第十五章磁场的源 b=dB lo ldl sin 6 L4兀 r=a/sml=-a·ctg P dl=ad6/sin e B=olr dl sin g ul re, 1 9s sin 0de 4T JL 4a B cOS cOS 47a第十五章 磁场的源   = L B dB  = L r Idl 2 0 sin 4    r = a /sin l = −a c tg   2 dl = ad /sin   = L r I dl B 2 0 sin 4     = 2 1 sin 4 0       d a I (cos cos ) 4 B 1 2 0      = − a I a P 1  l r   2 