An air-filled parallel plate capacitor of length L, width a, and plate separation d has its plates

Question:

An air-filled parallel plate capacitor of length L, width a, and plate separation d has its plates maintained at constant potential difference Vo. If a dielectric slab of dielectric constant ?r is slid between the plates and is withdrawn until only a length x remains between the plates as in figure, show that the force tending to restore the slab to its original position is