Question

What happens when current is passed through a metallic loop placed in magnetic field?
A. The ring will feel a force of attraction.
B. The ring will feel a force of repulsion.
C. It will move to and fro about its centre of gravity.
D. None of these

Answer 1

Hint:When a current carrying coil or loop is placed in a magnetic field, it experiences some torque and starts to rotate. Use this concept of torque experienced on a current carrying loop placed in a uniform magnetic field to get the solution of the above question.




Complete answer:
When current starts to flow through a metallic loop, it starts behaving like a bar magnet.
When this current carrying metallic loop is placed in a magnetic field, it starts to experience a torque.
This makes the loop rotate in the magnetic field.
Torque in a current carrying loop placed in a uniform magnetic field is given by:
$\tau = iAB\sin \theta $ … (1)
Where $i$ is the current flowing through the loop,
$A$ is the area of the loop,
$B$ is the intensity of the magnetic field and
$\theta $ is the angle between the magnetic field and the axis of the loop.
From the given options,
The loop does not move to and fro about its centre of gravity.
It also neither feels a force of attraction or repulsion.
It experiences a torque, making it rotate. The direction of this torque is determined by the cross product of $\vec A$ and $\vec B$ .
Thus, the correct option is D.



Note: A current carrying loop is similar to a bar magnet. This loop, when placed in a magnetic field, experiences a torque. As torque makes a body rotate, thus, the current carrying loop starts to rotate in the magnetic field, depending on the direction of the area vector and the direction of the magnetic field. It does not experience forces of attraction and repulsion.