E. E (1)延长线上 q E=E-E q E E +4兀o(r → 2 E q 2rl g 2Ir 1 2P 46(2-12/4)2]4r4zr 1<<r 定义P=q（1）延长线上 EP = E+ − E− → − + = 2 0 ) 2 ( 4 1 l r q E    + − = 2 0 ) 2 ( 4 1 l r q E   3 0 4 0 2 2 2 0 2 4 2 1 ( / 4) 4 2 4 r P r q lr r l q rl E       = =       − = l <<r 定义 P = ql