Magnetic Properties

MAGNETIC PROPERTIES

All the materials we use are not magnetic materials. Further, all the magnetic materials will not behave identically. So, in order to differentiate one magnetic material from another, we need to know some basic parameters. They are:

(a) Magnetising field

The magnetic field which is used to magnetize a sample or specimen is called the magnetising field. Magnetising field is a vector quantity and it denoted by  and its unit is A m^-1.

(b) Magnetic permeability

The magnetic permeability can be defined as the measure of ability of the material to allow the passage of magnetic field lines through it or measure of the capacity of the substance to take magnetisation or the degree of penetration of magnetic field through the substance.

In free space, the permeability (or absolute permeability) is denoted by µ₀ and for any medium it is denoted by µ.The relative permeability µ_r is defined as the ratio between absolute permeability of the medium to the permeability of free space.

Relative permeability is a dimensionless number and has no units. For free space (air or vacuum), the relative permeability is unity i.e., µ_r = 1. In isotropic medium, µ is a scalar but for non-isotropic medium, µ is a tensor.

(c) Intensity of magnetisation

Any bulk material (any object of finite size) contains a large number of atoms. Each atom consists of electrons which undergo orbital motion. Due to orbital motion, electron has magnetic moment which is a vector quantity. In general, these magnetic moments orient randomly, therefore, the net magnetic moment is zero per unit volume of the material.

When such a material is kept in an external magnetic field, atomic dipoles are created and hence, it will try to align partially or fully along the direction of external field. The net magnetic moment per unit volume of the material is known as intensity of magnetisation or magnetisation vector or magnetisation. It is a vector quantity. Mathematically,

The SI unit of intensity of magnetisation is ampere metre^-1. For a bar magnet of pole strength q_m, length 2l and area of cross-section A, the magnetic moment of the bar magnet is  and volume of the bar magnet is  The intensity of magnetisation for a bar magnet is

This means, for a bar magnet the intensity of magnetisation can be defined as the pole strength per unit area (face area).

(d) Magnetic induction or total magnetic field

When a substance like soft iron bar is placed in an uniform magnetising field , it becomes a magnet, which means that the substance gets magnetised. The magnetic induction (total magnetic field) inside the specimen 0 is equal to the sum of the magnetic field  produced in vacuum due to the magnetising field and the magnetic field _m due to the induced magnetisation of the substance.

e)Magnetic susceptibility

When a substance is kept in a magnetising field , magnetic susceptibility gives information about how a material respond to the external (applied) magnetic field. In other words, the magnetic susceptibility measures, how easily and how strongly a material can be magnetised. It is defined as the ratio of the intensity of magnetisation ( ) induced in the material due to the magnetising field ( )

It is a dimensionless quantity. For an isotropic medium, susceptibility is a scalar but for non-isotropic medium, susceptibility is a tensor. Magnetic susceptibility for some of the isotropic substances is given in Table 3.1.

EXAMPLE 3.10

Compute the intensity of magnetisation  of the bar magnet whose mass, magnetic  moment and density are 200 g, 2 A m² and 8 g cm^-3, respectively.

Solution

Density of the magnet is

EXAMPLE 3.11

EXAMPLE 3.12

Two materials X and Y are magnetised, whose intensity of magnetisation are 500 A m^-1 and 2000 A m^-1, respectively. If the magnetising field is 1000 A m^-1, then which one among these materials can be easily magnetized?.

Solution

The susceptibility of material X is

The susceptibility of material Y is

Since, susceptibility of material Y is greater than that of material X, material Y can be easily magnetized than X.