Substances Magnetic Properties
The motion of electron in its orbit acts like a current loop this gives it a magnetic dipole moment. Besides this, electron has a spin afloat momentum that contributes to dipole moment = 9.285 x 10-24 J/T. the nucleus may also have a magnetic moment but it is several thousand times smaller than the magnetic moment of an electron. The resultant magnetic moment of an atom is vector sum of all such magnetic moments.
Magnetization vector (1) = magnetic moment/volume = M/V
= 2ml/2/A = m/A
= pole strength / area unit Am-1
Magnetizing field intensity (H)
H = B/μ0 - I unit Am -1 or B = μ0 (H + I)
B = μ0H (1 + I/H) = μ0H (1 + y) = μ u0H = μmH
Susceptibility y = 1/H
Where μr = I + y is called relative permeability
μr = μm/μ0 where μm is permeability of the medium.
Thus B/μm =H
In vacuum or free space B0 = μ0ni
H = B0 / μ0 = nI
I = B / μ0 - H = B / μ0- nI.
Curler's law as the temperature increases randomization of individual atomic magnetic moments increases The proportional to the absolute temperature.
Y ∝ I / R or y = C/T where C is called the curie's constant paramagnetic beyond curie temperature. Thus for ferromagnetic substances y = C / T - T.
T is Curie temperature.
For iron It is 1043 k or 7700c.
For cobalt 1394 k or 11210C
For nickel 631 k or 3580C
For gaddinium 317 k or 440C.
CGS unit of B is gauss (G) IG = 10-4T.
CGS unit of H is oersted.
I oersted = IG/μ0 = 10-4 Am-1/4π x 10-7 = 80 Am-1
Neel's temperature (TN) is that temperature at which an ant ferromagnetic substance becomes paramagnetic.
Magnetic saturation:- the state of magnetic material in which value of I ir B becomes maximum.
Retentively when applied magnetizing field is removed the magnetism B or I that remains in the material is called retentively or remenant magnetisam.
Coercive force or coercively:- the magnetizing force or H applied in negative direction to make retentively zero is called coercively. In figure OY = OV coercively.
Alnico or carbons steel are used to make permanent magnets. Alnico is an alloy of Fe Ai, Ni and CO their coercively is high and retentively is low
Electromagnets are made from materials whose retentively is high and coercively is low. For example soft iron. Such materials are also used to make core of transformer, motors, dynamo and so on four per cent (Si) is added in soft iron core to further reduce the coercively. For transformer core perm alloy is preferred. For high frequency transformers μ-metal or radiometers are used to make core.
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