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Important Questions and Answers: Magnetic and Superconducting Materials

   Posted On :  03.12.2016 08:31 pm

Physics - Magnetic and Superconducting Materials - Important Questions and Answers: Magnetic and Superconducting Materials

SHORT QUESTIONS WITH ANSWERS

 

1.Define Magnetic Flux.

 

The total number of number magnetic lines of force passing through a surface is known as magnetic flux.

 

Unit: Weber.

 

2.Define magnetic induction (B) or magnetic flux density.

 

It is the number of magnetic lines of force passing through unit area of cross section.


 

3.Define Magnetic field strength or Magnetizing field.

 

It is the force experienced by a unit North Pole placed at a given point in a magnetic field. The magnetic induction B due to the magnetic field of intensity H applied in vacuum is related by


 

4.Define intensity of magnetization (I)

 

Magnetization is the process of converting a non-magnetic material in to a magnetic material. It is also defined as the magnetic moment per unit volume.

I = m / V.

 

Unit : Web / m2.

 

5.Define Magnetic permeability (μ)

It is the ratio of the magnetic induction (B) to the applied magnetic field intensity(H)

μ = B/H


Unit : Henry m–1.

 

6.Define relative permeability(μ)

 

It is defined as the ratio of permeability of the medium to the permeability of the free space(μr).

μr=μ/μo

 

7.Define magnetic susceptibility.

 

It is defined as the ratio of intensity of magnetization (I) and intensity of magnetic field (H). χ= I / H. The sign and magnitude of χ are used to determine the nature of the magnetic materials.


8.  Define Bohr magnetron(μB)

The orbital magnetic moment and the spin magnetic moment of an electron in an atom can be expressed in terms of atomic unit of magnetic moment called Bohr magneton.



9.  How the magnetic materials are classified?

The magnetic materials are classified into two categories.

 

The materials without permanent magnetic moment

 

Example:  1. Diamagnetic materials.

 

The materials with permanent magnetic moment.

 

Example:  1. Paramagnetic materials

 

          Ferromagnetic materials

 

          Anti-Ferromagnetic materials

 

          Ferrimagnetic materials.

 

10.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 i.e., having even number of electrons. Therefore, the electrons are 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.

 

11.Define Paramagnetic Materials.

 

Para magnetism is due to the presence of few unpaired electrons which gives rise to the spin magnetic moment. In the absence of external magnetic field, the magnetic moments (dipoles) are randomly oriented and possess very less magnetization in it. When an external magnetic field is applied to paramagnetic material, the magnetic moments align themselves along the field direction and the material is said to be magnetized.

Example: Platinum, CuSO 4 , MnSO4 Palladium, Chromium, Aluminum, etc

12. 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: Nickel, iron, Cobalt, Steel, etc.

 

13. What is the Effect of magnetic field on Ferromagnetic materials?

If a small external magnetic field is applied the magnetic moments align in the field direction and become very strong magnets.


 

14.What are the properties of Ferromagnetic Materials?

1.  All the magnetic lines of force pass through the material.


               The permeability is very much greater than one.

 

               They have enormous permanent dipole moment.

 

               When the temperature is greater than the Curie temperature, the Ferromagnetic material becomes paramagnetic material.

 

               The ferromagnetic material has equal magnitude dipole lying parallel to each other.

 

Examples: Nickel, iron, Cobalt, Steel, etc.

 

15.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 external magnetic field. The boundary line which separates two domains is called domain wall or Block wall.

 

16.Define Exchange Energy.

 

The energy which makes the adjacent dipoles to align themselves is known as exchange energy. It is also called as magnetic field energy or magneto static energy. It arises from interaction of electron spins and it depends upon the inter atomic distance.

 

17.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, weak field can be applied and in hard direction of magnetization, strong field should be applied.

 

The excess energy required to magnetize a specimen in particular direction over that required to magnetize it along the easy direction is called the crystalline anisotropy energy.

 

 

18.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 1000o A

             

19.What is Magnetostriction energy?

 

When the domain is 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.

 

20.Give the experimental evidence for Domain Structure?

 

An experimental evidence for domain structure was given by BITTER and is 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 the existence of domain structure.

 

21.Define Hysteresis.

 

Hysteresis means “Lagging” i.e., The Lagging of intensity of magnetization (I) behind the intensity of magnetic field (H) which is applied is called Hysteresis.

22. 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.

 

23. Define Retentivity and Coercivity.

 

During the process of demagnetization, the material retains some amount of magnetism, even though 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 is known as coercivity or coercive force.

 

24.What are reversible and irreversible domains?

 

When a magnetic field is applied, the domain walls are displaced and gives rise to small value of magnetization. Now if the field is removed, the domains return to its original state and are known as reversible domains.

 

When the field is removed certain domain boundaries do not come back to the original position due to the domain wall movement to a very large distance. These domains are called irreversible domains.

 

25.What are soft magnetic materials? Give 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

 

26.What are hard magnetic materials? Give its properties?

Hard Magnetic materials: Materials which retain their magnetism and are difficult to demagnetize are called hard magnetic materials.

 

These materials retain permanently the alignment of magnetic domains even 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

 

27. Define Energy product.

 

The product of residual magnetic induction (Br) and coercivity (HC) is called energy product (Br × HC). The value of energy product is very high for permanent magnets.

 

Importance:

 

               It is used to design powerful permanent magnets

 

               It is used to maximum amount of energy stored in the specimen

 

28.Distinguish between Soft and Hard magnetic material.


S. No Hard Magnetic Materials

1. Cannot be easily magnetized

2. It can be produced by heating and sudden cooling

3. Domain wall does not move easily and require large value of H for magnetization.

4. Hysteresis loop area is large Susceptibility and Permeability values are small.

5. Retentivity and Coercivity are large

6. High eddy current loss

7. Impurities and defects will be more

8. Examples: Alnico, Chromium steel, tungsten steel, carbon steel.

9. Uses: Permanent magnets, DC magnets.

 

Soft Magnetic Materials

1. Can be easily magnetized.

2. It can be produced by heating and slow cooling.

3. Domain wall move easily and requires small value of H for magnetization.

4. Hysteresis loop area is small Susceptibility and Permeability values are high.

5. Retentivity and Coercivity are small.

6. Low eddy current loss

7. No impurities and defects

8. Examples: Iron-silicon alloy, Ferrous nickel alloy, Ferrites Garnets.

9. Uses: Electro magnets, computer data storage. Transformer core.

 

29. Define Anti ferromagnetic materials.

Magnetic materials in which, the spins are aligned in anti-parallel manner due to un favorable exchange interaction among them resulting in zero magnetic moment are called as Anti ferromagnetic materials.

 

30. What are Ferrimagnetic materials or Ferrites?

 

Ferromagnetic materials are much similar to ferromagnetic materials in which the magnetic dipoles are aligned anti-parallel with unequal magnitudes. If small value of magnetic field is applied, it will produce the large value of magnetization.

 

31. What are the Applications of Ferrites?

 

               Ferrites are used to produce ultrasonic wave by Magnetostriction principle.

 

               Ferrites are used in audio and video transformers.

 

               Ferrites rods are used in radio receivers to increase the sensitivity.

 

               Ferrites are widely used in non-reciprocal microwave devices such as gyrator, circulator and Isolator.

 

               They are also used for power limiting and harmonic generation.

 

               Ferrites are used in parametric amplifiers so that the input can be amplified with low noise.

 

               They are used in computers and data processing circuits.

 

32. What Magnetic Bubbles?

 

Magnetic bubble is direct access storage medium. Magnetic bubbles are soft magnetic materials with magnetic domains of a few micrometers in diameter. These bubbles are the electrical analogue of the magnetic disk memories used in computers. The magnetic disk in the hard disk memory is moved mechanically where as the bubbles in a bubble memory device are moved electronically at very high speeds.

 

 

31.What is Superconductivity?

 

The ability of certain metals and alloys exhibit almost zero electrical resistivity when they are cooled to low temperature is known as superconductivity. (ie., maximum conductivity with zero resistance at zero Kelvin)

 

34. Define Critical Temperature.

Critical temperature (or) Temperature ( TC ) is defined as the temperature at which the resistivity falls to zero is called transition as temperature or Critical temperature. The temperature at which the normal conductor becomes a superconductor is know as critical temperature ( TC ) Below TC , the material (Tin) is in the superconducting state and above TC , it is in the normal state.

 

35.State BCS Theory?

 

(Bardeen, Cooper and Schrieffer theory). It states that an electron pair called cooper pair is formed at low temperature by overcoming the repulsive force. The cooper pair moves without scattering (without any resistance) in the lattice structure. Now the material becomes a super conductor.

 

This is also called as electron-phonon- electron interaction. these two electrons have equal and opposite momentum and spins.

 

This BCS theory is suitable only for low temperature superconductors.

 

36. Define Cooper pair.

 

The electron pairs called cooper pairs are formed at low temperature by overcoming the repulsive forces. The cooper pairs moves without scattering (without any resistance) in the lattice structure. Now the material becomes super conductor. This is also called as electron-phonon- electron interaction these two electrons have equal and opposite momentum and spins.

 

37.What is coherence length?

 

It is defined as the distance up to which two electrons combine to form a Cooper pair.

 

38.What are the important properties of superconductors?

1. Zero Electrical Resistance : The electrical resistivity drops to zero at the  transition temperature.( TC ) Dr. Collins observed that the current in a superconducting ring is unchanged for 2½ years.

2. Effect of magnetic field : Below TC , the superconducting property disappears when strong magnetic field is applied. i.e., it returns to normal state.

3. Diamagnetic property : The superconductor is a perfect diamagnetic.

4. Effect of heavy Current : The superconducting property disappears when a heavy current flows, since current flow will set up a magnetic field.

5. Isotope effect : The presence of isotopes in superconductors changes the transition temperature of the superconductors.

6. Thermal properties : Entropy and specific heat decreases at transition temperature and the thermoelectric effect disappear in the superconducting state.

7. Effect of Impurities : The impurities change the magnetic properties of the superconductors.

 

39.Define Critical magnetic field.

 

A very strong magnetic field applied to superconducting material it disappears superconducting property this is called as critical magnetic field.



When the temperature of the material increases, the value of critical magnetic field decreases.

 

40. What is Meissner effect? (or) Define Diamagnetic property.

When the superconducting material is subjected to a uniform magnetic field, under the condition T TC and H HC, the magnetic flux lines are excluded from the material. Thus the material exhibits perefect diamagnetism. This phenomenon is called as meissner effect.


 

41. What happens in a superconductor when a heavy Current flows?

 

The superconducting property disappears when a heavy current flows, since this current flow will set up a heavy magnetic field. According the Silsbee’s rule, for a superconducting wire


 

42. What is Isotope effect?

 

The presence of isotopes in superconductors change the transition temperature of the superconductor. The transition temperature is found to be inversely proportional to the square root of the atomic weight of the isotope.


The transition temperature of the heavier isotope is less than the that of the lighter isotopes.

 

43. How the superconductors are classified? or what are the types of superconductors.

Based on the value of HC we have,

1)      Type I (or) Soft superconductors        2)      Type II (or) Hard superconductors

Based on the value of TC we have,            

1)      High temperature superconductors    2)      Low temperature superconductors

 

44.What it Type I superconductors? Give its characteristics.

 

In type I superconductor, the magnetic field is completely excluded from the material below the critical magnetic field HC and the material loses its superconducting property abruptly at HC .

 

Characteristics

 

          They exhibit complete Meissner Effect.

 

          They have only one critical magnetic field value.

 

          Below the material behaves as superconductor and above the material behaves as normal conductor.

          These are called as Soft superconductors.

45. Distinguish between Type I and Type II Superconductors



 

            What it Type II superconductors? Give its characteristics.

 

In type II superconductor, the magnetic field is excluded from the material and the material loses its superconducting property gradually rather than abruptly.

 

Characteristics

 

          They do not exhibit a complete Meissner Effect.

 

          They have two critical magnetic field values. Lower critical magnetic filed [HC1] and Higher critical magnetic field [HC2].

 

          Below HC1 the material behaves as superconductor and above the material behaves as normal conductor. The region in between [HC1] and [HC2] is called mixed state or vortex region.

 

          These are called as Hard superconductors.

 

46.What are Low TC and High TC superconductors?

 

Low TC Superconductors

 

          The superconductors having the critical temperature less than 20 K are known as low TC Superconductors or elemental superconductors.

          The Superconductors by BCS theory.

 

          It is explained by BCS theory.

 

          It is not so useful due to its low temperature maintenance.

 

          It is called as N-type superconductor.

 

High TC Superconductors

 

          The superconductors having the critical temperature greater than 100 K are known as high TC Superconductors or ceramic or oxide superconductors.

          The Superconductors is due to hole states.

 

          It is explained by RVB theory proposed by Anderson.

 

          It is very useful for commercial and engineering purposes.

 

          It is called as P-type superconductor.

 

 

47.What are the characteristics of high temperature [high TC] superconductors?

 

               High Superconductors have high temperatures.

 

               They have a modified perovskite crystal structure.

 

               Superconducting state is direction dependent.

 

               These are oxides of copper with other elements.

 

               These are reactive, brittle, and cannot be easily modified or joined.

 

               For high superconductors, liquid Nitrogen is used instead of liquid helium.

 

48.What are the Applications of Superconductors?

 

               Electric generators can be made by using superconductors with smaller size, less weight and low energy consumption.

               Superconductors can be used for the transmission of power over very long distances.

               Superconductors transformers are available.

 

               Superconductors can be used in switching Devices.

 

               The superconductors can be used in sensitive electrical instruments.

 

               It can be used as a memory or storage element in computers.

 

               These are used to design Cryotron, Maglev, Josephson Devices and SQUID.

 

               DC superconducting motors are used in ship propulsion and in large mills.

 

               Superconducting magnetic field may be used to launch satellite into orbit directly from the earth without use of rockets.

               Ore separation can be done by using machines made of superconducting magnets.

               These are used in NMR (Nuclear Magnetic Resonance ) imaging equipments which is used for scanning purposes.

               Superconductors are used for the detection of brain tumor, defective cells, etc.,

               Superconducting solenoids are used in magneto hydrodynamic power generation to maintain the plasma in the body.

50.What is Cryotron?

 

               It is a magnetically operated current switch.

 

               The superconducting property disappear when the magnetic field is greater than critical field (HC).

 

               It consists of a superconducting material [A] and it is surrounded by an another super conducting coil of wire [B]. When the critical magnetic field of wire B exceed or less than that of superconducting material A, the current in A can be controlled by the current in the material B.

 

51. Define Magnetic levitation?

 

               When a magnet is placed over a superconductor, the magnet floats, this phenomenon is known as magnetic levitation. This is based on diamagnetic property of a superconductor. A diamagnetic material rejects the magnetic flux lines.

52.What is Maglev?

 

It is a magnetic levitated train. Electromagnetic induction principle is used here, this train cannot move over the rail. Instead it floats above the rails, so that it move faster with speed of 500 Km/hr without any frictional loss. It has two superconducting magnet on each side of the train and there is guiding system consisting of ‘S” shaped coils on each side. Due to actions of these magnets the train moves faster thro’ levitation.

 

53. What is SQUID?

 

SQUID stands for Superconductors Quantum Interference Device. It is a double junction quantum interferometer formed from two Josephson junctions mounted on a superconducting ring. SQUID is based on the flux quantization in a superconducting ring. The total magnetic flux passing through the ring is quantized. It is using to detect very minute magnetic field of the order of 10 14 tesla.


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