Choose the correct statements:
A. In uniform electric field E, the net force on electric dipole is zero.
B. The torque on dipole in electric field is \( \tau= PE\cos\theta \), where θ is angle between \( \vec P \) and \( \vec E \)
C. In non uniform Electric field, when dipole moment P is parallel to E, the dipole has a net force in the direction of decreasing field.
D. The direction of torque is perpendicular to both \( \vec P \) and \( \vec E \).
E. The net torque is zero if E is not uniform.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → A and D only
(A) In uniform electric field E, the net force on electric dipole is zero — Correct. The two charges of the dipole feel forces \( +q\vec{E} \) and \( -q\vec{E} \), which are equal and opposite in a uniform field, so they cancel.
(B) The torque on dipole in electric field is \( \tau= PE\cos\theta \) — Incorrect. The torque is \( \tau = pE\sin\theta \); it is zero when the dipole is along the field (\( \theta = 0 \)) and maximum when it is across it.
(C) In non uniform Electric field, when dipole moment P is parallel to E, the dipole has a net force in the direction of decreasing field — Incorrect. The force on the nearer charge and on the farther charge are now unequal. When \( \vec{p} \) is parallel to \( \vec{E} \), the positive charge sits where the field is stronger, so the net force points towards the region of increasing field.
(D) The direction of torque is perpendicular to both \( \vec P \) and \( \vec E \) — Correct. The torque is the vector product \( \vec{\tau} = \vec{p}\times\vec{E} \), and a cross product is perpendicular to both of its factors.
(E) The net torque is zero if E is not uniform — Incorrect. The torque still depends on the angle between \( \vec{p} \) and \( \vec{E} \), as \( \vec{p}\times\vec{E} \). It is zero only when they are parallel or anti-parallel, whether the field is uniform or not.
NCERT (2026-27):
Summary – “… 13. In a uniform electric field E, a dipole experiences a torque τ given by τ = p × E but experiences no net force.”
Dipole in a Uniform External Field – “… In either case, the net torque is zero, but there is a net force on the dipole if E is not uniform. Figure 1.20 is self-explanatory. It is easily seen that when p is parallel to E, the dipole has a net force in the direction of increasing field.”