P3-48 A parallel-plate capacitor of width w, length L, and separation d has a solid dielectric slab of permittivity in the space between the plates. The capacitor is charged to a voltage V, by a battery, as indicated in Fig. 3-45. Assuming that the dielectric slab is withdrawn to the position shown, determine the force acting on the slab a) with the switch closed, b) after the switch is first opened. Switch Vo FIGURE 3-45 A partially filled parallel-plate capacitor (Problem P.3-48).