Case: Read the passage and answer the question.
In a parallel plate air capacitor having plate separation 5 cm, an electric field of \( 4\times10^{4}\,\mathrm{V/m} \) is established between the plates. After the removal of the battery, a metal plate of thickness t = 2 cm is inserted between the plates of the capacitor.
Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): If a slab of a non-polar dielectric is used in place of the metal plate, its molecules develop induced dipole moments.
Reason (R): In a non-polar molecule, the centres of positive and negative charges are separated even when there is no external field.
In the light of the above statements, choose the most appropriate answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A) is correct but (R) is not correct.
Assertion (A) If a slab of a non-polar dielectric is used in place of the metal plate, its molecules develop induced dipole moments. — True. The field pulls the positive and negative charges of each molecule in opposite directions. Each molecule then gets a small dipole moment along the field, called an induced dipole moment.
Reason (R) In a non-polar molecule, the centres of positive and negative charges are separated even when there is no external field. — False. In a non-polar molecule the two centres coincide, so it has no permanent dipole moment. Centres that stay separated with no field describe a polar molecule, such as HCl.
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.”
Dielectrics and Polarisation – “In a non-polar molecule, the centres of positive and negative charges coincide. The molecule then has no permanent (or intrinsic) dipole moment.”