Introduction of Magnetic Properties of Materials

Introduction

A very large number of modern devices depend upon magnetic properties of materials for their working. For example, the speakers, electrical power generators, electrical machines, transformers, television, data storage devices like magnetic tapes and disks, magnetic compass etc.

MRI (Magnetic Resonance Imaging) scan is an important non-invasive diagnostic tool used in the medical field.

Understanding the origin of magnetism and the behaviour of magnetic materials will be helpful not only in the selection of suitable materials for a particular application but also in proper utilization of such devices.

Further, it is highly useful in designing new applications of these materials.

Magnetism in Materials

It arises from the magnetic moment or magnetic dipole of the magnetic materials. When an electron revolves around the positive nucleus, orbital magnetic moment arises. Similarly when the electron spins, spin magnetic moment arises.

Materials which can be magnetised by an external magnetic field are called magnetic materials.

The space around the magnet or the current carrying conductor where the magnetic effect is felt is called magnetic field.

Magnetic line of force is a continuous curve in a magnetic field as shown in fig. 2.22. The tangent at any point of this curve gives the direction of resultant intensity at that point.

All the molecules of a material contain electrons rotating around the nucleus. These orbits are equivalent to circulating currents. So they produce a magnetic motive force (m.m.f). m.m.f is a force which produces the magnetic effect.

In most of the molecules, each m.m.f due to an individual orbit is neutralised by an opposite one. But, in the magnetic materials like iron and steel, there are number of unneutralised orbits. Then, the resultant axis of m.m.f produces a magnetic dipole.

In unmagnetised specimens, the molecular m.m.f axes lie along continuous closed paths. Therefore, no external magnetic effect can be found.

In magnetic specimens, the magnetic dipoles will line up parallel with the exciting m.m.f.

When the exciting m.m.f. is removed, the magnetic dipoles may remain aligned in the direction of the external field. Thus, it produces permanent magnetism.

Basic Definitions

To understand the magnetic properties of materials in detail, we must study the basic terms and definitions involved in magnetism.

Magnetic Pole Strength

Magnetic pole strength of a pole is said to be unity it experiences a force of 1 Newton when placed at a distance of 1 meter from a similar one, in air (or vacuum).

Magnetic Dipole Moment (m)

Magnetic dipole moment m of a magnet is the product of magnetic pole strength and the distance between the two poles.

Magnetic flux (φ)

Total number of magnetic lines of force passing through a surface is known as magnetic flux.

It is represented by symbol (φ) and its unit is weber (Wb).

Magnetic Flux Density (or) Magnetic Induction (B)

Magnetic field is applied to the metals such as iron, steel, some alloys etc, they are magnetised to different degrees.

Magnetic flux density at any point in a magnetic field is defined as the magnetic flux (φ) passing normally through unit area of cross section (A) at that point.

It is denoted by the symbol 'B' and its unit is weber/metre2 (Wb/m²) m) or tesla (T).

Magnetic flux density is given by

B = φ / A weber/metre² or tesla (T).

It is also called magnetic induction.

Intensity of Magnetisation (I)

The term magnetisation means the process of converting a non magnetic material into a magnetic material.

When an external magnetic field is applied to the metals such as iron, steel, some alloys etc, they are magnetised to different degrees.

The intensity of magnetisation (I) is the measure of magnetisation of magnetised specimen.

It is defined as the magnetic moment per unit volume of the material.

where

M → Magnetic moment of the substance

V → Volume of the specimen

Magnetic Field Intensity (or) Strength (H)

Magnetic field intensity at any point in a magnetic field is the force experienced by a unit north pole placed at that point.

It is denoted by 'H' and its unit is newton per weber (N/Wb) or ampere turns per metre (A/ m).

Magnetic Permeability (μ)

Magnetic permeability of a substance measures the degree to which the magnetic field can penetrate through the substance.

It is found that magnetic flux density (B) is directly proportional to the magnetic field strength (H)

B α H

Β = μ Η

where μ is a constant of proportionality.

It is known as permeability or absolute permeability of the medium

μ = B/H

Thus, the permeability of a substance is the ratio of magnetic flux density (B) inside the substance to magnetic field intensity (H).

Absolute permeability of a medium or a material is also defined as the product of permeability of a free space (μ₀) and the relative permeability of the medium (μ_r)

i.e., μ = μ₀ × μ_r

Unit of permeability is henry / m (or) H/m

Relative Permeability

Relative permeability of a medium is defined as the ratio between absolute permeability of a medium (μ) to permeability of a free space (μ₀)

μ_r = μ / μ₀

Thus, the relative permeability is purely a number and it has no unit. For air and non - magnetic material, its value is '1'.

Magnetic Susceptibility (χ)

Magnetic susceptibility (χ) of a specimen is a measure of how easily a specimen can be magnetised in a magnetic field.

It is defined as the intensity of magnetisation produced in the substance per unit magnetic field strength (H).

χ = I / H

It is a dimensionless quantity because both I and H have same units.

Magnetic induction in a given magnetic material for the applied field strength 'H' is given by

But, we know that

From the eqns (1) and (2), we have