对B点:E=E q 4zEn(r2+P2/4) 2 E=E +e=2E y cos 8 √r2+124 2E. cos e E.=E+E=0 E=2E. cos 0 EE E 4兀6 Ax 4e r E 1 p 4兀E0 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 