无限长载流直导线 讨 B= 论 a1=0&2=π 半无限长载流直导线 (cosa-cosa2 4π0•无限长载流直导线 0 1  =  =  2 a I B   2 0 = (cos cos ) 4 1 2 0     = − a I B a P I B  讨 论 •半无限长载流直导线