# 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:

where

q 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
2

pr, and so the equation becomes:

B2pr = m_{o}I

therefore: B = m_{o}I/2pr