L单位正 电荷、r b dr a→>b一 dr 丌Enr 0 兀8 1-4=U-U 4丌 0 4丌anr4 L ( ) a b a b b a b a L a b U U r q r q r q r r r r q W = − = − =  = →   0 0 2 0 2 0 4 4 4 ˆ 4         d d 