Dielectric Properties

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Dielectric Properties, Electric Dipole, Dipole Moment, Dielectric Constant, Polarizability, Electric Susceptibility, Displacement Vector, Electronic, Ionic and Orientation Polarizations and Calculation of polarizabilities - internal Fields in Solids, Clausius-Mossotti Equation, Piezo-Electricity, Pyro-electricity and Ferro electricitty

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Dielectric Properties: 

Dielectric Properties

Introduction: 

Introduction Dielectrics are insulation materials. In dielectrics, all the electrons are bound to their parent molecules and there are no free charges. Even with normal voltage or thermal energy electrons are not released. Dielectrics are non metallic materials of high specific resistance and have negative temperature coefficient of resistance .

Fundamental definitions: 

Fundamental definitions Dielectric constant (  r ) : The ratio between the permittivity of the medium (  ) and the permittivity of free space (  0 ) .  r =/ 0 The dielectric characteristics of a material are determined by the dielectric constant or relative permittivity  r of the material . Since it is a ratio of same quantity.  r has no unit. It is a measure of polarization in the dielectric material.

Fundamental definitions: 

Fundamental definitions Electric Polarization:

Fundamental definitions: 

Fundamental definitions Electric Dipole : Let us consider an atom placed inside an electric field. The centre of positive charge is displaced along the applied field direction while the centre of negative charge is displaced in the opposite direction. Thus a dipole is produced. When a dielectric material is placed inside an electric field such dipoles are created in all the atoms inside.

Fundamental definitions: 

Fundamental definitions This process of producing electric dipoles which are oriented along the field direction is called polarization in dielectrics. Polarizability ( α ): When the strength of the electric field E is increased the strength of the induced dipole µ also increases. Thus the induced dipole moment is proportional to the intensity of the electric field. i.e. µ= α E

Fundamental definitions: 

Fundamental definitions Where α , the constant of proportionality is called Polarizability. It can be defined as induced dipole moment per unit electric field . Polarization vector P : The dipole moment per unit volume of the dielectric material is called Polarization vector P If µ is the average dipole moment per molecule and N is the number of molecules per unit volume the polarization vector P = Nµ

Fundamental definitions: 

Fundamental definitions The dipole moment per unit volume of the solid is the sum of all the individual dipole moments within that volume and is called the polarization P of the solid.

Fundamental definitions: 

Fundamental definitions Electric flux density (or) Electric displacement D: D =  r  0 E When E = electric field,  r = Dielectric constant or permittivity of the material  o = Dielectric constant or permittivity of the free space.

Fundamental definitions: 

Fundamental definitions As the polarization measures the additional flux density arising from the presence of the material as compared to free space, it has the same units as D and is related to it as D =  o E + P Since D =  r  0 E .:  r  0 E =  o E + P P =  o ( r – 1)E Electric flux density D is similar to magnetic induction B in magnetism

Fundamental definitions: 

Fundamental definitions Electric Susceptibility χ e :. The polarization vector P is proportional to the total electric flux density E and is in the same direction of E. Therefore the polarization vector can be written as P =  o χ e E Where the constant χ e is the electric susceptibility

Fundamental definitions: 

Fundamental definitions χ e = P /  o E =  o ( r -1)E /  o E χ e =  r -1

Polarization: 

Polarization The specimen is placed inside a d.c. electric field, polarization is due to four types of process Electronic polarization Ionic polarization Orientation polarization Space – charge polarization

Slide 14: 

Electronic Polarization: The displacement of the positively charged nucleus and the (negative) electrons of an atom in opposite directions, on application of an electric field, result in electronic polarization. Induced dipole moment µ α E µ= α e E Where α e is the electronic Polarizability. No field E Field Applied

Slide 15: 

Ze - Ze R Here the Nucleus of charge Ze is surrounded by an electron cloud of charge – Ze distributed in a sphere of radius R. The charge density 𝝆 is given by When an external field of intensity E is applied, the nucleus and the electrons experience Lorentz forces of magnitude ZeE in opposite directions. Hence the nucleus and electron cloud are pulled apart.

Slide 16: 

When they are separated a coulomb force develops between them, which tends to oppose the displacement. When these forces namely Lorentz force and Coulomb force are equal and opposite, equilibrium is reached. Let x be the displacement under the condition Since nucleus is much heavier than the electron cloud it is assumed that only the electron cloud is displaced when the external field is applied

Slide 17: 

Lorentz force = - ZeE Coulomb force = Ze x The charge enclosed = = =

Slide 18: 

Hence coulomb force is - ZeE = E = Or x = In the equilibrium position

Slide 19: 

Thus the displacement of the electron cloud is proportional to the applied field. Thus the two electric charges + Ze and – ze are separated by a distance x under the action of the applied field thus consisting induced electric dipoles. Induced electric dipole moment = i.e.,

Slide 20: 

Where is called electronic Polarizability. The dipole moment per unit volume is called electronic polarization. It is independent of temperature. = N N Where N is the number of atoms/ From = ( =N Or ( = =

Slide 21: 

Thanking You………. Ref: Engineering Physics by P.K.Palanisamy Author: Ganesh Billa Asst Prof. of Physics College: Vivekananda Groups of Institutions, Batasingaram , Hayathnagar , RR district. 9491318652