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Question 5: Differentiate between paramagnetic, diamagnetic and ferromagnetic materials with suitable examples.

Answer

Some definitions

  1. Permeability: Permeability μ is the ration of magnetic flux density in a body or medium to the external magnetic field inducing it.
    μ = B/H
    Its unit is henry/m.
    Permeability of free space μ0 is called magnetic constant.
  2. Relative permeability: Relative permeability, μr, is the ratio μ /μ0.
    • Now, if μr is less than unity, the material is diamagnetic.
    • If μr is greater than unity, the material is paramagnetic.
    • For ferromagnetic substances, the relative permeability is very high and varies with field strength.
  3. Magnetic susceptibility: Magnetic susceptibility is defined as μr – 1.

Paramagnetic materials:

In paramagnetic materials, the spin and orbital axes of electrons are so oriented that they support each other. Therefore, the atoms behave like tiny magnets.

Relative permeability of paramagnetic materials is slightly greater than 1. When such a substance is placed in a magnetic field, it is weakly magnetized in the direction of applied magnetic field.

Examples of paramagnetic substances are Aluminum and Manganese.

Diamagnetic materials

Diamagnetic materials have negative magnetic susceptibility, i-e, μr < 1. In diamagnetic materials, the orbital and spin axes of electrons are oriented in such a way that their magnetic fields cancel each other. Thus the atoms are magnetically neutral.

When a diamagnetic material is placed in a magnetic field, it is weakly magnetized in opposite direction to the applied magnetic field. Thus a strong magnet repels the diamagnetic material.

Examples of diamagnetic materials are lead, bismuth, Sodium Chloride.

Ferromagnetic materials

Solid substances having a large positive magnetic susceptibility are capable of being magnetized by weak magnetic fields are called ferromagnetic substances. Relative permeability of a ferromagnetic material is much greater than 1.

Ferromagnetic materials are able to retain a certain amount of magnetization when the magnetizing field is removed.

The characteristic features of ferromagnetism can be explained by the presence of domains. A domain is a region of crystalline matter whose volume may be between 10-12 and 10-8 m3. These domains contain atoms whose magnetic moments are aligned in the same direction. Domain is, thus, magnetically saturated and behaves like a magnet with its own magnetic axis and moment.

Now in an un-magnetized volume of a specimen, the domains are arranged in a random fashion with their magnetic axes pointing in all directions so that the specimen has no resultant magnetic moment. Under the influence of a weak magnetic field, the substance is strongly magnetized in the direction of the applied the applied magnetic field. Therefore, a ferromagnetic substance is attracted by a magnet.

(note: The ferromagnetic materials exhibit very strong magnetic effects and widely used in transformers and electric motors).

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