【例】圆电流轴线上的磁场 ldl dB r dbi dB R dB1=0 +0 dB B=dB dB=Aold/ IR sine 39 【例】圆电流轴线上的磁场  B= dB// d = 0  B⊥ 2 0 4 d d r I l B   = l r IR r I l B d 4 sin 4 d d 3 0 2 0 //      = = R r x B  d I I l  d dB// dB⊥   o