Question

How does the stored charge if air is replaced by medium of dielectric constant?

Answer 1

The equations are C=q/V=ereoA/d=q2/E because V=E/q. A dielectric has a higher er than air. So the capacitance goes up and the energy E goes down because the charge q stays the same. The charge q stays the same because the plates have the same area and separation distance d. Just follow the equations.