A non-polar dielectric material is placed in an external electric field. Choose the correct statement(s) from the following:
(A) There will be no change in the positions of positive and negative charges inside the dielectric.
(B) The positive charges of molecules will shift opposite to the direction of the electric field.
(C) The negative charges of molecules will shift in the direction of the electric field.
(D) The positive charges of the molecules will shift in the direction of the field and negative charges of the molecules will shift opposite to the field.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (D) only
In a non-polar molecule the centres of positive and negative charge coincide. An external field pushes positive charge along \( \vec{E} \) and negative charge against it, so the centres separate and each molecule gets an induced dipole moment along the field.
(A) There will be no change in the positions of positive and negative charges inside the dielectric. — Incorrect. The charges are displaced; that displacement is what polarises the dielectric.
(B) The positive charges of molecules will shift opposite to the direction of the electric field. — Incorrect. The force on a positive charge is \( q\vec{E} \), along the field.
(C) The negative charges of molecules will shift in the direction of the electric field. — Incorrect. The force on a negative charge is opposite to \( \vec{E} \).
(D) The positive charges of the molecules will shift in the direction of the field and negative charges of the molecules will shift opposite to the field. — Correct. This is the separation of charge that produces the induced dipole.
NCERT (2026-27):
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. … We consider only the simple situation when the induced dipole moment is in the direction of the field and is proportional to the field strength.”