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.
Suppose a dielectric slab is used in place of the metal plate. Consider the following statements about the slab:
(A) The charges induced on the two faces of the slab are bound charges of its molecules.
(B) Inside the slab, every small volume element carries a net charge.
(C) The dipole moment per unit volume of the slab is called its polarisation.
(D) If the slab has polar molecules, thermal agitation helps their dipoles to line up with the field.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (A) and (C) only
In an external field, a dielectric slab becomes polarised. Its molecules get dipole moments along the field.
(A) The charges induced on the two faces of the slab are bound charges of its molecules. — True. The charges of a dielectric cannot move through it. The unbalanced ends of the molecular dipoles at the two faces give the surface charges \( \pm\sigma_p \).
(B) Inside the slab, every small volume element carries a net charge. — False. Inside the slab, the positive end of one dipole sits close to the negative end of the next. A small volume element has a net dipole moment but no net charge.
(C) The dipole moment per unit volume of the slab is called its polarisation. — True. This is the definition of polarisation \( P \). For a linear isotropic dielectric, \( P=\varepsilon_0\chi_eE \).
(D) If the slab has polar molecules, thermal agitation helps their dipoles to line up with the field. — False. Thermal agitation keeps the dipoles in random directions. It works against the line-up produced by the field.
NCERT (2026-27):
Dielectrics and Polarisation – “We should note that the surface charge density ±σp arises from bound (not free charges) in the dielectric.”
Dielectrics and Polarisation – “Anywhere inside the dielectric, the volume element Δv has no net charge (though it has net dipole moment).”
Dielectrics and Polarisation – “The dipole moment per unit volume is called polarisation and is denoted by P.”
Dielectrics and Polarisation – “The extent of polarisation depends on the relative strength of two mutually opposite factors: the dipole potential energy in the external field tending to align the dipoles with the field and thermal energy tending to disrupt the alignment.”