Electrical polarization mechanisms

engineering

1. TOPIC :- ELECTRICAL POLARIZATION MECHANISMS MADE BY :- MALAY FALDU(150090119005) NAVED FRUITWALA(150990119006) UTKARSH GANDHI(150990119007) DIGVIJAYSINH GOHIL(150990119008) GUIDED BY :- SANJAY SIR SUBJECT :- PHYSICS

2. ELECTRICAL POLARISATION MECHANISM There are four different types of mechanisms through which electrical polarization can occur in dielectric materials when it is subjected to an external electric field. Electronic Polarization Ionic Polarization Orientation Polarization Space charge or Interfacial Polarization

3. Electronic Polarization When an EF is applied to an atom, + ve ly charged nucleus displaces in the direction of field and ẽ could in opposite direction. This kind of displacement will produce an electric dipole with in the atom. i.e , dipole moment is proportional to the magnitude of field strength and is given by, μ e = α e E where ‘ α e ’ is called electronic Polarizability constant.

4. Expression for Electronic Polarization Consider a atom in an EF of intensity ‘E’ since the nucleus (+ Ze ) and electron cloud (- Ze ) of the atom have opposite charges and acted upon by Lorentz force (F L ). Subsequently nucleus moves in the direction of field and electron cloud in opposite direction. When electron cloud and nucleus get shifted from their normal positions, an attractive force b/w them is created and the separation continuous until columbic force F C is balanced with Lorentz force F L , Finally a new equilibriums state is established.

5. Fig(2) represents displacement of nucleus and electron cloud and we assume that the - ve charge in the cloud uniformly distributed over a sphere of radius R and the spherical shape does not change for convenience.

6. Let σ be the charge density of the sphere

7. Force experienced by displaced nucleus in EF of Strength E is F L = Eq = ZeE -----(3) Hence electronic Polaris ability is directly proportional to cube of the radius of the atom.

8. Ionic polarization The dielectric material having ionic bonds such as Nacl, show ionic polarization. Polarization in such ionic crystals arises on account of the ions displaced from their equilibrium positions by the force of the applied electric field. The induced dipole moment due to ionic polarization is proportional to the applied electric field, i.e. For the most of the materials, the ionic polarizability is very less than the electronic polarizability ,

9. When a EF is applied to the molecule, the positive ions displaced by X 1 to the negative side electric field and negative ions displaced by X 2 to the positive side of field. The displacement between ions causes an increase or decrease in distance between the atoms, depending on the location of the ion pairs. The resultant dipole moment µ = e ( X 1 + X 2 ).. Restoring force constant depend upon the mass of the ion and natural frequency and is given by,

10. Where ‘M’ mass of anion and ‘m’ is mass of cat ion. It is independent of temperature.

11. Orientation Polarization Orientation polarization occurs in polar substances. These substances exhibit dipole moment even in the absence of external electric field. Due to random orientations of dipoles or molecules, the net dipole moment is zero . When such materials are subjected to an external electric field, the permanent molecular dipoles rotate about their axis of symmetry to align with the applied field. In electronic and ionic polarization processes, the force due to the externally applied field is balanced by elastic binding forces, but no such forces exist for orientation polarization. At thermal equilibrium with no external electric field, the permanent dipoles contribute no net polarization since they are randomly oriented.

12. With the application of external electric field, dipole alignment is largely offset by thermal agitation. The orientation polarization is strongly temperature dependent; it decreases with increase in the temperature. Expression for orientation polarization

13. Space Charge Polarization Sometimes due to the application of electric field to the dielectric material, charges accumulate at the electrodes or at the interface due to sudden change in conductivity shown in Fig. (a) and (b). Under the influence of applied electric field, the ions are diffused over appreciable distance, due to which redistribution of charges in the dielectric medium takes place. The tendency of redistribution of charges in the dielectric medium in the presence of an external electric field is known as space charge polarization.

14. Total Polarization Although the dielectric materials are classified in different groups based in the their mode of polarization, if a material can experience all forms of polarization, then its total polarizability can be given as the sum of electronic, ionic, and orientation polarizability , i.e. This is called Langevin – Debye equation for Total Polarizability in dielectrics. Here the contribution due to space charge polarization is not considered because it is almost negligible in most common dielectrics. In the above equations, first two terms on the right hand side are the functions of molecular structure of the dielectric material which are usually independent of temperature. Due to this reason, they are also known as deformation polarisability.

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