Question

A current loop in a magnetic field
A) Can be in equilibrium in one orientation
B) Can be in equilibrium in two orientations, both the equilibrium states are unstable
C) Can be in equilibrium in two orientations, one stable while the other is unstable
D) Experiences a torque where the field is uniform or non uniform in all orientations

Answer 1

This problem can be solved by applying the conditions so that a current loop in a magnetic field is in equilibrium and experiences no torque. This happens when the normal to the area of the current loop is parallel or antiparallel to the direction of the magnetic field.

Complete answer:
For a current carrying loop in a magnetic field, it experiences a torque due to the magnetic field when the angle made by the normal to the surface of the loop is non zero. In that case, there is a torque on the loop and it is not in equilibrium.
When the normal to the area of the loop is parallel or anti parallel to the magnetic field, then the loop experiences no torque and hence, is said to be in equilibrium. Therefore, there are two orientations of the loop in the magnetic field where it will be in equilibrium. When the normal to the area of the loop is parallel to the magnetic field, then the loop is said to be in stable equilibrium. On the other hand, when the normal to the area of the loop is anti parallel to the magnetic field, then the loop is said to be in unstable equilibrium.
Hence, there are two orientations for the loop where it can be in equilibrium, one is stable equilibrium and the other is unstable equilibrium.
Therefore, the correct answer is it can be in equilibrium in two orientations, one stable while the other is unstable.

So, the correct answer is “Option C”.

Note:
Students must note that in the orientation of unstable equilibrium, if the loop is disturbed a bit, then it progressively moves away from the orientation of the unstable equilibrium. This is why it is called the unstable equilibrium orientation. On the other hand, in the stable equilibrium orientation, if the loop is disturbed a bit, then it experiences a restoring torque that tries to bring it back to the stable equilibrium orientation. Hence, the orientation is called the stable equilibrium orientation.