Dielectric in Capacitor

Description:

It was observed by faraday that, air or free space between the two plates of parallel plate capacitor shows certain capacitance (say C₀). If an insulator or a dielectric is placed between the two parallel plates, the capacitance of capacitor increases. Let the capacitance due to dielectric is C_M.

Where (C_M > C₀)

Capacitance due to a medium increases because of polarization property of dielectric. Whenever a dielectric  material is placed between the parallel plates , the dielectric gets polarized, which decreases the electric field of the capacitor.

If E₀ is electric field due to free space within the plates of capacitor, then C₀ due to this will be:

C₀ = q/v = q/E₀ + d

Capacitor due to dielectric medium will be:

C_M = q/v = q/E_Md

We know that, when a dielectric is placed between parallel plates of capacitor, electric field decreases and hence the potential decreases. Since capacitance is inversely propotional to potential, so, capacitance of capacitor increases.

On using different dielectric, the capacitance decreases at a fixed ratio by C_M/C₀

This ratio C_M/C₀ is called as dielectric constant for that given material. It is symbolized by ‘k’.

Note

K is always greater than 1, because electric field always decrease in presence of dielectric.

So, capacitance with a dielectric = dielectric constance * capacitance with air

C_M = K * C₀

The ratio of decrease in electric field is given by :

K = E_M/E₀

Note

Dielectric strength − It is the property of a non conductor. It is the maximum strength of electric field after which a non conductor starts behaving like a conductor. Below dielectric strength,a  non conductor  behaves as an insulator.

Dielectric constant − It is the property ofa diaelectric. It gives the increased value of capacitance due to dielectric between parallel plate of capacitor.

This dielectric medium is always equal to the relative permittivity.

K = E_r

Where,

Relative permittivity (ε_r) = permittivity of a medium (ε_m)/permittivity in free space (ε₀)

We know, If A is the area of capacitor:

Then, C = Aε₀/d

and C_M = K Aε₀/d

So, K = ε_r = ε₀/ε_M = C_M/C₀ = E₀/E_M = F₀/F_M

The table shows ; the different values of dielectric constant and dielectric strength for different material.

For example:

If mica is used as a dielectric, then the capacitance of capacitor will be increased by six times with respect to capacitance due to free space.