•无限长载流直导线 讨 论 a1=0a2=π B •半无限长载流直导线 a1=π/2w2=π B= 4π •直导线延长线上 dB= Ho Idl sina 4π r2 a=0dB=0一B=0 cosa -cosaz 4π •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 I B  讨 论