载流直导线的磁场: B (cos a-cos a2) 4 无限长载流直导线 B P 2na 直导线延长线上: B=0 载流圆环 B B R 2R 载流圆弧 B=2R2兀3 载流直导线的磁场: 无限长载流直导线: a I B   2 0 = 直导线延长线上: B = 0 R I B 2  0 = 载流圆环 载流圆弧    2 2 0 =  R I B (cos cos ) 4 1 2 0     = − a I B I B   R