Part I - Electric & Magnetic Field In Material Space - Questions With Answers (Short Notes)
1. What Do You Mean By Dielectric Constant Of A Material ?
Answer: Dielectric constant (k) or relative permittivity (εr) is defined as the ratio of capacitance of a capacitor with dielectric to the capacitance of same capacitor without dielectric. It generally describes the ability of a material to polarize and store a charge.
Mathematically, k or εr = C/C0 .
[Since C = q/V, we have εr = E0/E = V0/V = ε/εr ].
2. Define Dielectric Medium.
Answer: Dielectric is a material in which energy can be stored by the polarization of the molecules. It is a material that increases the capacitance or charge storage ability of a capacitor. Ideally, a dielectric is an insulator and does not contain free charge. However, in the presence of external field it exhibits a relative displacement of opposite bound charges and hence the polarization of the medium. Due to polarization induced surface charges tend to weaken the original field within the dielectric.
[The resultant field in the presence of dielectric is given by E = E0 – E’ where E’ is the field due to induced charges and E0 is the field in the absence of dielectric.]
3. What Is The Difference Between Insulators & Dielectrics?
Answer: When electrically non-conducting materials like ceramics, polymers and paper are used for electrical insulation, they are called insulators. On the other hand, when these non-conducting materials are placed in an electric field, they modify the electric field due to induced charges on the surface of the material.
As a result these material act as a storage of electrical charges and materials are known as dielectrics. For a material to be a good dielectric, it must be an insulator. Thus all insulators are dielectric.
4. Give Relation For Electric Field Strength Due To Spherical Distribution Of Charge.
Answer:
)?For spherical distribution of charge,
6. Define Polarization Of Dielectric Medium.
Answer: The induced electric dipole moment per unit volume of dielectric is called dielectric polarization. Polarization vector (P) represents a measures of the extent of polarization in a unit volume of dielectric material. It is equal to the vector sum of dipole moments per unit volume. If the dipoles are randomly oriented then the vector sum is zero and so is the polarization vector. If p is the average dipole moment per molecule and n is the number of molecules per unit volume then P = np.
7. What Are Various Types Of Polarization Mechanisms?
Answer: Polarization in dielectric materials occurs due to following atomic mechanisms:
- Electronic Polarization
- Ionic Polarization.
- Orientational Polarization.
- Space Charge Polarization.
8. What Do You Mean By Space Charge Polarization?
Answer: In some materials the ions diffuse towards the electrodes in response to the electric field. Therefore, there is a redistribution of charges in the medium. The tendency of redistribution of charges in multiphase dielectric medium (or due to electrode polarization) in the presence of external electric field is known as space charge polarization.
9. What Is Clausius-Mossotti Relation?
Answer: Clausius-Mossotti equation relates the dielectric constant (εr), a macroscopic property, to the polarizability (α), a microscopic property of a dielectric material by the following relation
10. What Do You Mean By Electric Susceptibility?
Answer: Electric susceptibility (χe) is a material property that measures the extent of polarization in the material per unit field, i.e., χe = P/E. It relates the amount of polarization, P at a point in the dielectric to the field, E at that point via P = χeE = ε0(εr-1)E, here ε0 is the permittivity of free space and εr is the relative permittivity.
11. What Is Relaxation Time? How Does It Vary For Different Types Of Polarization?
Answer: The time required by dipoles to reorient in response to an external electric field known as relaxation time (τ). The relaxation time is different for each of the various polarization mechanisms contributing to the polarization of a dielectric. For electronic polarization, response is fast and relaxation time is small, for Ionic polarization response is slower, for dipolar polarization response is still slower and finally for space charge polarization response is quite slow and relaxation time is large.
12. What Is Dielectric Loss ? Why Does Dielectric Loss Occur?
Answer: Dielectric loss is the loss of energy in the form of heat due to internal friction of dipole in aligning themselves in the direction of electric field during charging and discharging of a capacitor. When the relaxation time and the time period of the applied field are similar, a phase lag occurs and energy is absorbed leading to loss of energy.
13. Define Loss Factor.
Answer: The product of real part of relative permittivity and tangent of loss angle is known as loss factor or dissipation factor of a dielectric.
Dissipation factor δ = εr' tan δ
where tan δ = εr " / εr '
14. What Do You Understand By Spontaneous Magnetization?
Answer: The spontaneous magnetization is the net magnetization that exists inside a uniformly magnetized microscopic volume in the absence of field. The magnitude of this magnetization, at 0 K, is dependent on the spin magnetic moments of electrons.
15. List Two Distinct Characteristics Of Ferromagnetic Materials.
Answer: Two distinct characteristics of ferromagnetic materials are their:
- Spontaneous Magnetization.
- Existence of magnetic ordering temperature (known as Curie temperature TC below which material is ferromagnetic) .
16. What Is The Relation Between Magnetic Field, Magnetic Induction & Magnetization?
Answer: B = μ0(H+M), where B is the magnetic induction, H is the magnetic field and M the magnetization.
17. What Is The Difference Between Diamagnetism & Para Magnetism?
Answer: In diamagnetic materials, the susceptibility χ < 0 while in paramagnetic materials, the susceptibility χ > 0
18. Why Are Not All Ferro- & Ferrimagnetic Materials Magnetized To Their Saturated States, Even In Zero Field?
Answer: According to domain theory of ferromagnetism, ferromagnets are subdivided into many small sub volumes, called domains. Each domain is spontaneously magnetized to saturation, but the direction of magnetization varies from domain to domain. The net vector sum of magnetization of all the domains therefore produces a total magnetization near to zero.
19. Why Do Magnetic Domains Form?
Answer: There are two competing energies in a ferromagnet: the exchange interaction between neighbouring spins which favours parallel spins, and the magnetic energy which favours small magnetisation. Domains of opposite magnetisation allow the magnetic energy to be reduced without creating too many neighbouring spins that are not parallel.
20. What Is The Importance Of Hysteresis Loop?
Answer: The lagging of magnetic flux density B with respect to the magnetising field H called Hysteresis. The complete cycle of magnetization for ferromagnetic material leads to formation of Hysteresis loop, commonly known as B-H curve. The area under this curve is proportional to the energy lost per unit volume per cycle of magnetization. The thin loop corresponds to soft magnetic materials and large one for hard magnetic materials. Thus, one can select a material suitable for making transformer core, permanent magnet etc. on the basis of hysteresis loop of the material.
21. What Informations Can Be Drawn From Hysteresis Loops?
Answer: Following important informations about a magnetic material can be drawn from hysteresis
loop:
- Value of saturation magnetization
- Retentivity or residual magnetization.
- Coercive field or coercivity.
- Reluctance (it is opposition that a ferromagnetic material disfavours the establishment of a magnetic field).
- Permeability (wider B-H curve shows lower permeability and narrower B-H curve shows higher permeability).
- Energy lost per unit volume per cycle of magnetization.
22. What Are The Characteristics Of A Material To Be Used For Making Permanent Magnets?
Answer: For making permanent magnets, materials should have below characteristics:
- High coercive force so that magnetism is retained even at strong external magnetic field and at high temperature.
- The retentivity should high.
23. What Are The Characteristics Of A Material To Be Used For Electromagnets?
Answer: Characteristics that a material should possess to be used as electromagnet are:
- High initial permeability.
- Low coercive force.
- Low hysteresis loss.
- High retentivity.
24 . What Are The Characteristics Of A Material To Be Used For Transformer Cores, Armatures & Chokes?
Answer: Some of the characteristics are:
- Low hysteresis loss.
- High specific resistance.
- High initial permeability.
Answer: F = Q(E + u * B) = m ( du / dt ) relates the force acting on a particle with charge Q in the presence of EM fields. It expresses the fundamental law relating EM to mechanics.
Based on the Lorentz force law, the force experienced by a current element Idl in a magnetic field B is dV = Idl * B From this, the magnetic field B is defined as the force per unit current element.
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