Question

(I) Why do magnetic lines of force form continuous closed loops?
(II) Why are the field lines repelled (expelled) when a diamagnetic material is placed in an external uniform magnetic field?

Answer 1

A magnetic monopole does not exist in reality. A magnet always exists in dipole form. The magnetic field lines emerge from the north pole of the magnet and merge at the south pole. In case of diamagnetic materials, an induced magnetic field is created in a direction opposite to the direction of the external magnetic field.

Complete answer:
(I) Magnetic lines of force are the imaginary lines in the magnetic field which describe the direction of the magnetic field at a point in the magnetic field. A magnet has two poles; the south pole and north pole. The poles of the magnet having the same polarity repel each other, while the ones having opposite polarity attract each other. Outside of a magnet, magnetic lines of force start from the north pole and end at the south pole.

Magnetic lines of force emerge from the south pole and end at the north pole inside of a magnet. It is experimentally found that a magnet always exists in dipole form. A magnetic monopole does not exist in reality. Therefore, closed and continuous loops or circles are formed by the magnetic lines.

(II) Diamagnetic materials are the materials whose atoms do not possess any permanent magnetic dipole moment due to the presence of electrons that are paired with each other, called paired electrons. This is the reason why when a diamagnetic material is placed in an external magnetic field, the magnetic field lines are repelled.

On the application of an external magnetic field, it creates a magnetic field in the diamagnetic material opposite to the direction of the applied field which causes a repulsive force.This is the reason why when a diamagnetic material is placed in an external magnetic field, the magnetic field lines are repelled.

Note: The tangent to the magnetic line of force at a point in a magnetic field depicts the direction of the magnetic field. This can be verified by placing a compass near a bar magnet. Diamagnetism is a property of all substances but it always makes weak contributions as compared to paramagnetism and ferromagnetism.