Electromagnetism- Substances Magnetic Properties

Substances Magnetic Properties

或电子的运动bit 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^-24J/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^-1or B = μ₀(H + I)

B = μ₀H (1 + I/H) = μ₀H (1 + y) = μ u₀H = μmH

Susceptibilityy = 1/H

Whereμr = I + yis called relative permeability

μr = μm/μ₀whereμmis permeability of the medium.

ThusB/μm =H

In vacuum or free spaceB₀= μ₀n _i

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/Twhere C is called the curie's constant paramagnetic beyond curie temperature. Thus for ferromagnetic substancesy = C / T - T.

T is Curie temperature.

For iron It is10⁴³k or 7700c.

For cobalt1394 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^-4Am^-1/4π x 10^-7= 80 Am ^-1

Neel's temperature (TN) is that temperature at which an ant ferromagnetic substance becomes paramagnetic.

Hysteresis

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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