Question

A rectangular coil of wire carrying a current is kept in a uniform magnetic field. The torque acting on the coil will be zero when

• A. the plane of the coil is perpendicular to the field
• B. the normal to the plane of the coil is making an angle of 45${}^o$ with the field
• C. the normal to the plane of the coil is perpendicular to the field
• D. the plane of the coil is parallel to the field

Answer 1

The correct option is A the plane of the coil is perpendicular to the field

$\overrightarrow{\tau }=\overrightarrow{M}\times \overrightarrow{B}$
$=niAB\sin \theta$
So, for torque to be zero $\sin \theta$ should be zero and for that $\theta$ should be zero, where $\theta$ is angle between normal to the plane of coil and field.
So the plane of the coil and field should be perpendicular to each other