MAGNETIC MATERIALS

Magnetic FieldsMagnetic Fields

The region where the magnetic forces act is The region where the magnetic forces act is called the “magnetic field”called the “magnetic field”

EARTH’S MAGNETISMThe Earth itself has a huge magnetic field - as if it had a huge bar magnet embedded at its centre. The Earth’s 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 and direction. The Earth’s magnetic field is strongest at the magnetic poles and weakest near the Equator.

Why does the Earth have a magnetic field?

The Earth has, at its centre, a dense liquid core, of about half the radius of the Earth, with a solid inner core. This core is though to be mostly made of molten iron and nickel perhaps mixed with some lighter elements. Circulating ions of iron and nickel in highly conducting liquid region of earth’s core might be forming current loops and producing earth’s magnetism.

The Earth’s magnetic field

Magnetic ElementsMagnetic Elements

Magnetic DeclinationMagnetic Declination Magnetic Inclination or MagnetiMagnetic Inclination or Magneti

c Dipc Dip

Magnetic DeclinationMagnetic Declination

The small angle The small angle between magnetic between magnetic axis and axis and geographic axis at geographic axis at a place is defined a place is defined as the magnetic as the magnetic declinationdeclination. .

Magnetic Inclination or Magnetic Inclination or Magnetic DipMagnetic Dip

The angle which The angle which the direction of the direction of total strength of total strength of earth’s earth’s magnetic field magnetic field makes with a makes with a horizontal line in horizontal line in magnetic magnetic meridian.meridian.

Atoms themselves have magnetic properties due to the spin of the atom’s electrons.

These areas of atoms are called “domains”

Groups of atoms join so that their magnetic fields are all going in the same direction

When an unmagnetized substance is placed in a magneticWhen an unmagnetized substance is placed in a magneticfield, the substance can become magnetized.field, the substance can become magnetized.

This happens when the spinning electrons line up in theThis happens when the spinning electrons line up in thesame direction.same direction.

The metals affected by The metals affected by magnetism consist of tiny magnetism consist of tiny regions called 'Domains' regions called 'Domains' which behave like tiny which behave like tiny magnets. Normally they are magnets. Normally they are arranged in the magnetic arranged in the magnetic material all pointing in material all pointing in different directions in a different directions in a completely random fashion completely random fashion and so their magnetic and so their magnetic effects cancel each other effects cancel each other out. If an object is out. If an object is magnetized it is because magnetized it is because the domains are all made to the domains are all made to point in the same direction. point in the same direction. This can be done by This can be done by stroking the magnetic stroking the magnetic material with a magnet (or material with a magnet (or magnets) as shown in the magnets) as shown in the diagram. When aligned the diagram. When aligned the domains reinforce one domains reinforce one another and create north another and create north and south poles at either and south poles at either end.end.

Classification of Classification of magnetic materialsmagnetic materials

Diamagnetic SubstancesDiamagnetic Substances Paramagnetic substancesParamagnetic substances Ferromagnetic substancesFerromagnetic substances

Diamagnetic substancesDiamagnetic substances

• The diamagnetic substances are The diamagnetic substances are those in which the individual atoms those in which the individual atoms or ions do not possess any net or ions do not possess any net magnetic moment on their own.magnetic moment on their own.

• When such substances are placed in When such substances are placed in an external magnetizing field, they an external magnetizing field, they get feebly magnetized in a direction get feebly magnetized in a direction opposite to a magnetizing field. opposite to a magnetizing field.

Paramagnetic SubstancesParamagnetic Substances

Paramagnetic substances are those Paramagnetic substances are those in which each individual atom or in which each individual atom or molecule or ion has a net non zero molecule or ion has a net non zero magnetic moment of its own.magnetic moment of its own.

When such substances are placed in When such substances are placed in an external magnetic field, they get an external magnetic field, they get feebly magnetized in the direction of feebly magnetized in the direction of the magnetizing field. the magnetizing field.

Ferromagnetic materialsFerromagnetic materials Ferromagnetic Ferromagnetic

Domains in MaterialsDomains in Materials

Ferromagnetic Ferromagnetic material are material are those in which those in which each individual each individual atom or atom or molecule has a molecule has a non zero non zero magnetic magnetic momentmoment

Hysteresis CurveHysteresis Curve

The relationship between magnetic field The relationship between magnetic field strength (H) and magnetic flux density (B) will strength (H) and magnetic flux density (B) will follow a curve up to a point where further follow a curve up to a point where further increases in magnetic field strength will result increases in magnetic field strength will result in no further change in flux density. This in no further change in flux density. This condition is called condition is called magnetic saturation magnetic saturation till till point (a)point (a)

• the plotted relationship will follow a the plotted relationship will follow a different curve back towards zero different curve back towards zero field strength at which point it will be field strength at which point it will be offset from the original curve by an offset from the original curve by an amount called the amount called the remanent flux remanent flux densitydensity or or RetentityRetentity as shown in as shown in graph at point (b) graph at point (b)

• The 'thickness' of the middle, The 'thickness' of the middle, describes the amount of hysteresis, describes the amount of hysteresis, related to the related to the coercivitycoercivity of the of the material as from (c) to (f)material as from (c) to (f)

Hysteresis curve of soft Hysteresis curve of soft and steeland steel

The retentivity of soft The retentivity of soft iron > retentivity of iron > retentivity of steelsteel

Soft iron is more Soft iron is more strongly magnetized strongly magnetized than steelthan steel

Coercivity of soft iron Coercivity of soft iron < Coercivity of steel< Coercivity of steel

Hence, soft iron loses Hence, soft iron loses its magnetism more its magnetism more rapidly than steel does.rapidly than steel does.

An An electromaelectroma

gnetgnet A soft iron rod has no A soft iron rod has no

magnetic fieldmagnetic field When current flows in When current flows in

the wire the soft iron the wire the soft iron becomes magnetized so becomes magnetized so a magnetic field is a magnetic field is detected by the plotting detected by the plotting compasses.compasses.

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