Magnetic Permeability and Susceptibility

MAGNETIC PERMEABILITY AND SUSCEPTIBILITY

1. Magnetic Susceptibility (X_m)

It is the measure of the ease with which the specimen can be magnetised by the magnetising force.

It is defined as the ratio between intensity of magnetisation (I) and the magnetic field intensity (H)

(i.e.,) 

2. Magnetic Permeability (µ)

It is defined as the ratio between the magnetic flux density (B) and the magnetic field intensity (H)

It is the measure of degree at which the lines of force can penetrate through the material.

3. Relative Permeability (µ_r)

It is the ratio between the permeability of the medium to the permeability of free space.

4. Relation between μ_r and X_m

When a magnetic material is placed in a magnetic field (H), then two types of lines of induction passes through the material. viz.,

(i) Due to magnetising field (H)

(ii) Due to material itself being magnetised by induction (1)

Total flux density B = µ_o (H+I) …….(1)

We know, μ = B/H ⇒ B = μ H …….(2)

Equating equation (1) and (2) we get

μH = μ_o (H+I)

Since μ = μ_o μ_r we have

Here I/H = X_m

5. Retentivity (or) Remanence (M_r)

When the external magnetic field applied to a magnetic material is removed, the magnetic material will not loss its magnetic property immediately. There exits some residual intensity of magnetisation in the specimen even when the magnetic field is cut off. This is called residual magnetism or retentivity.

6. Coercivity (-H_C)

The residual magnetism can be completely removed from the material by applying a reverse magnetic field. Hence Coercivity of the magnetic material is the strength of reverse magnetic field (-H_c) which is used to completely demagnetise the material.