q q X 2 X E i E 4兀E0(x-r0/2) 4πEo(x+rn/2)2 e=ee=9 2 xr 4丌o(x2-72/4) x>> E q 4兀E0x3l= 4兀E。xi x r q E   2 0 0 4π ( 2) 1     i x r q E   2 0 0 4π ( 2) 1      i x r q xr E E E                 2 2 0 2 0 0 ( 4) 2 4π  0 x  r i x r q E   3 0 0 2 4π 1   3 0 2 4π 1 x p     q  q E  E  r0 2 r0 2 A x O x