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.

capacitor in vaccum
Fig: Dielectric medium in vacuum

There will be uniform field intensity maintained,

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



    \[  The Electric Field intensity in parallel plates (E) = \frac{V}{d} Volts/Metre\]

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 =  Îµ0E

(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.

capacitotr in an dielectric medium
Fig: Dielectric medium introduced air/any material

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 εr = 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

Sl.NoDielectric materialDielectric constantDielectric strength
103 Volts per metre
1Paper3.716000
2Porcelain612000
3Silicon2.515000
4Teflon2.160000
5Vacuum1
6Water80
7Neoprine6.75T12000
8Bakelite4.934
9Wood3.34
10Paper3.7216000
Table : Dielectric Electric constant of different material

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