Engineering Physics Questions and Answers – Dielectric Polarisation

This set of Engineering Physics Multiple Choice Questions & Answers (MCQs) focuses on “Dielectric Polarisation”.

1. The forbidden energy gap of dielectrics is __________
a) Less than 1.2 eV
b) Greater than or equal to 1.2 eV
c) Less than 3 eV
d) Greater than or equal to 3 eV
View Answer

Answer: d
Explanation: Dielectrics are the materials devoid of free charges. An ideal dielectric is one which is completely devoid of charges. They have a forbidden energy gap of >=3eV.

2. Dipoles are created when dielectric is placed in __________
a) Magnetic Field
b) Electric field
c) Vacuum
d) Inert Environment
View Answer

Answer: b
Explanation: When the dielectric is placed in an electric field, like between the parallel plates of a capacitor, the dipoles are created and they tend to align themselves parallel to the direction of the field.

3. CO2 is a polar dielectric.
a) True
b) False
View Answer

Answer: b
Explanation: In CO2, the atoms are aligned in a symmetric way. Thus, the centers of positive and negative charge coincide and the molecule has no net charge and a zero dipole moment. Hence, it is a non-polar dielectric.
advertisement
advertisement

4. The torque induced on a dipole when placed in an electric field E is given by _________
a) Esinθ
b) pEsinθ
c) Ecosθ
d) pEcosθ
View Answer

Answer: b
Explanation: The torque on the dipole when it is placed in an electric field is given by: pEsinθ. It tends to align the dipole in the direction of the electric field.

5. The dipole is most stable when the angle between the dipole and the field is __________
a) 0°
b) 45°
c) 90°
d) 180°
View Answer

Answer: a
Explanation: The potential energy of a dipole is given by -pEcosθ. Thus the dipole would be most stable when potential energy would be minimum. Hence, when θ = 0, i.e., the dipole is aligned in the direction of the field, the dipole would be most stable.
Note: Join free Sanfoundry classes at Telegram or Youtube

6. What is the value of 1 Debye in cm?
a) 3.33 X 10-28 cm
b) 3.33 X 10-30 cm
c) 3.33 X 10-32 cm
d) 3.33 X 10-34 cm
View Answer

Answer: b
Explanation: Debye is the unit in which the dipole moment of a dipole is calculated. The value of 1 Debye is equal to 3.33 X 10-30 cm or 3.33 X 10-32 m.

7. Polarization is defined as the dipole moment __________
a) per unit length
b) per unit area
c) per unit volume
d) per unit time
View Answer

Answer: c
Explanation: When a dipole is placed in an electric field, the positive charges appear near the negative electrode and negative charges near the positive electrode. This phenomenon is called polarization which is defined as Dipole moment per unit volume.
advertisement

8. In a water drop of radius 1 mm all the molecular dipole points are in the same direction. If the dipole moment of a water molecule is 6 X 10-30 m, the polarization in the water drop is ___________
a) 6.4 X 10-13 m-2
b) 7.4 X 10-13 m-2
c) 8.4 X 10-13 m-2
d) 9.4 X 10-13 m-2
View Answer

Answer: c
Explanation: Molecular mass of water = 18 gm
18 gm of water contains 6.023 X 1023 molecules
18/103 m3 of water contains 6.023 X 1026/18 molecules
Volume of water drop = 4π/3 X (10-3)3 m3
No of molecules in the drop, N = 6.023 X 1026 X 4π X 10-9/18 X 3
= 1.4 X 1017 m-3
Polarization, P = Np
= 1.4 X 1017 m-3 X 6 X 10-30 m
= 8.4 X 10-13 m-2.

9. What is the direction of electric field?
Dipoles are arranged in a way that positive charge is on right & negative charge on left
a) Right to Left
b) Up to Down
c) Down to Up
d) Left to Right
View Answer

Answer: d
Explanation: As seen in the figure, the dipoles are arranged in such a way that the positive charge is on the right and negative charge on the left. Thus, the electric field is from left to right.
advertisement

10. In the formula, P = χeE, what is χe called?
a) Electric constant
b) Polarizing constant
c) Electric susceptibility
d) Polarizing susceptibility
View Answer

Answer: c
Explanation: In the given expression, χe is a constant characteristic of the dielectric and is known as the electric susceptibility of the dielectric medium.

11. H2O is a polar dielectric.
a) True
b) False
View Answer

Answer: a
Explanation: In H2O, the atoms are aligned in a symmetric way. However, as the electronegativity of oxygen is higher than hydrogen, it pulls the electronic cloud towards itself and it results in a net dipole moment.

12. What is the new capacitance of the plates, when a slab of Dielectric Constant K and thickness one-fourth of the separation of plate is inserted between the plates?
a) 4KCo/K + 2
b) 4KCo/1 + 2K
c) 4KCo/K + 3
d) 4KCo/1 + 3K
View Answer

Answer: d
Explanation: As the slab is inserted between the plates,
V = E0(d/4)/K + Eo(3d/4)
= Eod (1/4K + 3/4)
= Vo(1+3K)/4K
Now we know, C = Qo/V
= 4KCo/(1 + 3K).

13. The expression for electric susceptibility is ___________
a) εoE
b) εoEK
c) εoE (K + 1)
d) εoE (K – 1)
View Answer

Answer: c
Explanation: Electric susceptibility of a dielectric medium is a constant characteristic of the dielectric. The electric susceptibility is given by εoE (K – 1).

14. The relative dielectric constant of polystyrene is 2.5. What is the polarization produced when 0.5 mm thick sheet of polystyrene is subjected to 220 V?
a) 2.78 X 10-6 C/m
b) 3.91 X 10-6 C/m
c) 4.12 X 10-6 C/m
d) 5.84 X 10-6 C/m
View Answer

Answer: d
Explanation: We know, Polarization, P = εor – 1)E
Here, εr = 2.5
E = V/d
= 220 / 0.5 X 10-3
= 4.4 X 105 Vm
εo = 8.85 X 10-12 C/Vm
Hence, P = 8.85 X 10-12 X (2.5 – 1) X 4.4 X 105 C/m2
= 5.84 X 10-6 C/m.

Sanfoundry Global Education & Learning Series – Engineering Physics.

To practice all areas of Engineering Physics, here is complete set of 1000+ Multiple Choice Questions and Answers.

If you find a mistake in question / option / answer, kindly take a screenshot and email to [email protected]

advertisement
advertisement
Subscribe to our Newsletters (Subject-wise). Participate in the Sanfoundry Certification contest to get free Certificate of Merit. Join our social networks below and stay updated with latest contests, videos, internships and jobs!

Youtube | Telegram | LinkedIn | Instagram | Facebook | Twitter | Pinterest
Manish Bhojasia - Founder & CTO at Sanfoundry
Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

Subscribe to his free Masterclasses at Youtube & discussions at Telegram SanfoundryClasses.