Question

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

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

Answer 1

Answer: (C)

the normal to the plane of the coil is perpendicular to the field

$\overrightarrow{\tau }=\overrightarrow{M}\times \overrightarrow{B}$
$=niAB\sin \theta$
So, $\sin \theta$ is maximum when $\theta ={90}^o$
and $\theta$ is normal between normal of plane of coil and field.
So, the normal to the plane of the coil should be perpendicular to the field to produce maximum torque.