用有限差分法求电场电势分布，金属盒子接地，电位是0，中间三分之一处有电位是1的长方体，求解其电场电势分布(The electric potential distribution of the electric field is calculated by the finite difference method. The metal box is grounded, the potential is 0, and the 1/3 of the rectangular bodies with the potential is 1, and the electric potential distribution of the electric field is solved.)