Identify the correct statements among the following.
A. If the center of positive charges does not coincide with the center of negative charges in a molecule, it is a polar molecule.
B. When a non-polar molecule is placed in an external electric field, the field induces a dipole moment.
C. When a non-polar dielectric is placed in an external electric field, an electric field is induced in the dielectric, opposite to the external field.
D. A non-polar dielectric when placed in an external electric field between two charged plates, the net electric field between the plates increases.
Choose the correct option:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → A, B and C only
A molecule is polar when the centres of its positive and negative charges do not coincide. An external field gives a non-polar molecule an induced dipole moment, and these dipoles set up a field that opposes the applied field.
(A) If the center of positive charges does not coincide with the center of negative charges in a molecule, it is a polar molecule. — Correct. It has a permanent dipole moment.
(B) When a non-polar molecule is placed in an external electric field, the field induces a dipole moment. — Correct. The field pulls the charges slightly apart.
(C) When a non-polar dielectric is placed in an external electric field, an electric field is induced in the dielectric, opposite to the external field. — Correct. Bound surface charges produce a field opposite to the external field.
(D) A non-polar dielectric when placed in an external electric field between two charged plates, the net electric field between the plates increases. — Incorrect. The opposing field reduces the net field to \( E_0/K \).
NCERT (2026-27):
Dielectrics and Polarisation – “… The collective effect of all the molecular dipole moments is net charges on the surface of the dielectric which produce a field that opposes the external field. Unlike in a conductor, however, the opposing field so induced does not exactly cancel the external field. It only reduces it.”
Dielectrics and Polarisation – “… On the other hand, a polar molecule is one in which the centres of positive and negative charges are separated (even when there is no external field).”
Dielectrics and Polarisation – “… In an external electric field, the positive and negative charges of a non-polar molecule are displaced in opposite directions. … The non-polar molecule thus develops an induced dipole moment.”