A parallel plate capacitor is made of two plates of length l, width ω 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=−∂x∂U where U is the energy of the capacitor when dielectric is inside 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
(1) 2ωl2ϵ0Q2dK
(2) 2dl2ϵ0Q2ω(K−1)
(3) 2ωl2ϵ0Q2d(K−1)
(4) 2dl2ϵ0Q2ωK