2TR ldl B=B=dB cos0=[ Aold R 47r r r dB R R 27R dl POIR O 4 2(R2+x dB Idl 方向:+x(右螺旋法则) 轴线上 IR B 2(R2+x2)2 3 2( ) d 4 4 d d cos 2 2 2 0 2 0 3 0 2 0 2 0 / / R x IR l r IR r R r I l B B B R R + = = = = = 方向 : + x (右螺旋法则) i R x IR B 2 3 2( ) 2 2 2 0 + = 轴线上 x P R o r B d ' dB I l d ' I l d I