dL单位正E 电荷 b W a→b 兀8 兀nr q q =U-U 兀8 0a 4兀6ohb4 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 