A dielectric made of polar molecules is placed in an external electric field. The dielectric develops a net dipole moment mainly because
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → the field tends to align the permanent dipoles of the molecules along its direction
A polar molecule is already a small dipole, because its centres of positive and negative charge are apart. With no field, thermal agitation keeps these dipoles pointing in random directions, so their total dipole moment is zero.
Option (1) the field separates the positive and negative charges inside each molecule — Incorrect. This is how a non-polar molecule gets an induced dipole moment. A polar molecule may show this effect too, but it is small compared with the turning of its permanent dipoles.
Option (2) the field tends to align the permanent dipoles of the molecules along its direction — Correct. The field turns the permanent dipoles towards its own direction. Their moments no longer cancel, so the dielectric gets a net dipole moment along the field.
Option (3) the field increases the thermal agitation of the molecules — Incorrect. Thermal agitation comes from the thermal energy of the molecules. It acts against the field and tries to disturb the alignment of the dipoles.
Option (4) free electrons in the dielectric drift towards one face — Incorrect. A dielectric has no free charge carriers, or only a negligible number. Its charges stay bound inside the molecules.
NCERT (2026-27):
Dielectrics and Polarisation – “… When an external field is applied, the individual dipole moments tend to align with the field. … There may be, in addition, the ‘induced dipole moment’ effect as for non-polar molecules, but generally the alignment effect is more important for polar molecules.”