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Polarization Mechanisms
MCQ questions and answers.
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MCQ Questions
1. A non-polar molecule is placed in a uniform electric field. Which statement best describes the polarization mechanism involved?
A.
Permanent dipoles rotate until all molecular axes become exactly perpendicular to the field
B.
The positive and negative charge distributions shift slightly in opposite directions, producing an induced dipole
C.
The molecule loses electrons completely and becomes an ion
D.
The molecule acquires a permanent magnetic dipole moment
Correct Answer:
The positive and negative charge distributions shift slightly in opposite directions, producing an induced dipole
Explanation:
In electronic polarization, an external electric field causes a small relative displacement between the bound positive and negative charges of a non-polar molecule. This produces an induced dipole moment in the direction of the field without ionization.
2. In an ionic dielectric subjected to a weak electric field, which microscopic displacement primarily produces ionic polarization?
A.
Rotation of pre-existing molecular dipoles about their centers of mass
B.
Migration of free electrons across the entire dielectric
C.
Relative displacement of positive and negative ions from their equilibrium lattice positions
D.
Alignment of magnetic domains along the electric field
Correct Answer:
Relative displacement of positive and negative ions from their equilibrium lattice positions
Explanation:
Ionic polarization occurs when oppositely charged ions in an ionic solid are displaced slightly in opposite directions by the applied electric field. The resulting separation of charge creates a dipole moment.
3. A dielectric contains molecules with permanent dipole moments. When the temperature is increased while the applied electric field remains constant, the orientational polarization generally decreases because
A.
the permanent dipole moments disappear at higher temperature
B.
thermal agitation increasingly opposes the field-induced alignment of the dipoles
C.
the dielectric constant becomes exactly zero
D.
electronic polarization changes sign
Correct Answer:
thermal agitation increasingly opposes the field-induced alignment of the dipoles
Explanation:
Orientational polarization results from partial alignment of permanent molecular dipoles with the applied field. Increasing temperature enhances random thermal motion, reducing the degree of alignment and therefore reducing this polarization.
4. Which polarization mechanism can respond most rapidly to a suddenly applied electric field in an ordinary dielectric?
A.
Space-charge polarization
B.
Orientational polarization
C.
Ionic polarization
D.
Electronic polarization
Correct Answer:
Electronic polarization
Explanation:
Electronic polarization involves only a very small displacement of bound electrons relative to nuclei, so it has the shortest response time among the listed mechanisms. Orientational and space-charge mechanisms require much slower molecular rotation or charge migration.
5. A dielectric is exposed successively to alternating electric fields of increasing frequency. Which mechanism is expected to disappear first as the frequency becomes too high for dipoles to follow the field?
A.
Electronic polarization
B.
Orientational polarization
C.
Vacuum polarization
D.
Nuclear decay polarization
Correct Answer:
Orientational polarization
Explanation:
Orientational polarization requires physical rotation of permanent dipoles and therefore has a relatively long response time. At sufficiently high frequency, the dipoles cannot reorient rapidly enough, so their contribution decreases first.
6. A dielectric slab contains mobile charge carriers that are blocked at its opposite faces. Under a uniform applied electric field, charges accumulate near the faces. This is an example of
A.
electronic polarization
B.
ionic polarization
C.
space-charge polarization
D.
orientational polarization
Correct Answer:
space-charge polarization
Explanation:
Space-charge polarization arises when mobile charges or ions drift through part of the material and accumulate at interfaces, defects, or electrodes. The separated accumulated charges produce a macroscopic polarization.
7. For a dilute gas of non-interacting permanent dipoles in a weak electric field, the average dipole component along the field is proportional to E/T. This dependence is characteristic of
A.
electronic polarization controlled only by electron mass
B.
orientational polarization opposed by thermal agitation
C.
ionic polarization independent of temperature
D.
space-charge polarization caused only by electrode geometry
Correct Answer:
orientational polarization opposed by thermal agitation
Explanation:
For permanent dipoles in a weak field, the average alignment is approximately proportional to the ratio of field strength to temperature. Thus, orientational polarization varies approximately as E/T because thermal agitation randomizes dipole orientations.
8. A material has no permanent molecular dipoles, yet it becomes polarized in an external electric field. If the polarization vanishes immediately when the field is removed, the dominant mechanism is most likely
A.
electronic polarization
B.
remanent orientational polarization
C.
permanent space-charge polarization
D.
chemical ionization of every molecule
Correct Answer:
electronic polarization
Explanation:
Non-polar molecules can acquire temporary induced dipoles through displacement of their electronic clouds relative to their nuclei. When the field is removed, this displacement disappears rapidly, so the polarization is reversible.
9. In a dielectric, the polarization response continues to increase slowly after the electric field is applied because charges require time to drift and accumulate at internal interfaces. Which mechanism explains this delayed response?
A.
Electronic polarization
B.
Space-charge polarization
C.
Instantaneous nuclear polarization
D.
Magnetic domain polarization
Correct Answer:
Space-charge polarization
Explanation:
Space-charge polarization is a slow process because it depends on the transport of mobile charges over finite distances and their accumulation at interfaces or boundaries. Therefore, its development is delayed compared with electronic polarization.
10. Which comparison between electronic and ionic polarization is correct for a dielectric subjected to a static electric field?
A.
Electronic polarization involves displacement of electrons relative to nuclei, whereas ionic polarization involves displacement of oppositely charged ions
C.
Electronic polarization is caused by charge accumulation at electrodes, whereas ionic polarization is caused by magnetic alignment
D.
Electronic polarization occurs only in ionic crystals, whereas ionic polarization occurs only in gases
Correct Answer:
Electronic polarization involves displacement of electrons relative to nuclei, whereas ionic polarization involves displacement of oppositely charged ions
Explanation:
Electronic polarization is produced by distortion of each atom or molecule's electron cloud relative to its nucleus. Ionic polarization is produced by relative displacement of positive and negative ions in an ionic lattice.
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Polarization Mechanisms
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