Force on a Dielectric Slab

Capacitance of a parallel plate capacitor becomes $\frac{4}{3}$ times its original value, if a dielectric slab of thickness $t=\frac{d}{2}$ is inserted between the plates $[d$ is the separation between the plates]. The dielectric constant of the slab is

We have a parallel plate air capacitor of separation $d$ and the length and width of the plates are $\mathrm{L}$ and $b$ respectively. It is connected to a cell of emf $E$ Fig. A dielectric slab of

mass $m$ has the same dimension as the inner dimensions of the capacitor. We insert the slab into the space between the capacitor slowly upon a distance $l$ and then release it from rest. Show that in absence of friction it oscillates about that position and find the time-period.