证 B B=-Bk Ⅰdl Idi=Idxi+Idyj dF=ld1×B 大 Idx Idy 0= BIdxj-BIdy i 00-B F=BI(o dxj -ody i)=BILjI dl = Idx i + I dyj 证： B = B k dF =I dl × B = i j k Idx I dy 0 0 0 B = BIdxj BIdy i F =BI (  0 dxj  dy i ) 0 0 L =BI L j × × × × × × × × × × × × × × × × × × × × × × × × x y I dl L B