E q En fy 4兀6(r B B E q E 4兀60(r+ E E e A x E A4兀E0(r (r+ 2 2 4元 0 12p 兀EnF (1-)(1 2 2r 4丌Ei r l r l r qrl i l r q l r q EA    4 2 2 0 2 2 0 ) 2 ) (1 2 4 (1 2 ) 2 ) ( 2 ( 4 1 − + =           + − − =   3 0 3 0 2 4 1 2 4 1 r p i r ql EA      = = i ) l (r q E   2 0 2 4 − + =   i ) l (r q E   2 0 2 4 + − − =   l r y x • • B A l  r E+  E+  E−  E−  EB  EA 