十五章场的源 R +/=R CSC B 1= Rtg B4么 dl=-rcsc BaB R P 区88888888区888888 b=dB= 3句 sin BdB BI nl(COS B,-coS Bu)第十五章 磁场的源  2 2 2 2 R +l = R csc A1 A2 R P 1  2 l dl r  dl R d 2 = − csc l = Rctg    = L B dB  = − 2 1 sin 2 0      nI d (cos cos ) 2 2 1 0    = nI −