The molecules of a substance have permanent electric dipole moments. A strong uniform electric field lines up all the dipoles along it. The direction of the field is then suddenly turned through \(60^\circ\). As the dipoles line up with the new direction, the substance
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → loses potential energy, which is given out as heat
A dipole in a uniform field has potential energy \( U=-pE\cos\theta \), where \( \theta \) is the angle between \( \vec p \) and \( \vec E \). Just after the field turns, each dipole makes \( 60^\circ \) with it and has \( U=-pE/2 \). When it lines up again, \( U \) becomes \( -pE \).
Option (1) loses potential energy, which is given out as heat — Correct. Each dipole loses \( pE/2 \) of potential energy as it turns. This energy leaves the substance as heat.
Option (2) gains potential energy, which it takes from the surroundings as heat — Incorrect. Lining up with the field lowers the energy of a dipole. The substance has extra energy to give out, so it takes in no heat.
Option (3) keeps the same potential energy, because the field strength does not change — Incorrect. The energy depends on the angle with the field as well as on \( E \). Here the angle falls from \( 60^\circ \) to \( 0^\circ \).
Option (4) ends up with zero potential energy in its dipoles — Incorrect. Zero energy belongs to a dipole at \( 90^\circ \) to the field. A dipole along the field has \( U=-pE \), the lowest value.
NCERT (2026-27):
Example 2.6 – “… Initial potential energy, Ui = –pE cos θ = –6×10–6×106 cos 0° = –6 J …”
Example 2.6 – “So, there is loss in potential energy. This must be the energy released by the substance in the form of heat in aligning its dipoles.”