A dielectric constant of a substance is the factor by which the capacitance increases from its vacuum value when the dielectric is fully inserted in between the plates of the capacitor.
In the given diagram, a dielectric is inside a parallel plate capacitor.
The electric field inside a capacitor is given by
Ɛ is the permittivity of the substance.
Potential difference between the plates is given by
Where k is dielectric constant of the substance
K = 1 else
K = Ɛ/Ɛ0