Some Important Definitions :
Basically magnetic substances possess three important properties
Attractive or Repulsive property :
Magnetic substance have attractive and repulsive forces like poles are repelled and unlike poles are attracted.
Directive property :
When a magnet is suspended it comes to rest in north and south direction of the earth.
Inductive property :
A magnet can induce magnetism into another substances by inducing magnetic flux into it.
Attractive or Repulsive property :
Magnetic substance have attractive and repulsive forces like poles are repelled and unlike poles are attracted.
Directive property :
When a magnet is suspended it comes to rest in north and south direction of the earth.
Inductive property :
A magnet can induce magnetism into another substances by inducing magnetic flux into it.
Some basic definitions of magnetism :
Magnetic dipole : A system in which two equal and opposite poles are separated by a distance 2l is called "magnetic dipole".
Magnetic moment (M) : It is the product of pole strength (m) and the distance between the two poles (2l).
When a conductor is carrying current or motion of an electron around the nucleus develops magnetic moment
Magnetic flux ( ∅ ) : The number of magnetic lines of force is called Magnetic flux.
Units : Weber
Magnetic field induction or Magnetic flux density : It is defined as the force experienced by unit north pole when it is placed at a point in the magnetic field. or
Units : Weber
Magnetic field induction or Magnetic flux density : It is defined as the force experienced by unit north pole when it is placed at a point in the magnetic field. or
It is defined as the magnetic flux ( number of lines of forces ) per unit area.
Magnetic field intensity (H) : It is defined as 1/μ times of force acting on unit north pole at a point in the magnetic field.
Magnetic permeability : It is represented as conductivity for magnetic flux. If a material have high permeability is a good magnetic material. or
It is defined as the ratio of flux density to the magnetic field intensity.
It is defined as the ratio of flux density to the magnetic field intensity.
Relative permeability : It is the ratio of permeability of medium to the permeability of free space.
Intensity of magnetization (I) : It is the ratio of magnetic moment per unit volume.
units : A/m
units : A/m
susceptibility : It is the ratio of Intensity of magnetization to Magnetic field intensity.
Magnetization or Magnetic Polarizability : The process of converting a non magnetic material into magnetic material is called Magnetization.
Bohr magneton or Origin of magnetic moment
The magnetic moment contributed by the electron is known as Bohr magneton.
An electron of charge e is moving in an orbit of radius r around the nucleus with velocity v, the current developed
where T is time period
therefore the time period changes to
and hence the current developed changes to
and hence the magnetic moment changes to
by multiplying and dividing with m we get
and hence the magnetic moment is changed to
Classification of Magnetic materials :
Diamagnetic materials :
- Diamagnetic materials possess no permanent magnetic moment and it is repelled by the magnet.
- With the application of magnetic field all the magnetic dipoles allign in the direction opposite to the field.
- susceptibility is negative and it is independent of temperature.
- When a diamagnetic material is placed in a non-uniform magnetic field, it moves from stronger field to weaker field.
- Relative permeability is less than 1.
- When a diamagnetic liquid is placed in a U-shaped tube, liquid gets depressed when it is subjected to field.f
- Magnetic lines of forces do not pass through this specimen.
- Examples are Bismuth, Gold, Silver, water etc.
- These materials possess permanent magnetic moment and feebly attracted by the magnet.
- When paramagnetic substance is placed in uniform magnetic filed it comes to rest in the direction parallel to the field.
- When it is placed in non-uniform magnetic field, it moves from weaker field to stronger field.
- Relative permeability is just greater than 1.
- susceptibility is small and negative.
- When temperature increases paramagnetic nature decreases.
where T is absolute temperature and c is curie constant and this law is called curie-wiss law.
- When paramagnetic liquid is taken in u-shaped tube, liquid is raised between two poles.
- Examples are Alluminium, Platinum, Manganese etc.
- All the spins are in one direction in the presence of electric field in ferromagnetic substances.
- These materials are strongly attracted by magnet.
- In the absence of field magnetic dipoles are almost alinged due to spontaneous magnetization.
- When it is subjected to weak magnetic field the substance is strongly magnetized.
- Relative permeability is strictly greater than 1.
- susceptibility is positive and high.
- When it is placed in non-uniform magnetic field, it moves from weaker to stronger part of the field.
- When it is suspended in uniform magnetic field, it comes to rest in the direction of the field.
- Example : Cobalt, Nickle, Gadalonium etc.
- The temperature at which ferromagnetic substance changes to paramagnetic substance is called " curie temperature ".
- In anti ferromagnetic materials dipoles are anti parallel to each other with same magnitude such that its magnetic moment is zero.
- It's susceptibility is positive and very small.
- The temperature at which anti ferromagnetic material converts to paramagnetic material is known as Neel temperature.
- Examples for anti ferromagnetic materials are NiO, CoO, MnO etc.
- These ferrimagnetic materials are those in which equal number of opposite spins which have different magnitudes, such that it possess some net magnetism.
- susceptibility is positive and very large.
Hysteresis loop
- This loop shows the relation between the Magnetic induction (B) and Magnetic field (H) and it is often referred as B-H loop.
- A ferromagnetic substance when subjected to magnetic field the magnetic flux increases slowly by following the path OA. At A almost all magnetic domains are aligned in one direction and it is called " Saturation Point ".
- When H is reduced to zero , the curve will move from A to B. At this point it can be seen that some magnetic flux remains in the material even when the applied field is zero. This point is referred as " Retentivity " and that magnetization is called "Residual Magnetization ".
- As the magnetic field is reversed the curve moves to point C, where the flux has been reduced to zero. This is called " Coercivity "
- As the magnetic field is in the negative direction the material will again become magnetically saturated by following the path C to D.
- When magnetic field is reduced to zero, curve follows the path D to E. It will have a level of residual magnetism as before.
- Increasing H back in positive direction, curve follows the path E to F.
- If we further increase the field, curve will take the path F to A forming close loop.
Retentivity : It is defined as the magnetic flux density remained in the substance even when the magnetic field is zero.
Domain theory of ferromagnetism :
- If ferromagnetic substances which are not magnetized earlier is placed under the action of applied field. There may be two processes of magnetization.
- The growth in size of domains which are in the direction of field.
- Rotation of domains towards the field.
- When a weak electric field is applied, the domains which are in the field direction will grow in size and expand into other domains which are not in the field direction that is there is a movement of domain wall which is reversible indicated by the path OA in the figure.
- When the field becomes stronger, wall movement is sharp and it is irreversible, which is indicated by the path AB.
- With the application of higher fields the domains rotate in the direction of field. At C all the domains are in the direction of field and the specimen is said to be in Magnetic saturation.
- To get back the magnetization to zero, the specimen should tend to attain original configuration. Curve continuous from C to D to form a loop.
Relation between B, H and I
- When the specimen is placed in the presence of magnetic field, induction is developed in that will of two ways.
- One is due to the magnetizing field and
- The other is due to the material itself being magnetized by induction.
Relation between relative permeability and susceptibility
we also know the value of B from the relation between B, H, I. By equation both the equations we get
Types of magnetic materials
soft magnetic materials :
properties :
- They are easily magnetized and demagnetized.
- They have small hysteresis loss due to small loop area.
- coercivity and retentivity are small.
- These materials have large values of permeability and susceptibility.
- In the soft magnetic materials the domain walls must be able to move easily and should be reversible, so that the magnetization changes by large amounts.
Hard magnetic substances :
properties :
- They cant be easily magnetized or demagnetized.
- These materials have large hysteresis loss due to large loop area.
- These materials have large coercivity and retentivity.
- They have small values of permeability and susceptibility.
- In these materials, the domain wall movement is difficult.