3.若改变积分绕行方向 doe P di B di=f,B cosadz=f, B cost 0)du B cose dl=-[at Hol 0 2r do =-1CI3. 若改变积分绕行方向 = cos θ dl  l B (π ) = cosθ dl  l B I 2π =  d μ o j 2π 0 = μ o I  l B.dl = cos dl  l B a . r B dl I dj P O l θ a θ