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 = μ_oI
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: