Question

A current carrying coil is placed in a constant uniform magnetic field B. Torque is maximum on this coil when plane of coil is:

• A. perpendicular to B
• B. parallel to B
• C. at ${45}^{\circ }$ to B
• D. at ${60}^{\circ }$ to B

Answer 1

The correct option is B parallel to B

The equation for torque is $T=\overrightarrow{m}\times \overrightarrow{B}=mBsin\left(\theta \right)$.

The sine is the main factor here. Since sin(90) gives you 1, the torque is multiplied by one, which has the maximum value.
To sum it up, max torque is achieved by a magnetic field perpendicular to the magnetic moment, and min torque is achieved by a magnetic field parallel to the magnetic moment.
Therefore a current-carrying coil placed in a constant uniform magnetic field experiences maximum torque when the plane of the coil is parallel to the direction of the magnetic field (B).