# 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

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

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

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.

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

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°

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.
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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

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

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.

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

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?

a) Right to Left
b) Up to Down
c) Down to Up
d) Left to Right

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.

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

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

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

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)

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

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.

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