- Angular motion of charge is responsible for magnetism when a magnetic material is put into a magnetic field then either it is attracted by the field or it is repelled away from the field that is material gets magnetized.
- All material show some magnetic effect but in many substances effect are so weak that the material are often considered to be non magnetic however a vacuum is the only true non-magnetic medium.
Magnetization.
- Total magnetic flux density in a magnetic material is due to
- Applied magnetic field
- Magnetism
- Angular momentum of charge is responsible for creation of a magnetic dipole. In general at the atomic level there are three factors contributing to the total permanent magnetic dipole moment of an atom.
- Electron orbital angular momentum
- Electron spin angular momentum
- Nuclear spin angular momentum
- Most of the properties of magnetic material are mainly affected by electron spin angular momentum.
- Elements having completely filled orbitals have zero resultant electron spin dipole moment.
- A group of elements having incompletely filled inner orbits pocess net electron spin dipole moment.
- If z is the number of unpaired electrons in inner three orbit of element then electron spin dipole moment is.
- Diamagnetic material
- Paramagnetic material
- Ferromagnetic material
- Antiferromagnetic material
- Ferrimagnetic material
- These materials do not contain permanent magnetic dipoles.
- Magnetic susceptibility of diamagnetic material is small, negative and independent of temperature.
- These materials are repelled away from applied magnetic field.
- In a diamagnetic material
- Example Bismuth, copper, gold, diamond ,water, etc.
- These materials contained permanent magnetic dipoles which are equal in magnitude but randomly oriented.
- Because of the random orientation of dipole net magnetization in the absence of field is zero.
- On the application of magnetic field we get a small value of magnetisation along the direction of applied magnetic field so these materials are having small and positive value of magnetic susceptibility.
- With increase in temperature randomness in dipole increases because of which net magnetisation along the field direction decrease so in permanent paramagnetic materials magnetic susceptibility decreases with increase in temperature.
- Example aluminium ,calcium ,lithium ,oxygen ,platinum ,etc.
- Some paramagnetic materials do not obey curie law instead they follow curie weiss law.
- Ferromagnetic materials are characterized by the presence of parallel alignment of dipole in a particular direction.
- These materials are strongly magnetized along the direction of applied magnetic field and remained magnetized even after the removal of field.
- This property of ferromagnetic material is known as spontaneous magnetization.
- These materials remain ferromagnetic upto curie temperature and above curie temperature they start behaving like paramagnetic material.
Note: with increase in temperature area of the hysteresis loop decreases and above curie temperature the slope merges into a straight line.
1.Eddy current loss
- Due to change in magnetic flux an EMF is developed across the core material which leads to flow of circulating current on the surface of core such circulating currents are known as eddy currents and they contribute a loss in the core material given as.
- To reduce eddy current losses laminated cores are used.
- Silicon steel is used which has higher resistivity than iron.
- Hysteresis loss is directly proportional to the area of the hysteresis loop .
- To reduce hysteresis loss soft magnetic materials are used.
Note: magnetostriction is responsible for humming noise in the transformer.
Villari affect
- It is converse of magnetostriction.
- Longitudinal deformation leads to change in magnetic permeability of the material along the direction of applied mechanical stress.
- This material contained antiparallel arrangement of dipoles which are equal in magnitude hence net magnetization in the absence of field is zero.
- These materials have small and positive value of magnetic susceptibility.
- These materials remain antiferromagnetic up to Neel temperature and above Neel temperature they start behaving like paramagnetic material.
- Example manganese oxide , Cao and ferrous oxide etc.
- This material contained anti parallel arrangement of dipoles but dipoles are not equal in magnitude because of which net magnetization in the absence of field is non-zero.
- These materials remain ferrimagnetic up to curie temperature and above curie temperature they start behaving like paramagnetic material.
- The advantage of ferrites is their highest resistivity then ferromagnetic material because of which ferrites are having lesser eddy current losses than ferromagnetic material and are used in designing of core of high frequency inductors and transformers.
- Inorganic material
- High resistivity.
- Low eddy current loss.
- High permeability.
- High dielectric constant.
- Low dielectric clauses.
1. Hard magnetic materials.
- These materials have wide hysteresis loop.
- These are difficult to demagnetize.
- These are permanent magnetic materials.
- Properties: high retentivity ,high coercivity ,high hysteresis, low permeability and susceptibility.
- Example Carbon steel, tungsten steel, alnico.
- This material have narrow hysteresis loop.
- These can be easily magnetized and demagnetized.
- These are used in high frequency applications.
- Property : low retentivity and coercivity ,lower hysteresis loss ,high permeability and susceptibility.
- Example iron silicon alloy ,iron nickel alloy, ferrites, etc.
Superconductivity
- State of material in which it has zero resistivity is known as superconductivity.
Meissner's effect
- The repulsion of magnetic flux lines from the interior of a piece of superconducting material as the material undergoes transition to the superconducting state is known as meissner's effect.
- In a superconductor
- Resistivity should be zero
- Perfect diamagnetism.
- Superconductors are the materials which exhibit zero resistivity and meissner's effect independently below a critical temperature.
- It is defined as minimum required magnetic field at a given temperature to destroy superconductivity.
- If a superconducting wire carries a current such that the magnetic field which it produces is equal to critical magnetic field than superconducting disappears this is known as silsbe rule.
- To destroy superconductivity we need not to apply external magnetic field even the internal develop field can also destroy it.
- According's to ampere's law
Type 1 super conductors:
- Ideal or soft superconductor
- Low transition temperature
- Low critical magnetic field
- Change of state is abrupt
- Exhibit complete meissner's effect and silsbe rule
- eg: mercury , zn , lead , pd , etc.
Type 2 super conductor:
- Non ideal or hard super conductor
- High transition temperature
- High critical magnetic field
- Change of state is gradual
- Exhibit incomplete meissner's effect and silsbe rule
- eg : Nb ti, Nb3Sn , etc
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