A loop of flexible wire of irregular shape carrying current is placed in an external magnetic field. Identify the effect of the field on the wire.

When a flexible current-carrying loop of irregular shape is placed in an external magnetic field, every small element of the wire experiences a force. These forces act outward on the wire since the current-carrying loop tends to maximize the magnetic flux through it.

The loop tends to maximize its area to maximize the flux linkage, and for a given perimeter the shape that encloses maximum area is a circle. Furthermore, the torque on the loop orients it so that its plane is perpendicular (normal) to the magnetic field, as this is the orientation of minimum potential energy ($$U = -\vec{m} \cdot \vec{B}$$ is minimized when the magnetic moment is parallel to the field).

Therefore the loop assumes a circular shape with its plane normal to the field.