DIELECTRIC CONSTANT

If q 1 & q 2 are two charges separated by a distance r in vacuum then as per Coulomb Law the magnitude of Force F is

The permittivity of the medium measures ability of the medium to store energy in an electric field in it.

If vacuum is replaced by another medium or a dielectric then the force F would be less than force between them in vacuum . It will be again given by equation (1) ,where

Ratio between permitivity of medium to free space is called dielectric constant.

K = 1 for vacuum . Air (1.00054) ,MICA (4) Glass (5-10) , Water (80) . The dielectric with smaller value of K is preferred as insulator . However for capacitor use higher dielectric constant materials are employed.

If A & B are two parallel plate capacitors and if A is in vacuum and dielectric is placed between the plates of B. Then we find charge Qb stored in capacitor B is higher than charge Qa stored in capacitor A

Where
C = Capacitance
V = Voltage

Hence

It follows that capacity to hold electric charges increase if dielectric is placed between plates of capacitor.The ratio of capacitance with the dielectric to that of free space is also denoted by dielectric constant K of the material.

Dielectric constant is of significant importance for insulating materials used in high voltage motors. The insulation with higher dielectric constant will support partial discharge. Even very small voids would have sufficient voltage gradients to ionise the air and produce partial discharge.

Usually high voltage motors contain mica epoxy insulation .The system has dielectric constant appox 4. With this system insulation thickness is maintained to provide 60-80 volts/mil gradient. It is a very safe practice. Now if we were to use another insulation system having dielectric constant of 10, then permissible voltage gradient will be limited to 25-30 volts/mil ,thus necessitating insulation thickness to go up by 2.5 times.

BACK