Each of the following questions consists of two statements, one labelled ‘Assertion (A)’ and the other labelled the ‘Reason (R)’. Examine the two statements carefully and decide if the Assertion (A) and Reason (R) are individually true and if so whether the Reason (R) is the correct explanation of the Assertion (A). Select your answers to these questions using the codes given below:
Codes:
(a) Both A and R are true and R is a correct explanation of A
(b) Both A and R are true but R is not a correct explanation of A
(c) A is true but R is false(d) A is false but R is true

(d) A is false but R is true

1. (A): At absolute zero degrees Kelvin temperature the semiconductor materials behave as insulators

(R): At absolute zero degrees Kelvin temperature, there is no energy available to generate the current carriers in the semiconductor.

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Answer. a

2. (A): Electron mobility in metals decreases with increasing temperature.
(R): In metals electron concentration is high.

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Answer. b

3. (A): A heavily doped semiconductor can exhibit positive temperature coefficient of resistance.
(R): The carrier mobility decreases with an increase of temperature.

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Answer. a

4. (A): An electric or mechanical input makes the flux jump from one superconductor to another, generating large direct current.
(R): The electric resistivity of superconductors depends upon the magnetic field.

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Answer. d

5. (A): A superconductor is a perfect diamagnetic material.
(R): A superconductor is a perfect conductor.

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Answer. c

6. (A): Superconductors cannot be used as coils for production of strong magnetic fields.
(R): Superconductivity in a wire may be destroyed if the current in the wire exceeds a critical value.

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Answer. a

7. (A): Superconductivity of a superconducting material can be destroyed by application of an external magnetic field.
(R): 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.

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Answer. c

8. (A): Cryotron is a switch constructed out of superconducting materials.
(R): Switching action is characteristic of superconductors only.

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Answer. c

9. (A): if a piece of metal is made to have a temperature gradient between its two ends, an emf exist between these ends.
(R): Electrons at the hot end move towards the cold end.

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Answer. a

10. (A): Thermal noise in metallic resistors can be characterized by Gaussian probability density function.
(R): Power spectral density of thermal noise is essentially constant for a very large frequency range.

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Answer. b

11. (A): Ammonium dihydrogen phosphate is a piezoelectric material.
(R): There is no centre of symmetry in ammonium dihydrogen phosphate.

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Answer. a

12. (A): The Diamagnetic effect is present in all the materials.
(R): The applied magnetic field alters the orbital motion of the constituent electrons of the material.

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Answer. d

13. (A): The spins within a magnetic domain are aligned permanently below Curie temperature in a ferromagnetic material.
(R): Ferromagnetic material is magnetic only when the domains are aligned by an external field.

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Answer. c

14. (A): Iron, cobalt and nickel are ferromagnetic.
(R): Spin imbalance of electrons in the 3rd band is responsible.

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Answer. a

15. (A): Soft magnetic material is used in making electromagnets.
(R): Soft magnetic materials have a high coercive field.

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Answer. c

16. (A): Soft magnetic materials cannot be used for the construction of cores for the electrical machines.
(R): Soft magnetic materials have relatively small and narrow hysteresis loop.

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Answer. d

17. (A): Alnico (aluminium-nickel-iron-cobalt) alloys are the mass importation of hard magnetic materials.
(R): Alnico magnet alloys have the lowest energy per unit of volume of any permanent magnet material available commercially.

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Answer. c

18. (A): Hard magnetic materials are used for making permanent magnets.
(R): Hard magnetic materials have relatively small and narrow hysteresis loop.

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Answer. c

19. (A): DC resistance of ferrites is much higher than that of dia-, para- or ferromagnetic materials.
(R): 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.

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Answer. b

20. (A): Ferrites are useful at very high frequencies.
(R): Ferrites have high permeability and high resistivity.

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Answer. a

21. (A): Ferrites cores are used in inductances for high frequencies.
(R): Ferrites provide high flux density with minimum eddy current loss and are insulators.

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Answer. a

22. (A): Magnetic cores are generally used in main memory of a digital computer.
(R): Magnetic cores are slow and volatile.

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Answer. c

23. (A): Storage of energy in a dielectric is due to shift in relative positions of internal charges against normal atomic and molecular forces.
(R): All dielectrics are characterised by the presence of molecules having a permanent displacement between the centres of gravity of the positive and negative charges.

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Answer. a

24. (A): An insulator has a high value of resistivity, and with increasing temperature, the value of its resistivity decreases exponentially.
(R): With increasing temperature, the value of the energy band gap decreases.

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Answer. d

25. (A): The relative dielectric constant of an insulator decreases with increase in the frequency of the applied alternating field.
(R): With an increase in the frequency of the applied field, the polarization process increases in number.

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Answer. c

26. (A): Mica is usually not used for slot linings of high voltage machines.
(R): Space factor in high voltage machines is high.

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Answer. c

27. (A): Skin depth is the depth by which electromagnetic wave has been increased to 37% of its original value.
(R): The depth of penetration of wave in a lossy dielectric increase with increasing wavelength.

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Answer. c

28. (A): Sulphur hexafluoride gas is highly inflammable.
(R): Sulphur hexafluoride gas is used in switchgear.

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Answer. d

29. (A): Mica has very high dielectric strength.
(R): Mica cannot be used for the large commutator of a dc machine.

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Answer. b

Electrical Engineering Materials: Assertion/Reason Questions

One thought on “Electrical Engineering Materials: Assertion/Reason Questions

  • August 30, 2017 at 2:26 PM
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    Thanks sir. Its useful for ies preparation.

    Reply

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