dB= lord 2(r2+x2) B 0 OR oSin ed6·Rsin26 RQ 2R3 0 R uogasin6d6 2 方向如图 b=dB=DRools 2 23000=-FoRoo 写成矢量式:B=2nRa R o x r dI sin d 2 2 sin d sin 2( ) d d 3 0 3 2 2 2 0 2 2 2 0 2 3 R R R R r x r I B = = + = B B R 0 R 0 0 3 3 2 sin d 2 d = = = B 0 R 3 2 写成矢量式: = B d 方向如图