对B点:E,=E=4AEn(r2 +12/4) 2 E=E +E=2E y cOS 6 √r2 +124 2E, cos 0 E B E.=E+E=0 E=2E. coS 6 E EAEX 4兀6 r I p 4丌Enr 1 E B 4兀6 34 ( 4) 1 2 2 0 r l q E E + + = − =  Ex = E+ x + E−x = E+ x 2 4 2 2 2 r l l + cos = = −2E+ cos = + = 0 Ey E+ y E− y 对B点： 2 3 2 0 2 4 4 1 2 ( ) cos l r ql EB E + = = +    3 0 4 1 r p   3 4 0 1 r p EB     = − l r y x • • B A l  r E+  E+  E−  E−  EB  EA 