Magnetic properties of materials
The study of magnetic properties of materials assumes significance since these properties decide whether the material is suitable for permanent magnets or electromagnets or cores of transformers etc. Before classifying the materials depending on their magnetic behaviour, the following important terms are defined.
(i) Magnetising field or magnetic intensity
The magnetic field used to magnetise a material is called the magnetising field. It is denoted by H and its unit is A m?1.
(Note : Since the origin of magnetism is linked to the current, the magnetising field is usually defined in terms of ampere turn which is out of our purview here.)
(ii) Magnetic permeability
Magnetic permeability is the ability of the material to allow the passage of magnetic lines of force through it.
Relative permeability ?r of a material is defined as the ratio of number of magnetic lines of force per unit area B inside the material to the number of lines of force per unit area in vacuum Bo produced by the same magnetising field.
Relative permeability ?r = B/B0
?r = ?H / ?0 ? = ?/ ?0
The magnetic permeability of the medium ? = ?o?r where ?o is the permeability of free space.
Magnetic permeability ? of a medium is also defined as the ratio of magnetic induction B inside the medium to the magnetising field H inside the same medium.
(iii) Intensity of magnetisation
Intensity of magnetisation represents the extent to which a material has been magnetised under the influence of magnetising field H.
Intensity of magnetisation of a magnetic material is defined as the magnetic moment per unit volume of the material.
I = M/V
Its unit is A m-1.
For a specimen of length 2l, area A and pole strength m,
I = 2lm/2lA
Hence, intensity of magnetisation is also defined as the pole strength per unit area of the cross section of the material.
(iv) Magnetic induction
When a soft iron bar is placed in a uniform magnetising field H, the magnetic induction inside the specimen B is equal to the sum of the magnetic induction Bo produced in vacuum due to the magnetising field and the magnetic induction Bm due to the induced magnetisation of the specimen.
B = Bo + Bm
But Bo= ?oH and Bm = ?oI
B = ?oH + ?oI
B = ?o (H + I)
(v) Magnetic susceptibility
Magnetic susceptibility χm is a property which determines how easily and how strongly a specimen can be magnetised.
Susceptibility of a magnetic material is defined as the ratio of intensity of magnetisation I induced in the material to the magnetising field H in which the material is placed.
Thus χm = 1/H
Since I and H are of the same dimensions, χm has no unit and is dimensionless.
Relation between χm and ?r
χm = 1/H
We know B = ?o (H + I)
B = ?o (H + χmH)
B = ?oH (1 + χm)
If ? is the permeability, we know that B = ?H.
∴ ?r = 1 + χm
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