十五章场的源 b=l dB Mo ldl sin 8 4兀 r=a/sn8 l=-actg0 P dl=ad0/sin 0 BuoI r dsin b uoI re2 sin ede 4兀 47a Je, B=2(coS 6 4 Icos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 