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MILHDBK419A
6.2.2.2 Inductive Coupling. The magnetic field surrounding a long, straight, currentcarrying wire as shown in
Figure 67 is the means for inductive coupling. This field can be determined from Ampere's law (61):
(61)
where
is the magnetic field strength and
is a small element of length along the path of integration (any
closed loop around the current i(t)). Choosing a circle of radius r for the integration path in Equation 61 allows
one to derive an expression for the magnetic field:
(62)
Thus the magnetic field strength surrounding a long straight wire carrying a current i is inversely proportional
to the distance r from the wire, i.e., H decreases as the distance from the wire increases.
Figure 67. Magnetic Field Surrounding a CurrentCarrying Conductor
68


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