Magnetism
Print this page
Email this Page
| Discuss Magnetism MAGNETIC PROPERTIES OF MATTER
COULOMB’S LAW
The force of attraction or repulsion two magnetic poles is proportional to the product of the pole strengths of the two poles and inversely proportional to the square of the distance between them. Thus,
|
F = m0qm1qm2/4pr2 |
The magnetic scalar potential is defined as the work done in taking a unit north pole from infinity to a point in the magnetic field. For a point pole it is given as
|
V = m0qm/4pr |
The magnetic potential energy is defined as the work done in bringing two poles from infinity to a distance r. Thus
|
U = m0qm1qm2/4pr |
It is defined as the magnetic moment per unit volume of a magnet. It is a measure of the amount of magnetisation in a magnet.
|
I = M/V |
Where I is the intensity of magnetisation; M, the magnetic moment of the magnet and V its volume.
In a bar magnet the electronic current may be assumed to be flowing in surface loops. So if the total current flowing in surface loops is ie, and the cross sectional area of the magnet is A, then,
M = ieA
I = ieA/Al
|
I = ie/l |
So according to the above result, the intensity of magnetisation can also be described as the total electronic current flowing in surface loops per unit length.
MAGNETIC FIELD INTENSITY
The magnetic field intensity or magnetic intensity is defined as the magnetic effect of current per unit length of the magnetic line.
It is mathematically given by
|
H = (B/m0 ) - I |
The unit of H is Ampere/metre, and is dimensionally equal to that of the intensity of magnetisation.
MAGNETIC SUSCEPTIBILITY
The magnetic susceptibility of a magnet describes the contribution made by a substance when subjected to a magnetic field to the total flux density present. It is a dimensionless quantity, and is equal to mr - 1 where is mr is the relative permeability of the substance. It is denoted by c.
It is also given by
|
c= I/H |
DIAMAGNETIC SUBSTACES
Those substances in which the magnetisation occurs in the opposite direction to that of the applied field are known as diamagnetic substances. The susceptibility of diamagnetic substances is thus negative. All substances in general are diamagnetic, although the amount of diamagnetism may be less than the amount of other types of magnetism and thus it is masked by some other, stronger type of magnetism. Diamagnetism results from the changes induced in the orbits of electrons in the atoms of a substance by the applied field. The direction of the change opposes the flux, according to Lenz’s law. The susceptibility of these types of substances is of the order of -10-8m3mol-1. The relative permeability is slightly less than one.
PARAMAGNETIC SUBSTANCES
In the case of Paramagnetism the atoms or molecules of the substances have net orbital or spin magnetic moments that are capable of being aligned in the direction of the applied field. These substances therefore, have a small but positive susceptibility. The relative permeability is slightly more than one. All molecules and atoms that have unpaired electrons exhibit paramagnetism.
FERROMAGNETIC SUBSTANCES
In ferromagnetic substances there are net atomic magnetic moments, which line up in such a way that magnetisation, persists even after the removal of the applied field. This property is a function of temperature. Below a certain temperature, called the Curie temperature, an increasing magnetic field applied to a ferromagnetic substance will cause an increase in the magnetisation to a high value. This value is called the saturation magnetisation. These substances are used in the manufacture of permanent magnets.
