How many of the following statements are correct?
(a) A dielectric has no free charge carriers, or only a negligible number of them.
(b) Oxygen \(\mathrm{(O_2)}\) and hydrogen \(\mathrm{(H_2)}\) molecules have a permanent dipole moment.
(c) In an external field, the charges of a non-polar molecule stop moving apart when the force of the field on them is balanced by the restoring force inside the molecule.
(d) When a conductor is placed in an external field, the field of its induced charges only reduces the external field inside it.
(e) In a linear isotropic dielectric, the induced dipole moment is proportional to the strength of the field.
(f) The two faces of a polarised dielectric slab carry induced charges of the same sign.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Three
A dielectric is an insulator. In an external field its charges cannot flow through it. Its molecules are only stretched or turned, and the dielectric gets a net dipole moment.
The statements that are correct:
(a) — correct, dielectrics are non-conducting substances
(c) — correct, the internal fields of the molecule give the restoring force
(e) — correct, a dielectric of this kind is called linear isotropic
The statements that are not:
(b) — \(\mathrm{O_2}\) and \(\mathrm{H_2}\) are symmetric, non-polar molecules with no dipole moment
(d) — in a conductor the two fields cancel each other, so the net field inside is zero
(f) — one face gets \(+\sigma_p\) and the opposite face gets \(-\sigma_p\)
Number of correct statements = 3
NCERT (2026-27):
Dielectrics and Polarisation – “Dielectrics are non-conducting substances. In contrast to conductors, they have no (or negligible number of ) charge carriers.”
Dielectrics and Polarisation – “… Examples of non-polar molecules are oxygen (O2) and hydrogen (H2) molecules which, because of their symmetry, have no dipole moment.”
Dielectrics and Polarisation – “… The displacement stops when the external force on the constituent charges of the molecule is balanced by the restoring force (due to internal fields in the molecule).”
Dielectrics and Polarisation – “… This happens until, in the static situation, the two fields cancel each other and the net electrostatic field in the conductor is zero.”
Dielectrics and Polarisation – “… 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. (Substances for which this assumption is true are called linear isotropic dielectrics.)”
Dielectrics and Polarisation – “Thus, the polarised dielectric is equivalent to two charged surfaces with induced surface charge densities, say σp and –σp.”