Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): When a dielectric slab is placed in a uniform external electric field, the total field inside the slab becomes less than the external field.
Reason (R): With no external field, the permanent dipoles in a polar dielectric point in random directions because of thermal agitation.
In the light of the above statements, choose the most appropriate answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Both (A) and (R) are correct but (R) is NOT the correct explanation of (A).
Assertion (A) When a dielectric slab is placed in a uniform external electric field, the total field inside the slab becomes less than the external field. — True. The external field polarises the slab. Induced charges \( +\sigma_p \) and \( -\sigma_p \) appear on the two faces normal to the field. Their field points against the external field, so the total field inside is smaller.
Reason (R) With no external field, the permanent dipoles in a polar dielectric point in random directions because of thermal agitation. — True. Thermal agitation is the random motion of molecules due to heat. It keeps the dipoles in random directions, so the total dipole moment is zero.
Why Reason (R) does not explain Assertion (A)?
R describes the dielectric before any field is applied. It says nothing about the field inside the slab.
The field is reduced by the induced surface charges \( \pm\sigma_p \). These appear for non-polar dielectrics also, where there are no permanent dipoles at all.
NCERT (2026-27):
Dielectrics and Polarisation – “Thus, the polarised dielectric is equivalent to two charged surfaces with induced surface charge densities, say σp and –σp. Clearly, the field produced by these surface charges opposes the external field. The total field in the dielectric is, thereby, reduced from the case when no dielectric is present.”
Dielectrics and Polarisation – “… In the absence of any external field, the different permanent dipoles are oriented randomly due to thermal agitation; so the total dipole moment is zero. …”