A parallel plate capacitor is made of two plates of length l , width w and separated by distance d . A dielectric slab (dielectric constant K) that fits exactly between the plates is held near the edge of the plates .It is pulled into the capacitor by a force F=−∂U∂x where U is the energy of the capacitor up to distance x (See figure) .If the charge on the capacitor is Q then the force on the dielectric when it is near the edge is :
