if a dielectric slab of dielectric constant K is introduced between the plates of a parallel plate capacitor completely , how does the energy density of the capacitor change?

For a normal parallel plate capacitor the energy density is given as

u = (1/2)ε₀E²

and

for a capacitor filled with dielectric the energy density is given as

u' = (1/2)εE²

where

ε = Kε₀

K - dielectric constant

so,

u' = (1/2)Kε₀E²

thus, we get

u' = Ku

so, change in energy density will be

du = u' - u = Ku - u

thus,

du = u.(K-1)