At
m
NI
=
ampere-turns (At)
L
=
length between poles of coil (m)
FM
=
Magnetomotive force (mmf)
Example 1:
Find field intensity of an 80 turn, 20 cm coil, with 6A of current.
Solution:
N = 80, I = 6A, and NI = 480 At
H
480 At
0.2 m
2400 At
m
Example 2:
If the same coil in Example 1 were to be stretched to 40 cm with wire length and
current remaining the same, find the new value of field intensity.
Solution:
N = 80, I = 6A, and NI = 480 At
H
480 At
0.4 m
1200 At
m
ES-01
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