Suppose that the capacitor is connected to a battery, an electric field E_{0 }is produced. Now, if we insert the dielectric slab of thickness t = d/ 2, the electric field reduces to E.

Now, the gap between plates is divided in two parts, for distance t there is electric field E and for the remaining distance (d—t) the electric field is E_{0} If V be the potential difference between the plates of the capacitor, then