Characteristics of dielectric materials behaves in a capacitor

Capacitor a static electrical device used to store electrical energy for short time. In general capacitor made with two electrodes and a dielectric insulating medium kept between those electrodes. There are various type of material used as dielectric medium. Each makes different effect in capacitance and accumulating charge in a capacitor. Here we shall discuss about characteristics of dielectric materials behaves in a capacitor

In Vacuum medium

Let consider two parallel plates are connected across a potential difference ‘V’ and kept in a vacuum chamber. As there is no other material used between these plates vacuum act as dielectric medium. On apply of potential difference between both plates, they will become positive and negatively charged (Q₀) with uniform electric intensity.

There will be uniform field intensity maintained,

Electric field intensity in parallel plates (E) =  V/d Volts/metre

The magnitude of charge accumulated is proportional to applied voltage between plates.

Q₀ = V

Q₀ =C₀ V

where

Q₀ = Charge accumulated in vacuum

C₀ = Capacitance when vacuum used as di electric medium

On application of Gauss theorem, it states that electric flux in a closed surface is proportional to net electric charge in that surface (Q) and 1/ _, where  is known as relative permittivity of free space.

Electric Flux (D) =Q/ε₀

Electric field strength and flux density are related to each other

D =  ε₀E

(or)

The field strength between plates (E) = D/ε₀

Electric flux density (D) =Q/ε₀ x 1/A = Q/ε₀A

Since V = Ad capacitance of plates given as Q₀ =^ε₀ A/d

where ε₀ is absolute permittivity in the free space expressed in farads /metre

Absolute permittivity in free space =8.85 × 10^-12

When dielectric medium introduced

Now let consider that some dielectric material is introduced between both parallel plates of capacitor.

By keeping potential difference between plates constant, its charge has increased to considerable amount

Charge between plates when dielectric medium introduced (Q) = C/V

The capacitance value increased in insertion of die electric medium C =εA/d

Where  is relative permittivity of dielectric medium

Relative permittivity of a dielectric medium defined as ratio of actual permittivity of a medium to absolute permittivity in vacuum

Relative permittivity ε_r = ε/ε₀

It also known as dielectric constant of a material

It can be measured with and without dielectric medium by using proper experimental setup

It varies 1 to 10 for normal solid substances and more than 10 for liquid like alcohol and glycerin. For air it is more in high frequencies

The dielectric constant of a medium will be constant state, if medium does not vary from point to point. Definitely, at boundary between two medium dielectric constant changes and bodies are non-homogenous with respect to density and other properties are usually non homogenous with the dielectric constant as well