− | In the special case of a wire in free space, the integral is easy to evaluate. At a radius r, the field is uniform over a circular path round the wire , the path length is 2πr. So for the magnetic field B at a radius r from a wire in free space, B2πr=u0I. | + | In the special case of a wire in free space, the integral is easy to evaluate. At a radius r, the field is uniform over a circular path round the wire , the path length is 2πr. So for the magnetic field B at a radius r from a wire in free space, [[File:B2Pir-equals-uoI.jpg]] |