∮BD=H=Hd5 B·dS=0 dF,=A2dC2×(1d1×2)=ld2×dB 4丌 F=F+FB=qE+qr×B0 2 2 1 1 21 21 2 2 2 21 ( ) ˆ 4 I d I d r dF I d dB r     = =  0 0 S B dl I j ds =  =    F F F qE qv B = + = +  E B B ds = 0 