HC Verma Questions and Solutions: Chapter 37: Magnetic Properties of Matter- 1 | HC Verma Solutions - JEE PDF Download

If the magnetic field is increased, magnetisation of paramagnetic material placed in magnetic field is also increases. Hence, option (a) is correct.
Magnetization (I) is given by,

As the temperature is increased, magnetic moments of paramagnetic material becomes more randomly aligned due to incresed thermal motion. This leads decrease in the magnetization I.

Magnetisation becomes constant i.e all the magnetic moments have got aligned in the direction of the applied field. So now, if the temperature is decreased, thermal vibration of the paramagnetic material will reduce, But as all the magnetic moments are already aligned in the direction of the field so no further alignment can take place due to reduced thermal motion. Thus, there will be no negative effect of decreasing the temperature on the magnetisation.

Atoms of ferromagnetic material in unmagnetised state form domains inside the ferromagnetic material. These domains have large magnetic moments due to the magnetic moment of atoms. In the absence of magnetic field, these domains have magnetic moment in different directions. But when the magnetic field is applied, domains aligned in the direction of the field grow in size and those aligned in the direction opposite to the field reduce in size.

From the Biot-Savart law, magnetic field (B) at a point P close to the wire carrying current i is given by,

Magnetising field intensity (H) will be,

Now, as the cylindrical rod is brought close the wire such that centre of the rod is at P, then distance of point P from the wire(r) will remain same. Hence, magnetic field intensity will remain almost constant. Also even when the rod is carrying any current then B will be zero at the centre of the rod so the value of Magnetising field intensity will remain the same at point P.

Magnetic susceptibility is defined as the ratio of the intensity of magnetisation induced in the material to magnetising foorce applied on it.
Magnetic susceptibility is negative for diamagnetic materials. Magnetic susceptibility is positive for paramagnetic and ferromagnetic materials.

Permanent magnets should have high retentivity so that the magnet is strong and high coercive force, so that magnetisation is not erased by stary magnetic fields, temperature change or due to rough handling etc.

Electromagnets are made of soft iron because soft iron has
low retentivity -  When soft iron is placed inside a solenoid to make an electromagnet and current is passed through the solenoid,magnetism of the solenoid is incresed thousand folds. When the current is switched off, the magnetism is removed instantly because of the low retentivity of soft iron.
Low coercivity - it has low coercivity so that area under the hysteresis cureve for soft iron is very small hysterisis loss in case of soft iron is small.

Electrons of an atom moves in circular path around the nucleus and constitute electric current. Since a current loop has magnetic moment, therefore electron also has magnetic moment due to this orbit motion. It also has magnetic moment due to the spinning about its own axis.
Orbital motion of electron around the nucleus give rise to magnetic field around the proton (nucleus). This create a torque and thus magnetic dipole moment on proton.
All nuclei also have magnetic moment but it is several thousand times smaller than the magnetic moment of the electron so it can be ignored in comparison to the magnetic moment of an eletron.
Generally, an atom has no magnetic moment. Because the magnetic moments of electrons of an atom have a tendency to cancel in pairs.

Diamagnetic material have zero magnetic moment on their own.
Paramagnetic and ferromagnetic materials have non zero permanent magnetic moment. But we are not sure about the material, whether it is Paramagnetic or ferromagnetic. If the material has large value of  permanent magnetic moment, then it will be ferromagnetic and if it has small value of  permanent magnetic moment, then it will be Paramagnetic.

Paramagnetic and ferromagnetic materials have non zero permanent magnetic moment on their own. Only atoms of diamagnetic materials have zero permanent magnetic moment.

Dimension of Magnetic field B is given by,
F=Bqv

Dimension of magnetising field intensity H is given by,

Dimension of intensity of magnetisation I is also  as both Magnetising field intensity H and intensity of magnetisation I are measured in Am^-1.
Hence, option (a) and option (b) is incorrect. Option (c) is correct.
As Longitudinal strain and magnetic susceptibility both are dimensionless quantity. Hence, option (d) is correct.

We know,
Magnetic susceptibility = I/H
At point B, the value of H is zero but I is non zero. Magnetic susceptibility is infinity here. At point C, value of magnetic susceptibility will be negative. Here Magnetic field is applied in opposite direction to reduce the intensity of magnetization to zero. The applied field to reduce the residual magnetization to zero is called coercivity. At point D, I is zero but H is not zero so susceptibility of the material will be zero.

When a material is placed in magnetic field, dipole moment are induced in the atoms by the applied magnetic field. Since the direction of magnetic field due to induced dipole moment is opposite to the applied magnetic field. Therefore, resultant magnetic field is smaller than the applied magnetic field. This process is called diamagnetism. As this process takes place for all the material, therefore all the material exhibit diamagnetism. Hence, option (a) and (d) are correct.
Diamagnetic material do not have permanent magnetic moment on their own. When they are placed in magnetic field, dipole moments are induced by the applied magnetic field. Thus, there is no net alignment of permanent magnetic moment so these mterials do not have any permanenet magnetic momentof their own. Hence, option (b) is incorrect.
Magnetic field intensity is not zero in free space. Hence, option (c) is incorrect.