5. (5%, 本題計算過程須計分) Find an equivalent resistor Z =      (5)      (Fig. 3 lower) for the infinite blocks of resistors (Fig. 3 upper).

6. (7%, 本題計算過程須計分) Two spherical conductors of radii r₁ and r₂ are separated by a distance much greater than the radius of either sphere. The spheres are connected by a conducting wire as shown in Fig. 4. The charges on the spheres in equilibrium are q₁ and q₂, respectively, and they are uniformly charged. Find the ratio of the magnitude of the electric field at the surface of the sphere 1 to the one of the sphere 2 is (6a) (3%). Please use the meaning of the ratio to explain the principle of a lightning rod (避雷計). Your explanation: (6b) (4%)

5. (5%, 本題計算過程須計分) Find an equivalent resistor Z = (5) (Fig. 3 lower) for the infinite blocks of resistors (Fig. 3 upper).

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