1. What are Magnetic Materials ?
The materials which can be made to behave like a magnet and which are easily magnetized by a magnetic field are called as magnetic materials.
2. How the Magnetism arises in a material?
The magnetism arises from the orbital and spin magnetic moments of the magnetic materials. When an electron revolves around the mucleus the orbital magnetic moment arises and due to the spin motion of the electrons spin magnetic moment arises.
3. Define Magnetic Flux [Φ]
The total number of number magnetic lines of force passing through a surface is known as magnetic flux. Unit is Weber.
4. Define magnetic induction (B) or magnetic flux density
It is the number of magnetic lines of force passing through unit area. UNIT: weber/m2 or tesla.
5. Define Intensity of magnetic field or Magnetic field strength or Magnetising field
It is the force experienced by an unit north pole placed at the given point in a magnetic field. H = F/m, UNIT: ampere/metre. or N/Wb.
6. Define magnetic moment (m)
The product of pole strength and magnetic length is called magnetic moment. (m = p x 2l) UNIT: weber metre.
7. Define intensity of magnetisation (I)
Magnetisation is the process of converting a non-magnetic material into a magnetic material. It is defined as the magnetic moment per unit volume. I = m/V UNIT : weber .
8. Define magnetic permeability (μ)
It is the ratio of the magnetic induction (B) to the applied magnetic field intensity (μ). μ = B/H UNIT: henry/m.
9. Define relative permeability (μr )
It is defined as the ratio of permeability of the medium to the permeability of the free space. μr = μ/μ0 .
10. Define magnetic susceptibility (χ)
It is defined as the ratio of intensity of magnetization and intensity of magnetic field. χ = I/H and The sign and magnitude of χ are used to determine the nature of the magnetic materials.
11. Define Bohr magneton (μB )
It is the magnetic moment produced by an unpaired electron in an atom. It is the fundamental unit of magnetic moment. One Bohr magneton, μB = 9.27 x 10-24 A m2 .
12. Derive relation between susceptibility [χ] and permeability [μr].
We know that when a current is supplied through a coil, magnetic field develops.
When a magnetic material is placed inside a magnetic field, the magnetic flux density [B] arises is due to applied magnetic field (H) and also due to the induced magnetization [I]
i.e. The total flux density,
B = μ0(I+H) ……..(1)
But we know that μ = B/H ………(2)
From equations (1) and (2) we get,
μH = μ0 (I+H)
μ/μ0 = (I+H)/H
μr = 1+(I/H)
μr = 1+ χ
χ = (μr – 1) this is the required relation
13. How the magnetic materials are classified?
The magnetic materials are classified into two categories:
- Not-having permanent magnetic moments. Example: i) Diamagnetic materials.
- Having permanent magnetic moments.
Example: i) Paramagnetic materials
ii) Ferromagnetic materials
iii) Anti-ferromagnetic materials
iv) Ferrimagnetic materials
14. Define Diamagnetic materials.
In a diamagnetic material the electron orbits are randomly oriented and the orbital magnetic moments get cancelled. Similarly, all the spin moments are paired (having even no.of electrons). Therefore, the electrons spinning in two opposite directions and hence the net magnetic moment is zero. These materials are called as diamagnetic materials.
Example: Gold, germanium, silicon, antimony, bismuth, silver, lead, copper, hydrogen, Water and alcohol
15. Define Paramagnetic Materials.
Paramagnetism is due to the presence of a few unpaired electrons which gives rise to the spin magnetic moment. In the absence of external magnetic field, the magnetic moment (dipoles) are randomly oriented and possess very less magnetization in it. When an external magnetic field is applied, the magnetic moments align themselves along the field direction and the material is magnetized.
Example: Platinum, CuSO4, MnSO4 , Palladium, Chromium, Aluminium, etc.
16. Define Ferromagnetic materials.
Ferromagnetism is due to the presence of more unpaired electrons. Even in the absence of external field, the magnetic moments align parallel to each other. So that it has large magnetism. This is called spontaneous magnetization.
Example: Iron, Cobalt, Ni
17. What is the Effect of magnetic field in ferromagnetic materials?
If a small external magnetic field is applied the magnetic moments align in the field direction and become very strong magnets.
18. What are the properties of Ferromagnetic materials?
- All the magnetic lines of force pass through the material.
- Its susceptibility is high positive and it is given by χ = [C /(T – θ)]
- The permeability is very much greater than one.
- They have enormous permanent dipole moment.
- When the temperature is greater than the Curie temperature, then the Ferromagnetic becomes paramagnetic.
- The ferromagnetic material has equal magnitude dipoles parallel to each other.
19. What are magnetic domains?
A ferromagnetic material is divided into a large number of small regions called domains. ( 0.1 to 1 mm2 of area). Each direction is spontaneously magnetized. The direction of magnetization varies from domain to domain and the net magnetization is zero in the absence of external magnetic field. The boundary line which separates two domains is called domain wall or Bloch wall.
20. How the external magnetic field helps the magnetization in ferromagnetic material?
When the magnetic field is applied to the Ferromagnetic material, the magnetization produces by two ways. 1]. By the motion of domain walls. [2]. By the rotation of domains.
21. Define Exchange Energy.
It is the energy which makes the adjacent dipoles to align themselves is known as exchange energy. It is also called as magnetic field energy or magnetostatic energy. It arises from interaction of electron spins and it depends upon the interatomic distance.
22. What is Anisotropy energy?
Crystals are anisotropic. The energy arises from the difference of energy required for magnetization along any two different directions in a single crystal. These are two directions of magnetization.
1. Easy direction 2. Hard direction
In easy direction of magnetization, a weak field can be applied. and in hard direction, strong field should be applied.
The excess energy required to magnetize a material in particular direction over that required to magnetize it along the easy direction is called Anisotropy energy.
23. What is Domain wall energy (or) Bloch wall energy?
It is the sum of the exchange and anisotropy energy in the domain wall. Thickness of the wall is approximately 1000 Å.
24. What is Magnetostriction energy?
When the domains are magnetized in different directions, they will either expand or shrink. i.e change in dimension when it is magnetized. The energy produced in this effect is called magnetostriction energy. It is the energy due to the mechanical stresses generated by domain rotations.
25. What is the experimental Evidence of Domain Structure?
An experimental evidence for domain structure was given by BITTER called Bitter powder pattern. In this method, a drop of colloidal suspension of finely divided ferromagnetic powder is allowed to spread over the surface of the ferromagnetic material. It is found that through the microscope, the colloidal particles are collected along the domain boundaries. Which shows that the existence of domain structure.
26. Define Hysteresis.
Hysteresis means “Lagging behind”. i.e., The Lagging of magnetic induction behind the intensity of magnetic field (H) which is applied.
27. What is meant by Hysteresis Loss?
When the specimen is taken through a cycle of magnetization, there is loss of energy in the form of heat. This is known as Hysteresis Loss.
28. Define Retentivity and Coercivity.
During the process of demagnetization, the material retains some amount of magnetism, eventhough when intensity of magnetic field is zero. It is known as Retentivity or residual magnetism.
The amount of intensity of magnetic field applied in the reverse direction to remove the retentivity known as coercivity or coercive force.
29. What are reversible and irreversible domains?
When a magnetic field intensity is applied, the domain walls are displaced and gives rise to small value of magnetization. Now, the field is removed, the domains return to its original state known as reversible domains.
When the field is removed the domain boundaries do not come back to the original position due to the domain wall moved to a very large distance. These domains are called irreversible domain.
30. What are soft magnetic materials? What are its properties?
Soft Magnetic materials
Materials which are easy to magnetize and demagnetize are called soft magnetic materials.
These materials do not retain the alignment of magnetic domains after the removal of external magnetic field.
Properties
- They have high permeability
- The storing of magnetic energy is less
- Retentivity and Coercivity values are small
- They have low hysteresis loss.
31. Distinguish between Soft and Hard magnetic material.
| Hard Magnetic | Soft Magnetic |
| Cannot be easily magnetised | Can be easily magnetised |
| Domain wall does not move easily and require large value of H for magnetisation. | Domain wall move easily and requires small value of H for magnetisation. |
| Hysteres is loop area is large | Hysteresis loop area is small |
| Permeability values are low | Permeability values are high |
| Retentivity and Coercivity are large | Retentivity and Coercivity are Small |
| High eddy current loss | Low eddy current loss |
| Irregularities will be more
Examples: Cunife, Cunico, Alnico, |
No irregularities
Examples: Iron- silicon alloy, Ferrous nickel |
| Chromium steel, tungsten steel, carbon steel. | alloy, Ferrites, Garnets |
| Uses: Permanent magnets data storage | Uses: Electro magnets, computer |
32. What are hard magnetic materials? What are its properties?
Materials which retain their magnetism and are difficult to demagnetize are called hard magnetic materials.
These materials which retain permanently the alignment of magnetic domains after the removal of external magnetic field.
Properties
- They have low permeability
- The storing of magnetic energy is more
- Retentivity and Coercivity values are high
- They have high hysteresis loss.
33. Define Energy product.
It is the product of retentivity (Br) and coercivity (Hc) . Energy product = Br x Hc. It gives the maximum amount of energy stored in the specimen.
34. Define Antiferromagnetic materials.
In a magnetic material, the spins are aligned in anti-parallel manner due to unfavorable exchange interaction among them, resulting in zero magnetic moment. This material is called as Anti ferromagnetic materials.
35. What are Ferrimagnetic materials or Ferrrites?
Ferrimagnetic materials are much similar to ferromagnetic materials in which the magnetic dipoles are aligned in antiparallel with unequal magnitudes. If small value of magnetic field is applied, it will produce the large value of magnetization.
36. What are Regular spinel and Inverse spinel?
In the regular spinel, each trivalent metal ion occupies an octahedral site [OHS] and each divalent ion occupies a tetrahedral site [THS] of FCC oxygen lattice. [FCC – oxygen, and 3+– OHS and 2+ – THS]. Eg: Ferrous ferrite (Fe2+ Fe23+ O4 ).
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