Download Sri Venkateswara College Of Engineering Department of Applied

Sri Venkateswara College Of Engineering
Department of Applied Physics
CAT-II Question Bank
PART-A
1. What is Bohr magneton? Give its value.
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 as Bohr magneton. [B =9.27 * 10-24 Am2 ]
2. What is the origin of magnetic moment?
When a charged particle has an angular momentum, it contributes to the magnetic moment. In general there
are three contributions to the magnetic moment of an atom.
(a) Orbital angular momentum of the electrons
(b) Electron spin angular momentum
(c) Nuclear Spin angular momentum.
3. What are Ferrites?
Ferrimagnetic materials or Ferrites are much similar to Ferromagnetic materials. The magnetic dipoles are
aligned anti-parallel with unequal magnitudes. If small value of magnetic field is applied, large value of
magnetization is produced.
3. What do you understand by the term magnetic domains and domain walls?
According to Weiss, a ferromagnetic material consists of a number of regions which are spontaneously
magnetized. These regions are called domains. The boundaries between different domains are called domain
walls.
4. What are the four types of energies involved in the growth of magnetic domains?
The total internal energy of the domain structure in a ferromagnetic material is made up from the following
contributions.
1. Exchange energy (or) Magnetic field energy.
2. Crystalline energy (or) Anisotropy energy.
3. Domain wall energy (or) Bloch wall energy.
4. Magnetostriction energy.
5. What is magnetic hysteresis?
The lagging of magnetic induction (B) behind the applied magnetic field (H) is known as hysteresis.
6. Define coercivity and Retentivity.
“The residual magnetism which is left out even after the removal of the external magnetic field” is called as
the Retentivity or Remanence.
“The reverse magnetic field which is used to completely remove the residual magnetism” is called as the
coercivity.
7. What do you mean by energy product?
The product of retentivity and coercivity is known as energy product.
8. Distinguish between soft and hard magnets.
Hard magnets
Soft magnets
They are not easily magnetized They are easily magnetized and
and demagnetized
demagnetized
Hysteresis loop is large
Hysteresis loop is small
Coercivity and Retentivity is large
Coercivity and Retentivity is small
Permeability and Susceptibility is Permeability and Susceptibility is
small
large
Domain wall movement is difficult Domain wall movement is easier.
Hysteresis loss is large
Used in the manufacture
permanent magnets
Hysteresis loss is small
of Used in the manufacture
temporary magnets
of
9. Define superconductivity.
Certain metals and alloys exhibit almost zero electrical resistivity when they are cooled to sufficiently low
temperature. This phenomenon is known as superconductivity.
10. What do you understand by the terms ‘critical temperature’ and ‘critical field’ of a superconductor?
The temperature at which the transition from normal state to superconducting state takes place on cooling in
the absence of magnetic field is called the critical temperature (TC).
Below critical temperature in the absence of any magnetic field, the material is in superconducting state. At
any temperature the minimum field required to destroy superconductivity is called Critical field (HC).
11. What is Meissner effect?
In superconducting state, the material becomes a perfect diamagnet. The flux lines of a magnetic field are
ejected out of a superconductor. This effect is known as Meissner Effect.
12. What is an isotope effect?
In superconducting materials the transition temperature varies with the average isotopic mass M of its
constituents. The variation follows the general form
TC  M-
TC M = Constant
Where  is called the isotope effect coefficient.
13. What is Cooper pair of electrons?
At very low temperature, an electron passing though crystal lattice attracts lattice ions and lattice gets
deformed. The phonon emitted by the electron is absorbed by the vibrating lattice. The vibration passes from
one ion to another until another electron absorbs the phonon. These two electrons form a pair called Cooper
pair.
14. What are High Tc superconductors? Give examples.
Any superconductor with a transition temperature above 30K is in general called high Tc superconductors.
YBa2Cu3O7, La2-xSrxCuO4.
15. Mention the applications of superconductors.
SQUID-Superconducting Quantum Interference Device.
Cryotron
Magnetic Levitation
16. What is Meissner Effect in superconductors?
In superconducting state, the material becomes a perfect diamagnet. The flux lines are ejected out of the
superconductors. This effect is known as Meissner effect.
17. Super conducting tin has a critical temperature of 3.7K at zero magnetic fields and a critical field of
0.0306tesla at 0K.find critical field at 2K.
  T 2 
H C  H 0 1    
  TC  
18. Define effective mass of the electron.
When an electron moves through a crystal lattice, it experiences a periodic potential and hence an internal
force. The effect of this force is incorporated in the mass of the electron and such a mass is called effective
mass of the electron.
19. What is GMR?
If the charge in electrical resistance is very high compared to the magnetization. It is called Giant MagnetoResistance (GMR) and this effect is called GMR.
20. What is meant by persistent current?
When d.c. current of large magnitude is once induced in a super conducting ring, then due to the diamagnetic
property of the superconductor, the magnetic flux is trapped inside the ring and hence the current persists in
the ring for longer time. This current is called as persistent current.
PART-B
1. (a)Discuss the domain structure in ferromagnetic materials. Show how the hysteresis curve is explained on the
basis of domain theory (16)
2. (a) Briefly explain the structure, properties and applications of Ferrites.(10)
(b) Explain various types of energy involved in the process of domain growth in detail.(6)
3. (a)Distinguish Type I and Type II superconductors.(8)
(b) Explain the various properties of superconducting materials. (8)
4. (a)Explain in detail high Tc superconductors with examples. (6)
(b)Discuss the applications of superconductors. (6)
(c) In a Hall Experiment, a current of 25A is passed through a long foil of silver which is 0.1mm thick and
3cm wide. A magnetic field of flux density of 0.14Wb/m2 is applied perpendicular to the foil. Calculate the
Hall voltage developed and estimate the mobility of electrons in silver. The electrical conductivity of silver is
6.8*107ohm-1m-1 and hall coefficient is -8.4*10-11 m3/C.
5. What is Hall Coefficient? Explain with necessary theory the Hall Effect and the experimental method to
Determine the electrical conductivity of a semiconductor. Explain any four applications. (16)
5. Obtain an expression for density of electrons in the conduction band of n-type semiconductor and explain the
variation of Fermi level in n-type semiconductor. (16)