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…

A parallel plate capacitor is made of two plates of length $\displaystyle l$, width $\displaystyle w$ and separated by distance $\displaystyle d$. 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 $\displaystyle \mathrm{F}=-\frac{\partial \mathrm{U}}{\partial x}$ where U is the energy of the capacitor when dielectric is inside the capacitor up to distance $\displaystyle x$ (See figure). If the charge on the capacitor is Q then the force on the dielectric when it is near the edge is :