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 Dipole moment (μ)
The product of the magnitude of one of the charge (q) and distance between two charges (d) is called as dipole moment.
Dipole moment = Magnitude of charge x distance
μ = qd coulomb – metre.
3. Define Polarization,
The separation of negative and positive charge is called polarization. i.e., the process of producing electric dipoles by an electric field is called polarization
μ = αE
where α = constant of proportionality called polarisability.
4. Define Polarization vector
It is defined as the average dipole moment per unit volume of a dielectric. If μ is the average dipole moment per molecule and N is the number of molecules per unit volume then the polarization vector
\overrightarrow{\mathrm{P}}=\mathrm{N} \vec{\mu} \text { coulomb } / \mathrm{m}^25. Define Polarisability
It is defined as the ratio of average dipole moment to the electrical field applied
\alpha=\frac{\vec{\mu}}{\mathrm{E}}6. Define Dielectric constant or Relative Permitivity (εr)
It is defined as the ratio of the permitivity of the medium (ε) to the permittivity of the free space or vacuum (ε0)
\begin{gathered} \varepsilon_{\mathrm{r}}=\frac{\text { Permitivity of the medium }}{\text { Permitivity of free space }} \\ \varepsilon_{\mathrm{r}}=\frac{\varepsilon}{\varepsilon_{\mathrm{o}}} \end{gathered}7. What is meant by electric susceptibility?
The magnitude of polarisation is directly proportional to the intensity of the applied electric field (E).
So, the polarisation vector
\overrightarrow{\mathrm{P}} \quad \alpha \quad \overrightarrow{\mathrm{E}} \therefore \overrightarrow{\mathrm{P}}=\varepsilon_{\mathrm{o}} \chi_{\mathrm{e}} \overrightarrow{\mathrm{E}}Where χe is a scalar quantity (proportionality constant) known as electric susceptibility and is characteristic of every dielectric.
χe = εr – 1
8. Compare active and passive dielectrics
| S.No | Active dielectrics | Passive dielectrics |
| 1. | Dielectrics which can easily adapt themselves to store the electrical energy in it is called active dielectrics | Dielectrics which resulting the flow of electrical energy in it are called passive dielectrics. |
| 2. | They are used in the production of ultrasonics | They are used in production of sheets |
| 3. | Ex. Piezo electrics, ferro electrics | Ex: Glass, mica and Plastic |
9. What are the differences between polar and non-polar molecules?
| S.No | Polar molecules | Non-polar molecules |
| 1. | Polar molecules have permanent dipole moments even in the absence of an applied field. | Non-polar molecules do not have permanent dipole moments. |
| 2. | Polarization of polar molecules is highly temperature dependent. | Polarization of this kind of molecules is independent of temperature. |
| 3. | These molecules do not have symmertrical structure and they do not have centre of symmetry. | These molecules have symmetrical structure and they have centre of symmetry. |
| 4. | For this kind of molecules, there is absorption or emission in the infrares range.
Examples: CHCl3, H2O,HCl |
For these molecules, there is no absorption or emission in the range of infrared.
Examples: CCl4, CO2, H2 |
10. Define Electronic Polarisation
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.
αe = 4π ∈o R³
11. What is ionic polarization?
The displacement of cations (+ve) and anions (-ve) in opposite directions is called ionic polarisation. It occurs in ionic solids in the presence of electric field
\alpha_1=\frac{\mathrm{e}^2}{\omega_{\mathrm{o}}{ }^2}\left(\frac{1}{\mathrm{~m}}+\frac{1}{\mathrm{M}}\right)12. 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.
\alpha_1=\frac{\mu^2}{3 \mathrm{~K} \mathrm{~T}}13. 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.
14. List out of frequency dependence of polarization.
- A optical frequencies (−1015 Hz) electronic polarization alone is present.
- At 1013 Hz range ionic polarization occurs in addition to electronic polarization.
- At 106 to 1010 Hz range contribution due to orientation polarization gets added with the above two.
- At 102 Hz range space charge polarization also contributes in addition to all the above.
15. Write a note on temperature dependence of polarization. (or) How temperature effect the polarization of dielectrics?
The electronic and ionic polarizabilities are independence of temperature (for normal temperatures). The orientation polarization and space charge polarization are affected by temperature. With increase of temperature, due to randomizing action of thermal energy, orientation polarization decreases. With increase of temperature, due to increase of diffusion of ions, space charge polarization increases.
16. 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 [Ei] The internal field Ei 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
Ei=E+ The field due to all other dipoles.
17. Define Dielectric Loss
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.
18. Define dielectric breakdown.
When a dielectric material loses its resistivity and permits very large current to flow through it, them the phenomenon is called dielectric breakdown.
19. What are the different mechanisms involved in dielectric breakdown?
The different mechanisms involved in dielectric breakdown are
(i) Intrinsic breakdown
(iv) electrochemical breakdown
(ii) Thermal breakdown
(v) defect breakdown
(iii) Discharge breakdown
20. What is discharge breakdown?
Discharge breakdown is classified as external or internal. External breakdown is caused by a glow or corona discharge. Such discharge deterioration of the adjacent solid dielectric medium resulting in breakdown. Dust and/or moisture on the surface of the dielectric may also cause external discharge breakdown. Internal breakdown occurs when the insulator contains occluded gas bubbles. Gas molecules ionize and bombard solid dielectric causing breakdown.
21. Explain electro chemical breakdown in dielectric.
Temperature rise in an insulating material accelerates the chemical deterioration. When temperature rises, mobility of ions increase and hence electrochemical reaction takes place. When ionic mobility increases leakage current also increases and this may lead to dielectric breakdown. Field induced chemical reaction gradually decreases the insulation resistance and finally results in break-down.
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