Choose the correct option for the following electrical engineering materials assertion & reason questions.
- Both P and Q are true and Q is a correct explanation of P
- Both P and Q are true but Q is not a correct explanation of P
- P is true but Q is false
- P is false but Q is true
(Q): At absolute zero degrees Kelvin temperature, there is no energy available to generate the current carriers in the semiconductor.
2. (P): Electron mobility in metals decreases with increasing temperature.
(Q): In metals electron concentration is high.
3. (P): A heavily doped semiconductor can exhibit positive temperature coefficient of resistance.
(Q): The carrier mobility decreases with an increase of temperature.
4. (P): An electric or mechanical input makes the flux jump from one superconductor to another, generating large direct current.
(Q): The electric resistivity of superconductors depends upon the magnetic field.
5. (P): A superconductor is a perfect diamagnetic material.
(Q): A superconductor is a perfect conductor.
6. (P): Superconductors cannot be used as coils for the production of strong magnetic fields.
(Q): Superconductivity in a wire may be destroyed if the current in the wire exceeds a critical value.
7. (P): Superconductivity of a superconducting material can be destroyed by application of an external magnetic field.
(Q): if the applied magnetic field is greater than the critical magnetic field at a given temperature which is more than the transition temperature, superconductivity can be destroyed.
8. (P): Cryotron is a switch constructed out of superconducting materials.
(Q): Switching action is characteristic of superconductors only.
9. (P): if a piece of metal is made to have a temperature gradient between its two ends, an emf exist between these ends.
(Q): Electrons at the hot end move towards the cold end.
10. (P): Thermal noise in metallic resistors can be characterized by Gaussian probability density function.
(Q): Power spectral density of thermal noise is essentially constant for a very large frequency range.
11. (P): Ammonium dihydrogen phosphate is a piezoelectric material.
(Q): There is no centre of symmetry in ammonium dihydrogen phosphate.
12. (P): The Diamagnetic effect is present in all the materials.
(Q): The applied magnetic field alters the orbital motion of the constituent electrons of the material.
13. (P): The spins within a magnetic domain are aligned permanently below Curie temperature in a ferromagnetic material.
(Q): Ferromagnetic material is magnetic only when the domains are aligned by an external field.
14. (P): Iron, cobalt and nickel are ferromagnetic.
(Q): Spin imbalance of electrons in the 3rd band is responsible.
15. (P): Soft magnetic material is used in making electromagnets.
(Q): Soft magnetic materials have a high coercive field.
16. (P): Soft magnetic materials cannot be used for the construction of cores for the electrical machines.
(Q): Soft magnetic materials have relatively small and narrow hysteresis loop.
17. (P): Alnico (aluminium-nickel-iron-cobalt) alloys are the mass importation of hard magnetic materials.
(Q): Alnico magnet alloys have the lowest energy per unit of volume of any permanent magnet material available commercially.
18. (P): Hard magnetic materials are used for making permanent magnets.
(Q): Hard magnetic materials have relatively small and narrow hysteresis loop.
19. (P): DC resistance of ferrites is much higher than that of dia-, para- or ferromagnetic materials.
(Q): Ferrites are obtained by replacing divalent ferrous ion in ferrimagnetic magnetite by another divalent metal such as Mg, Zn etc., but the exchange interactions are the antiferromagnetic type.
20. (P): Ferrites are useful at very high frequencies.
(Q): Ferrites have high permeability and high resistivity.
21. (P): Ferrites cores are used in inductances for high frequencies.
(Q): Ferrites provide high flux density with minimum eddy current loss and are insulators.
22. (P): Magnetic cores are generally used in the main memory of a digital computer.
(Q): Magnetic cores are slow and volatile.
23. (P): Storage of energy in a dielectric is due to shift in relative positions of internal charges against normal atomic and molecular forces.
(Q): All dielectrics are characterised by the presence of molecules having a permanent displacement between the centres of gravity of the positive and negative charges.
24. (P): An insulator has a high value of resistivity, and with increasing temperature, the value of its resistivity decreases exponentially.
(Q): With increasing temperature, the value of the energy band gap decreases.
25. (P): The relative dielectric constant of an insulator decreases with increase in the frequency of the applied alternating field.
(Q): With an increase in the frequency of the applied field, the polarization process increases in number.
26. (P): Mica is usually not used for slot linings of high voltage machines.
(Q): Space factor in high voltage machines is high.
27. (P): Skin depth is the depth by which electromagnetic wave has been increased to 37% of its original value.
(Q): The depth of penetration of wave in a lossy dielectric increase with increasing wavelength.
28. (P): Sulphur hexafluoride gas is highly inflammable.
(Q): Sulphur hexafluoride gas is used in switchgear.
29. (P): Mica has very high dielectric strength.
(Q): Mica cannot be used for the large commutator of a dc machine.
