Part A - 2 Marks Questions and Answer

Part - A '2' Marks Q & A

ANNA UNIVERSITY Q&A

1. Define dielectric constant. (A.U. Jan 2014)

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

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

ε_r= ε / ε₀

2. Define polarisation of a dielectric material. (A.U. June 2015)

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

Induced dipole moment (μ) = α E

E  → Applied electrical field

α → Polarizability

3. Name the four polarisation mechanisms. ( A. U. May 2016 )

(i) Electronic polarisation

(ii) Ionic polarisation

(iii) Orientational polarisation

(iv) Space charge polarisation.

4. What is electronic polarisation? (A.U. May 2016)

The induced dipole moment produced in an atom by the application of electrical field is known as electronic polarization. It is due to shifting of negatively charged electron and positively charged nucleus charges of atom in the material by the applied electric field.

5. What is ionic polarisation? (A.U. May 2014)

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 orientational polarisation? (A.U. May 2012)

When an electrical field is applied on the dielectric medium with polar molecules, the dipoles 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 orientational polarisation.

7. What is space - charge polarisation? (A.U. May 2013)

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

8. What is meant by local field in a dielectric? (A.U. Jan 2012)

When a dielectric is kept in an external electric field (E), two fields are exerted due to (i) external field and (ii) dipole moment created

These long range coulomb forces which are created due to the dipoles are called local field in dielectric

It is given by E_int =E + p / 3 ε₀

where P→ polarisation

ε₀ → permitivity in free space.

9. Define dielectric loss and loss tangent. (A.U. Jan 2012, June 2013)

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 90°.

But for commercial dielectric, this phase angle is less than 90° by an angle δ and is called dielectric loss angle. Tan 8 is taken as a measure of dielectric loss and is known as loss tangent.

10. Define dielectric breakdown and dielectric strength. (A.U. Jan 2013)

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.

11. Mention the various breakdown mechanisms. (A.U. Dec. 2012)

(i )Intrinsic breakdown and avalanche breakdown

(ii)The Thermal breakdown

(iii) Chemical and Electrochemical breakdown

(iv) Discharge breakdown

(v) Defect breakdown

12. 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.

13. What is thermal breakdown?

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

Due to excess of heat, the temperature inside the dielectric increases and may produce local melting in the dielectric material.

This type of breakdown is known as thermal breakdown.

14. What is chemical and electrochemical breakdown?

Electrochemical 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.

15. What is discharge breakdown? ortay

Discharge breakdown occurs when a dielectric contains occluded gas bubbles. When this type of dielectric is subjected to electrical field, the gases present in the material will easily ionise and thus produce large ionisation current.

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

16. What is defect breakdown?

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

17. Mention any two active and passive dielectrics with their applications (or) Compare active and passive dielectrics. (A.U. April 2013)