Case: Read the passage and answer the question.
Conductors contain mobile charge carriers. In metallic conductors, these charge carriers are electrons. In a metal, the outer (valence) electrons part away from their atoms and are free to move. These electrons are free within the metal but not free to leave the metal. The free electrons form a kind of ‘gas’; they collide with each other and with the ions and move randomly in different directions. In an external electric field, they drift against the direction of the field. The positive ions made up of the nuclei and the bound electrons remain held in their fixed positions. A dielectric (an insulator) behaves differently. Its charges are held to their atoms or molecules and cannot move freely through it. An external electric field can only shift these charges a little or turn the molecules. The dielectric is then said to be polarised.
Match List-I with List-II.
| List-I | List-II |
|---|---|
| (A) Polarisation of a dielectric | (I) Bound charges of the dielectric |
| (B) A non-polar molecule | (II) Dipole moment per unit volume |
| (C) Induced surface charge on a polarised dielectric | (III) Both positive and negative ions |
| (D) Charge carriers in an electrolytic conductor | (IV) No permanent dipole moment |
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (A)-(II), (B)-(IV), (C)-(I), (D)-(III)
(A) Polarisation of a dielectric — This matches (II) dipole moment per unit volume. In a field, each small volume of the dielectric gets a net dipole moment, and this per unit volume is the polarisation \( P \).
(B) A non-polar molecule — This matches (IV) no permanent dipole moment. Its centres of positive and negative charge coincide, as in \( \mathrm{O_2} \) and \( \mathrm{H_2} \).
(C) Induced surface charge on a polarised dielectric — This matches (I) bound charges of the dielectric. The unbalanced ends of the molecular dipoles at the faces give \( \pm\sigma_p \), and these charges cannot move through the slab.
(D) Charge carriers in an electrolytic conductor — This matches (III) both positive and negative ions. In a metal the carriers are electrons, but in an electrolyte they are ions of both signs.
NCERT (2026-27):
Dielectrics and Polarisation – “The dipole moment per unit volume is called polarisation and is denoted by P.”
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.”
Dielectrics and Polarisation – “We should note that the surface charge density ±σp arises from bound (not free charges) in the dielectric.”
Electrostatics of Conductors – “In electrolytic conductors, the charge carriers are both positive and negative ions; but the situation in this case is more involved …”