无限长载流直导线 B a1=0a=兀B 2za 半无限长载流直导线 201,=丌B 0 刀L 论|·直导线延长线上 dB-uo Id sina=0 B=40l (cos a, -cosa,) 兀r arT a=0dB=0-B=0 國(下一页)•无限长载流直导线 0 1  =  =  2 a I B   2 0 = •半无限长载流直导线 2 1  =   =  2 a I B   4 0 = •直导线延长线上 2 0 4 r Idl sin dB    =  = 0 dB = 0 B = 0 (cos cos ) 4 1 2 0     = − a I B 讨 论 I B  （下一页） = 0