2.平面内环路 rdo=dl cose B Ⅰd9 2 dl 乐,B.dl=,Boa1=51Brd =1rd9-n1∫ma2. 平面内环路 =μ o I 2π r I =  rd μ o j r dj = dl cosθ = 2 I π μ o r B  l B.dl = cosθ dl  l B =  l B r dj I 2π =  d μ o j 2π 0 . θ r B dl I dj P O l