B=可B=了g=6经a =4o0迟= 4o⊙y 2 2nR 线圈磁矩 如图取微元pm=SdI=m2σordf Pacordr-zog 方向： =  =  =  R rdr r r dI B dB 0 0 0 2 2     B q R  r dr R R q       2 2 0 0 = = 线圈磁矩 p m ISn   = 如图取微元 dp m SdI r  rdr 2 = = 4 4 0 2 R p dp r rdr R m m  =  =     = 方向： 