Define dielectric constant of a medium. Briefly explain why the capacitance of a parallel plate capacitor increases, on introducing a dielectric medium between the plates.

Explanation

Dielectric constant (K) of a medium is defined as the ratio of absolute permittivity (`epsi`) of the medium to the permittivity of free space (`epsi_0`). Alternately dielectric constant of a medium is defined as the ratio of the capacitance of a parallel plate capacitor when filled with that medium to its capacitance with free space as the dielectric.
When a dielectric medium is introduced between the plates of a parallel plate capacitor, the dielectric is polarised by the electric field between the plates. As a result, the electric field and hence potential difference between the plates of capacitor decreases. Consequently, the capacitance of the capacitor increases.