Important Questions and Answers: Dielectric Materials

DIELECTRIC MATERIALS

1. Define dielectric constant?

It is the ratio between the absolute permittivity of the medium (ε)and the permittivity of free space (ε0).

Dielectric constant εr =   Absolute permittivity (ε) / Permittivity of free space (ε0)

2. Define polarization of a dielectric material.

The process of the producing electrical dipoles inside the dielectric by the application an external electrical field is called polarization in dielectrics.

Induced dipole moment (µ) = αE E →Applied electrical field

α → Polarizability

3. Name the four polarisation mechanisms. i. Electronic polarisation.

ii.          Ionic polarisation.

iii.         Orientational polarisation.

iv.          Space- charge polarisation.

v.

4. What is electronic polarisation?

Electronic polarisation means production of electric dipoles by the applied electric field .It is due to shifting of charges in the material by the applied electric field.

5. What is ionic polarisation?

Ionic polarisation is due to the displacement of cations (negative ions) and anions (positive ions) in opposite direction due to the application of an electrical field. This occurs in an ionic solid.

6. What is orientation polarisation?

When an electrical field is applied on the dielectric medium with polar molecules, the dipole align themselves in the field direction and thereby increases electric dipole moment.

Such a type of contribution to polarisation due to the orientation of permanent dipoles by the applied field is called orientation polarisation.

7. What is space- charge polarisation?

In some materials containing two or more phases, the application of an electrical field causes the accumulation of charges at the interfaces between the phases or at the electrodes.

As result of this, polarisation is produced. This type of polarisation is known as space

charge polarisation.

8. Define dielectric loss and loss tangent.

When a dielectric material is subjected to an A.C voltage, the electrical energy is absorbed by the material and is dissipated in the form of heat. This dissipation of energy is called

dielectric loss.

In a perfect insulator, polarisation is complete during each cycle and there is no consumption of energy and the charging current leads the applied voltage by 90⁰. But for

commercial dielectric, this phase angle is less than 90⁰ by an angle and is called dielectric loss angle. Tan is taken as measure of dielectric loss and is known as loss tangent.

9. Define dielectric breakdown and dielectric strength.

Whenever the electrical field strength applied to a dielectric exceeds a critical value, very large current flows through it. The dielectric loses its insulating property and becomes conducting. This phenomenon is known as dielectric breakdown.

The electrical field strength at which dielectric breakdown occurs is known as dielectric

Strength.

10.            Mention the various breakdown mechanisms.

i)                   Intrinsic breakdown and avalanche breakdown

ii)                Thermal breakdown

iii)              Chemical and Electrochemical breakdown

iv)              Discharge break down

v)                Defect breakdown

11.            What is intrinsic breakdown?

For a dielectric, the charge displacement increases with increasing electrical field strength. Beyond a critical value of electrical field strength, there is an electrical breakdown due to physical deterioration in the dielectric material.

12. What is thermal breakdown?

When an electrical field is applied to a dielectric material, some amount of hear is produced. This heat must be dissipated from the material.

In some cases, the amount of hear produced is very large as compared to the heat dissipated. Due to excess of heat the temperature inside the dielectric increases and may produce local melting in the dielectric material.

During this process, a large amount of current flows through the material and causes their dielectric to breakdown. This type of breakdown is known as thermal breakdown.

13. What is chemical and electrochemical breakdown?

Electro chemical breakdown is similar to thermal breakdown. When the temperature of a dielectric material increases, mobility of ions increases and hence the electrochemical reaction may take place.

This leads to leakage current and energy loss in the material and finally dielectric breakdown occurs.

14. What is discharge break down?

Discharge breakdown occurs when a dielectric contains occluded gas bubbles. When this type of dielectric is subjected to electric field; the gases present in the material will easily ionize and thus produces large ionization current.

The gaseous ions bombard the solid dielectric. This causes electrical deterioration and leads to dielectric breakdown.

15. What is defect breakdown?

The surface of the dielectric material may have defects such as cracks, porosity and blowholes. Impurities like dust or moisture may collect at these discontinuities (defects). This will lead to a breakdown in a dielectric material.

16. What are requirements of good insulating materials?

The good insulating materials should have

i)                   High electrical resistivity to reduce leakage current.

ii)                High dielectrical strength to with stand higher voltage.

iii)              Smaller dielectric loss

iv)              Sufficient mechanical strength.

17 Compare active and passive dielectrics.

S.No Active dielectrics

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

2. Examples. Piezo electric ,Ferro electrics

3. It is used in the production of ultrasonics.

Passive dielectrics.

Dielectric which restricts the flow of electrical energy in it are called passive dielectrics.

Examples. Glass, mica, plastic

It is used in the production of sheets, pipes, etc.,

18. What are ferro-electric materials? Give examples.

Materials which exhibit electronic polarization even in the absence of the applied electrical field are known as ferro-electric materials.

Example.

Barium Titanate (BaTiO3)

Potassium Dihydrogen Phosphate (KH2PO4)

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

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S.No  Polar molecule    

1. These molecules have permanent dipole moments even in the absence of an applied field.

2. The polarization of polar molecules is highly temperature dependent.

3.  These molecules do not have symmetrical structure and they do not have centre of  symmetry.

4. For this kind of molecules, there is absorption or emission in the infrared range.

5. Examples: CHCl3,HCl

Non-polar molecules

1. These molecules do not have permanent dipole moments

2. The polarization of polar molecules is temperature independent.

3. These molecules have symmetrical structure and they have centre of symmetry.

4. For these molecules, there is no absorption or emission in the infrared range.

5. Examples: CCl4, CO2

20. What is meant by pyro-electricity?

It means that, the creation of electronic polarization by thermal stress.