2.任意积分回路 fB.dl=∮Bcos0dl = cos Odl jB●d=HoI    2 2 0 I = 2. 任意积分回路  B dl =  Bcosdl   =  dl r I    cos 2 0 =     rd r I 2 0 B dl I  • = 0   r I •  d dl  B   P N