Magnetic Materials 2-1

7/31/2019 Magnetic Materials 2-1

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Magnetic Fields

The region where the magnetic forces act is

called the magnetic field


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The Earth itself has a hugemagnetic field - as if it had a

huge bar magnet embedded

at its centre. The Earths

magnetic field lines emerge

from near the geographical

north pole and re-enter it at

the south pole. The nature of

the field around the Earth

varies in both strength anddirection. The Earths

magnetic field is strongest at

the magnetic poles and

weakest near the Equator.


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Why does the Earth have a magnetic field?

The Earth has, at its centre, adense liquid core, of abouthalf the radius of the Earth,with a solid inner core. Thiscore is though to be mostly

made of molten iron andnickel perhaps mixed withsome lighter elements.Circulating ions of iron and

nickel in highly conductingliquid region of earths coremight be forming currentloops and producing earthsmagnetism.


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The Earths magnetic field


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Magnetic Elements

Magnetic Declination

Magnetic Inclination or Magnetic Dip


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Magnetic Declination

The small angle

between magnetic

axis andgeographic axis at

a place is defined

as the magneticdeclination.


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The angle whichthe direction of

total strength ofearths magneticfield makes with

a horizontal linein magneticmeridian.


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Atoms themselves have magnetic propertiesdue to the spin of the atoms electrons.

These areas of atoms are called domains

Groups of atoms join so that their

magnetic fields are all going in the samedirection


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The metals affected bymagnetism consist of tinyregions called 'Domains'which behave like tiny

magnets. Normally they arearranged in the magneticmaterial all pointing indifferent directions in acompletely random fashionand so their magnetic effectscancel each other out. If anobject is magnetized it isbecause the domains are allmade to point in the samedirection. This can be done bystroking the magneticmaterial with a magnet (ormagnets) as shown in thediagram. When aligned thedomains reinforce oneanother and create north and

south poles at either end.


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Diamagnetic Substances

Paramagnetic substances

Ferromagnetic substances


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Diamagnetic substances

The diamagnetic substances are those inwhich the individual atoms or ions do not

possess any net magnetic moment ontheir own.

When such substances are placed in an

external magnetizing field, they get feeblymagnetized in a direction opposite to amagnetizing field.


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Paramagnetic Substances

Paramagnetic substances are those inwhich each individual atom or molecule or

ion has a net non zero magnetic momentof its own.

When such substances are placed in an

external magnetic field, they get feeblymagnetized in the direction of themagnetizing field.


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Ferromagnetic materialsFerromagnetic

Domains inMaterials Ferromagnetic

material are thosein which eachindividual atom ormolecule has anon zero magnetic

moment


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Hysteresis Curve

The relationship between magnetic field

strength (H) and magnetic flux density (B) will

follow a curve up to a point where further

increases in magnetic field strength will result

in no further change in flux density. This

condition is called magnetic saturation till

point (a)


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the plotted relationship will follow adifferent curve back towards zero fieldstrength at which point it will be offsetfrom the original curve by an amountcalled the remanent flux densityor

Retentity as shown in graph at point (b) The 'thickness' of the middle, describes

the amount of hysteresis, related to the

coercivity of the material as from (c) to (f)


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Hysteresis curve of soft and steel

The retentivity of softiron > retentivity of steel

Soft iron is more strongly

magnetized than steel Coercivity of soft iron

Magnetic Materials 2-1

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