NEET Physics Notes Magnetism-Magnetic Behaviour of Materials
Magnetic Behaviour of Materials
Magnetic Behaviour of Materials
The magnetic behaviour of metals can be considered by using the description of different type of materials i.e. paramagnetic, diamagnetic and ferromagnetic as given below
Diamagnetic Materials
These are materials which show a very small decrease in magnetic flux, when placed in a strong magnetising field. Hydrogen, water, copper, zinc, antimony, bismuth etc. are examples of diamagnetic materials.
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- In a diamagnetic material, the net magnetic moment (sum of that due to orbital motion and spin motion of electrons) of an atom is zero. The external magnetic field B distorts the electron orbit and thus, induces a small magnetic moment in the opposite direction.
- Diamagnetic materials are feebly repelled in an external magnetic field and thus, have a tendency to shift from the stronger to weaker regions of the magnetic field.
- The relative permeability of any diamagnetic substance is slightly less than 1 (µ)r < 1 and susceptibility has a small negative value.
- Diamagnetism is an intrinsic property and does not vary with magnetic field B or temperature.
Paramagnetic Materials
These are the materials which show a small increase in the magnetic flux when placed in a magnetising field. Oxygen, air, platinum, aluminium etc. are examples of paramagnetic materials.
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- In a paramagnetic material, the net magnetic moment of every atom is non-zero.
- Paramagnetic materials are feebly attracted in an external magnetic field and thus, have a tendency to shift from the weaker to the stronger regions of magnetic field.
- The relative permeability (µ)r of a paramagnetic material is slightly greater than one (µ)r > 1 Magnetic susceptibility
of paramagnetic materials is positive.
- Paramagnetism is temperature dependent. According to the Curie’s law, the magnetic susceptibility of a paramagnetic substance is inversely proportional to its temperature T.
- Mathematically
- where, B is the external magnetic field and C is a constant, known as the Curie constant.
Ferromagnetic Materials
These are the materials which are strongly attracted by a magnetic field and can themselves be magnetised even in a weak magnetising field. Iron, steel,nickel and cobalt are ferromagnetic.
- These materials show a large increase in the magnetic flux, when placed in a magnetic field. Thus, for them (µ)r >> 1 Accordingly, is positive and large.
- Ferromagnetic materials exhibit all properties exhibited by paramagnetic substances and by a much larger measure.
- Magnetic susceptibility of ferromagnetic materials decreases steadily with a rise in temperature.
- Above a certain temperature Tc (known as Curie temperature), the substance loses its ferromagnetic character and begins to behave as a paramagnetic substance.
- Above the Curie temperature Tc, the magnetic susceptibility of a ferromagnetic material varies as
- where, C is a constant. It is known as the Curie-Weiss law.
Hysteresis Curve
A great deal of information can be learned about the magnetic properties of a material by studying its hysteresis loop. A hysteresis loop shows the relationship between the induced magnetic flux density (B) and the magnetising force (H). It is often referred as B-H loop.
A ferromagnetic material can be easily magnetised by placing it in an external magnetising field H.
Initially as H is increased, the intensity of magnetisation I developed in the material increases non-linearly along the curve OA and reaches a maximum, known as saturated magnetism. Now, on reducing H, I follows the path AB. Point B corresponds to H = 0, but I has a finite positive value. This value of I is called remanence or retentivity or residual magnetisation.
If direction of H is reversed and is gradually increased, H curve is along BC. Point C represents the situation, when I is finally reduced to zero. The reversed value of I, represented by OC, is known as the coercivity of the material.
On increasing H in the reverse direction further, graph CD is obtained. Now, if H is taken back from its negative saturation value to its original positive saturation value, a similar I – H curve represented by DEFA, is traced.
The whole graph ABCDEFA is a closed loop and known as hysteresis loop.
Electromagnet
When a soft-iron road is placed in the solenoid, the magnetism of the solenoid increases hundreds of times. Then, the solenoid is called an electromagnet. It is a temporary magnet.