A loop of soft, flexible wire has an irregular shape and carries a steady current. It is placed in a uniform magnetic field. The loop slowly takes a circular shape, with its plane normal to the field. This happens because, in this position,
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → the magnetic flux through the loop is the largest
A current loop that is free to move settles where the flux of the total field through it is the largest. The length of the wire is fixed, so the loop can change only its shape and its position.
Option (1) the magnetic force on every part of the wire becomes zero — Incorrect. Each part of the wire is perpendicular to the field, so each part still feels a force \( IB\,dl \). These forces stretch the wire in its own plane. They add up to zero only for the loop as a whole.
Option (2) the torque on the loop is the largest — Incorrect. With the plane normal to the field, the area vector is along the field. Then \( \tau=IAB\sin\theta \) with \( \theta=0 \), so the torque is zero.
Option (3) the magnetic moment of the loop becomes zero — Incorrect. The magnetic moment is \( m=IA \). A circle gives the largest area for this wire, so the moment is the largest. It is not zero.
Option (4) the magnetic flux through the loop is the largest — Correct. For a given length of wire, a circle encloses more area than any other shape. With its plane normal to the field, the whole of this area faces the field. So the flux through the loop is the largest.
NCERT (2026-27):
Example 4.11 – “(c) It assumes circular shape with its plane normal to the field to maximise flux, since for a given perimeter, a circle encloses greater area than any other shape.”