Important Short Questions and Answers: Dielectric Materials

SHORT QUESTIONS WITH ANSWERS

1.What is a dielectric material? State their property.

Dielectrics are the insulating materials having electric dipole moment permanently or temporarily by applying the electric field. These are mainly used to store electrical energy and used as electrical insulators. All dielectrics are electrical insulators. But all electrical insulators need not to be dielectrics. Dielectrics are non-metallic materials of high specific resistance and have negative temperature coefficient of resistance.

2.Define electric flux density (D) or Electric Displacement Vector.

The number of electric lines passing through the unit area of cross section.

Unit: Coulomb / m²

3.Define Permittivity

It is the ratio of electric displacement vector in a dielectric medium to the applied electric field strength. ε= D / E

4.Define Dielectric constant or Relative Permittivity

It is the ratio of permittivity of the medium to the permittivity of the free space.

5. Define Dipole moment

Dipole moment is defined as the product of charge and distance.

Unit : Coulomb meter.

6.Define Polarization, Polarization vector and Polarisability

Polarization

The separation of negative and positive charges is called polarization. i.e., the process of producing electric dipoles by an electric field is called polarization.

Polarization vector

If is the average dipole moment per molecule and N is the number of molecules per unit volume then the polarization of the solid is given by the polarization vector P and it can be written as

The polarization vector is the dipole moment per unit volume of the dielectric material.

Polarisability

The polarization depends on electric field.

the proportionality constant is called as polarisability.

7. Define Electronic Polarization.

Electronic polarization occurs due to the displacement of positively charged nucleus and negatively charged electron in opposite directions by an external electric field. It creates a dipole moment in the dielectric. This is called electronic polarization.

8.What is ionic polarization.

The displacement of cations (+ve) and anions (-ve) in opposite directions is called ionic polarization. It occurs in ionic solids in the presence of electric field

9.  Define orientation polarization.

When an electric field is applied on the dielectric medium with polar molecules, the electric field tries to align these dipoles along its field direction, due to that there is a resultant dipole moment in that material and this process is called orientation polarization.

10. What is Space charge polarization?

The space charge polarization occurs due to the diffusion of ions along the field direction giving rise to redistribution of charge in the dielectrics. Normally this type of polarization occurs in ferrites and semiconductors and it is very small when compared to other types of polarization.

11.What are polar and non-polar molecules?

Molecules which are having permanent dipole moment even in the absence of an applied field are called polar molecules. Example: H₂ O, HC, CO.

Molecules which do not have permanent dipole moment, but they have induced dipole moment in the presence of applied electric field are called non - polar molecules.

Example: O₂ , H₂ , N₂

12. Define Local or internal or Lorentz field.

In a dielectric material, the field acting at the location of an atom is called local field or internal field [ E_i ]

The internal field E must be equal to the sum of applied field and the field due to location of the atom by the dipoles of all other atoms

ie, E_i = E + The field due to all other dipoles.

13. Define Dielectric Loss and Dielectric breakdown.

When a dielectric material is subjected to an alternating electric field, some amount of energy is absorbed by the material and is dissipated in the form heat. This loss of energy is called Dielectric loss.

When a dielectric material loses its property and permids the flow of a large current, it is said to be dielectric breakdown.

14.What is optical absorption and infrared absorption in a dielectric?

The dielectric losses in the optical region, associated with the electrons are referred to as optical absorption. This absorption leads to color of materials. The dielectric losses in the infra-red region, associated with the ionic vibrations are referred to as Infrared absorption.

15. Define Dielectric breakdown and what are its types?

When a dielectric material loses its property and permits the flow of a large current, it is known a dielectric breakdown.

Types

         Intrinsic breakdown.

         Thermal breakdown.

         Electrochemical breakdown.

         Defect breakdown.

         Discharge breakdown.

16. What are Ferro electric materials?

Materials which exhibit electric polarization even in the absence of electric field are known as Ferro electric materials.

Crystalline dielectric materials which posses a permanent electric polarization are called ferroelectric materials have electric dipole moment even in the absence of any field. Normally they are anisotropic crystals which exhibit spontaneous polarization.

Examples : Parium Titanate [Ba TiO₃ ], Potassium Dihydrogen Phosphate [K H₂ P O₄ ], Lithium Niobate [LiNb O₃ ] and Rochelle salt.

17. What are the differences between polar and non - polar molecules?

Polar molecules

These molecules have permanent dipole moment even in the absence of on applied field.

The polarization of polar molecules is highly temperature dependent.

There is absorption or emission in the infrared range for these molecules

Example: H₂ O, HC1, CO

Non - Polar molecules

These molecules do not have permanent dipole moment

The polarization in these type of molecules is independent of temperature.

There is no absorption of emission in infrared range for these molecules

Example: O₂, H₂, N₂

18. Compare active and passion dielectrics.

Active dielectrics

Dielectrics which can easily adapt to store the electrical energy in it is called active dielectrics.

They are used in the production of ultrasonics.

Example : Piezo electrics, Ferro electrics

Passive dielectrics

Dielectrics which resulting the flow of electrical energy in it are called passive dielectrics.

They are used in production of sheets.

Example : Glass, mica and Plastic

19. What are the applications of ferroelectric materials?

         Ferroelectric materials are used to make pressure transducers, ultrasonic transducers, microphones and gas filters.

         They are used as memory cores in computers.

         They are used to measure and control temperature.

         Ferroelectric ceramics are used as capacitors to store electrical energy

         They are used to make very good infrared detectors.

         Rochelle salt is used in devices like microphones, strain gauges, phonograph pickups and SONAR devices.

         In optical communication, the ferroelectric crystals are used for optical modulation.

         Ferro electric materials are used to produce ultrasonics

         Electrets are also used to bond the fractured bones in human body.

They are used as frequency stabilizers and crystal controlled oscillators.