(ii) Force required to pull the dielectric slab from parallel plate capacitor (when battery is disconnected from the capacitor): Let us consider the capacitor has rectangular plates of length l and width b. The length of the dielectric slab inside the plates is x. The capacitance of parallel plate capacitor is
C =
 +
 =
[Kx + (l – x)] =Â
[l + x(K – 1)].
When the battery is disconnected the charge of the capacitor (q) remain constant.
The energy of the capacitor is U =
 =
.
The electrostatic force Fe = external force F required to pull the slab.
So, F = –
=
.