Which of the following statements are correct?
A. Dipole field depends on the distance r and also on the angle between the position vector \( \vec{r} \) and the dipole moment \( \vec{p} \)
B. A comb run through dry hair acquires charge through friction
C. If surface charge density of a plate is positive, the electric field is directed towards the plate.
D. Electrostatic field lines cannot form closed loops.
E. In a uniform electric field, a dipole experiences a torque and a net force also.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → A, B and D Only
The statements cover the field of a dipole, charging by rubbing, the field near a charged plate, field lines and a dipole in a uniform field.
(A) Dipole field depends on the distance r and also on the angle between the position vector \( \vec{r} \) and the dipole moment \( \vec{p} \) — Correct. On the axis the field is \( 2p/(4\pi\varepsilon_0 r^{3}) \) and on the equatorial line \( p/(4\pi\varepsilon_0 r^{3}) \), so it depends on both the distance and the angle.
(B) A comb run through dry hair acquires charge through friction — Correct. Rubbing transfers electrons between the hair and the comb, so both become charged.
(C) If surface charge density of a plate is positive, the electric field is directed towards the plate. — Incorrect. The field of a positive charge points away from it, so it is directed away from a positively charged plate.
(D) Electrostatic field lines cannot form closed loops. — Correct. The electrostatic field is conservative: lines start on positive charges and end on negative charges, and never close on themselves.
(E) In a uniform electric field, a dipole experiences a torque and a net force also. — Incorrect. The forces \( +q\vec{E} \) and \( -q\vec{E} \) on the two charges are equal and opposite, so the net force is zero; only a torque \( \vec{p}\times\vec{E} \) acts.
NCERT (2026-27):
The Field of an Electric Dipole – “… Further, the magnitude and the direction of the dipole field depends not only on the distance r but also on the angle between the position vector r and the dipole moment p.”
Dipole in a Uniform External Field – “… The comb, as we know, acquires charge through friction.”
Electric Field Lines – “… (iv) Electrostatic field lines do not form any closed loops.”
Dipole in a Uniform External Field – “… The net force on the dipole is zero, since E is uniform. However, the charges are separated, so the forces act at different points, resulting in a torque on the dipole.”