This set of Engineering Physics Multiple Choice Questions & Answers (MCQs) focuses on “Properties of Dielectric Materials”.
1. What is the process of producing electric dipoles inside the dielectric by an external electric field?
a) Polarisation
b) Dipole moment
c) Susceptibility
d) Magnetisation
View Answer
Explanation: When an external magnetic field is applied to the dielectrics, the field exerts a force on each positive charges in its own direction while negative charges are pushed in the opposite direction. Consequently, an electric dipole is created in all the atoms. Thus the process of producing electric dipoles inside the dielectrics by an external electric field is called polarisation.
2. Which of the following easily adapt itself to store electrical energy?
a) Passive dielectric
b) Superconductor
c) Active dielectric
d) Polar molecules
View Answer
Explanation: When a dielectric is subjected to an external electric field, if the dielectric actively accepts electricity, then they are termed as active dielectrics. Thus active dielectrics are the dielectrics which can easily adapt itself to store the electrical energy in it.
3. Which of the following restricts the flow of electrical energy?
a) Superconductors
b) Passive dielectrics
c) Polar molecules
d) Active dielectric
View Answer
Explanation: Passive dielectric acts as an insulator; conduction will not take place through this dielectrics. Thus passive dielectrics are the dielectrics that restrict the flow of electrical energy in it.
4. For non-polar molecules, there is no absorption or emission in the range of infrared.
a) True
b) False
View Answer
Explanation: These molecules possess centre of symmetry and hence the centres of positive and negative charges coincide. Therefore the net charges and net dipole moment of these molecules will be zero and hence these non-polar molecules will not posses any dipole moment in it. Hence there is no absorption or emission in the range of infrared.
5. How does ionic polarisation occur?
a) Splitting of ions
b) Passing magnetic field
c) Displacement of cations and anions
d) Never occurs
View Answer
Explanation: Ionic polarisation occurs due to the displacement of cations and anions from its original position in the opposite directions, in the presence of an electric field.
6. Polar molecules have permanent dipole moments even in the absence of an electric field.
a) False
b) True
View Answer
Explanation: In the absence of an electric field the polar molecules posses some dipole moment. These dipoles are randomly arranged and they cancel each other. Hence the net dipole moment is very less.
7. Which of the following polarisations is very rapid?
a) Electronic polarisation
b) Ionic polarisation
c) Space charge polarisation
d) Orientation polarisation
View Answer
Explanation: Electronic polarisation is very rapid and will complete at the instant the voltage is applied. The reason is that the electrons are very light particles. Therefore even for high frequency this kind of polarisation occurs.
8. Which of the following is the slowest polarisation method?
a) Ionic polarisation
b) Orientation polarisation
c) Electronic polarisation
d) Space charge polarisation
View Answer
Explanation: Space charge polarisation is very slow because in this case, the ions have to diffuse over several interatomic distances. Also, this process occurs at a very low frequency.
9. When does a dielectric become a conductor?
a) At avalanche breakdown
b) At high temperature
c) At dielectric breakdown
d) In the presence of magnetic field
View Answer
Explanation: When a dielectric is placed in an electric field and if the electric field is increased, when the electric field exceeds the critical field, the dielectric loses its insulating property and becomes conducting. This is called dielectric breakdown.
10. Which of the following breakdowns occur at a higher temperature?
a) Avalanche breakdown
b) Thermal breakdown
c) Electrochemical breakdown
d) Dielectric breakdown
View Answer
Explanation: When a dielectric is subjected to an electric field, heat is generated. This generated heat is dissipated by the dielectric. In some cases, the generated heat will be very high compared to the heat dissipated. Under such conditions, the temperature inside the dielectric increases and heat may produce breakdown. This is thermal breakdown.
11. When mobility increases, insulation resistance decreases and dielectric becomes conducting.
a) True
b) False
View Answer
Explanation: If the temperature is increased, the mobility of ions increases and hence electrochemical reaction may be induced to take place. Therefore when the mobility of ions is increased, insulation resistance decreases and hence dielectric becomes conducting.
12. Which of the following materials exhibit Ferro-electricity?
a) Iron
b) Platinum
c) Hydrogen
d) Rochelle salt
View Answer
Explanation: When a dielectric exhibits electric polarisation even in the absence of an external field, it is known as ferro-elecricity and these materials are termed as Ferro-electrics. They are anisotropic crystals that exhibit spontaneous polarisation. Hence only Rochelle salt exhibits Ferro-electricity.
13. Calculate the electronic polarizability of an argon atom whose ɛr = 1.0024 at NTP and N = 2.7×1025 atoms/m3.
a) 0.0024 Fm2
b) 7.87 ×10-40 Fm2
c) 7.87 Fm2
d) 1.0024×10-40 Fm2
View Answer
Explanation: Electronic polarisabilty αe = (ε0 (εr-1))/N
Electronic polarisability = 7.87 × 10-40 Fm2.
14. Calculate the dielectric constant of a material which when inserted in parallel condenser of area 10mm × 10mm and distance of separation of 2mm, gives a capacitance of 10-9 F.
a) 8.854×10-12
b) 100
c) 2259
d) 5354
View Answer
Explanation: C = (εr ε0 A)/d
εr = Cd/(ε0 A) = 2259.
15. Find the capacitance of layer of A12 O3 that is 0.5μm thick and 2000mm2 of square area εr = 8.
a) 1000μF
b) 0.283μF
c) 16μF
d) 2.83μF
View Answer
Explanation: C = (εr ε0 A)/d
Capacitance = 0.283μF.
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