Magnetism

## Analogy in Magnetism & Electrostatics

There is an analogy between permittivity and permeability of free space in electrostatics and magnetism .

 Revolving Electron

The ratio of magnetic dipole moment with angular momentum of revolving electron is called as gyromagnetic ratio .

Gyromagnetic Ratio = M/L = 8.8 x 1010 C / kg

magnetic moment = m = evr / 2

## Magnetization

The net magnetic dipole moment per unit volume is called as Magnetization (Mz) .

Mz = Mnet / Volume

Also , Mz = C x Bext / T

Consider a torroid with iron core on it . As current flows through the torroid it sets up a magnetic field around itself . This magnetic fild now magnetizes the iron core .

Magnetic Field due to torroid Bo = μonI

Magnetc Field due to iron core BM = μoMz

Now , B = B0 + Bm

.....B = μonI + μoMz

## Magnetic Intensity

Magnetic Intensity (H) is a quantity used in describing magnetic field strength of a field . It is defined as follows :

B0 = μoH= μonI ; i.e. H = nI

Now , B = μonI + μoMz

B = μoH + μoMz

B = μo(H + Mz)

## Susceptibility

The relation between magnetization(Mz) and intensity(H) is given as :

Mz = XH ; where X is the magnetic susceptibility of the substance (iron core in example)

Now , B = μo(H + Mz) = B = μo(H + XH)

B = μo(1+X)H

B = μoμrH

μ = μ0μr = μo(1+X)

## Domain Theory

The region in which all the magnetic moments are aligned in the same direction are known as domains .

## Diamagnetism

Silver , lead , silicon , nitrogen , sodium chloride , copper , antimony , gold , mercury , water , air , hydrogen , etc.

1) All the orbitals are completely filled .

2) The spins are paired and the magnetic field is repelled . The diamagnetic materials move away from stronger magnetic field to weaker magnetic field .

3) If a thin rod of diamagnetic substance is freely suspended in external magnetic field , it comes to rest with it's length perpendicular to the field .

4) In absence of external magnetic field , the net magnetic moment is zero .

## Paramagnetism

Aluminium , Manganese , Chromium , Platinum , oxygen , sodium , calcium , lithium , copper chloride , tungsten , niobium , etc.

1) Atom contains unpaired electron ; i.e. their orbitals are not completely filled .

2) The spins do not cancel each other and the magnetic field is attracted . The paramagnetic materials move towards the stronger magnetic field .

3) If a thin rod of paramagnetic substance is suspended freely in a magnetic field , it comes to rest , parallel to the magnetic field .

4) In absence of external magnetic field , the dipole moments of atoms are randomly oriented , and hence the net dipole moment is zero .

## Ferromagnetism

Iron , Nickel , Cobalt , gadolinium , dysprosium and their alloys , etc.

1) Atom contains a large number of unpaired electrons , which results in large magnetic moment .

2) The ferromagnetic materials move towards the stronger magnetic field .

3) When ferromagnetic substances are kept in magnetic field , they get strongly magnetized in the direction of magnetic field .

4) In the absence of external magnetic field , these materials have a resultant magnetic moment .

## Curie Temperature

The temperature at which the domain structure is destroyed and the ferromagnetic substance loses its magnetism , is called Curie Temperature .

 Iron
 Nickel
 Gadolinium
 Fe2O3 | 893

Above Curie temperature , ferromagnetic substances become paramagnetic .