Ampere's law
This law is an alternative to the Biot-Savart law for deriving
the expressions for the magnetic flux density of a field. It states that the line integral of the flux
density around a closed path is equal to the product of the current enclosed by the path and the
permeability of the material, that is:
∫B cos θ dL = μoI
where θ is the angle between a short length of path dL and the
direction of the field at that point.
For example the line integral for a straight wire is
2πr, and so the equation becomes:
B2πr = μoI
therefore: B = μoI/2pr
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