Electrical Engineering ⇒ Topic : Dielectric Constant or Relative Permittivity

Dielectric Constant or Relative Permittivity

The insulating material between the plates of a capacitor is called dielectric. When the capacitor is charged, the electrostatic field extends across the dielectric. The presence of dielectric* increases the concentration of electric lines of force between the plates and hence the charge on each plate.The degree of concentration of electric lines of force between the plates depends upon the nature of dielectric.

The ability of a dielectric material to concentrate electric lines of force between the plates of a capacitor is called dielectric constant or relative permittivity of the material.Air has been assigned a reference value of dielectric constant (or relative permittivity) as 1. The dielectric constant of all other insulating materials is greater than unity. The dielectric constants of materials commonly used in capacitors range from 1 to 10. For example, dielectric constant of mica is 6. It means that if mica is used as a dielectric between the plates of a capacitor, the charge on each plate will be 6 times the value when air is used; other things remaining equal. In other words, with mica as dielectric, the capacitance of the capacitor becomes 6 times as great as when air is used.

Let                  V  = Potential difference between capacitor plates

                       Q = Charge on capacitor when air is dielectric

Then,          C_AIR = Q/V

When mica is used as a dielectric in the capacitor and the same p.d. is applied, the capacitor will now hold a charge of 6Q. 

Hence dielectric constant (or relative permittivity) of a dielectric material is the ratio of capacitance of a capacitor with that material as a dielectric to the capacitance of the same capacitor with air as dielectric.