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Magnetism

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Magnetism

It is a property of materials that respond to an applied magnetic field. Permanentmagnets have persistent magnetic fields caused by ferromagnetism.

Magnetism can be made by things we call magnets, or can also be madeby electricity in a wire called an electromagnet.

Magnetic attraction

It is when magnets are put near to magnetic objects, the magnet will attract the

magnetic object and pull it towards the magnet until it is as near as it can get or touching

it. Magnets can also repel other magnets. Most objects that are attracted to magnets

have iron in them. Most other metals, such as aluminum, are not attracted to magnet.

HISTORY OF MAGNETISM

The term comes from the ancient Greek city of Magnesia, at which many naturalmagnets were found. We now refer to these natural magnets aslodestones, lodemeans to lead or to attract) which contain magnetite, a natural magnetic material

Fe3O4.

Pliny the Elder (23-79 AD Roman) wrote in a hill near the river Indus that wasmade entirely of a stone that attracted iron.

Chinese as early as 121 AD knew that an iron rod which had been brought nearone of these natural magnets would acquire and retain the magnetic property andthat such a rod when suspended from a string would align itself in a north-south

direction.

Use of magnets to aid in navigation can be traced back to at least the eleventhcentury.

William Gilbert (1540-1603)an Englishman who first investigate thephenomenon of magnetism systematically using scientific methods. He also

discovered that Earth is itself a weak magnet.

Carl Friedrich Gauss (1777-1855)a German who carried out early theoreticalinvestigations into the nature of Earth's magnetism.

Charles Coulomb (1736-1806)a Frenchman who initiated quantitative studies ofmagnetic phenomena in the eighteenth century. He also established the inverse

square law of force, which states that the attractive force between two magnetized

objects is directly proportional to the product of their individual fields and

inversely proportional to the square of the distance between them.

Hans Christian Oersted (1777-1851) - a Danish physicist who observed that acompass needle in the vicinity of a wire carrying electrical current was deflected!

Michael Faraday (1831)an Englishman who discovered that a momentarycurrent existed in a circuit when the current in a nearby circuit was started orstopped. He discovered that motion of a magnet toward or away from a circuit

could produce the same effect.


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SUMMARY: Oersted showed that magnetic effects could be produced by moving

electrical charges; Faraday and Henry showed that electric currents could be produced by

moving magnets

Andre Marie Amperea Frenchman who first suggested in 1820 that magneticproperties of matter were due to tiny atomic currents.

Origin of magnetism

Magnetism arises from two types of motions of electrons in atoms one is the

motion of the electrons in an orbit around the nucleus, similar to the motion of the planets

in our solar system around the sun, and the other is the spin of the electrons around its

axis, analogous to the rotation of Earth about its own axis.

Sources of magnetism

Electric currents or more generally, moving electric charges create magnetic fields. Many particles have nonzero "intrinsic" magnetic moments. Just as each particle,

by its nature, has a certain mass and charge, each has a certain magnetic moment,

possibly zero.

MAGNETS

It is a material or object that produces a magnetic field.

Permanent magnets - are objects that produce their own persistent magnetic fields. They

are made of ferromagnetic materials, such as iron and nickel, that have been magnetized,

and they have both a north and a south pole.

MAGNETIC POLES

There are North Poles and South Poles.

Every magnet has at least one North Pole and one South Pole. The magnetic fieldlines leave the North end of a magnet and enter the South end of a magnet.

Like poles repel, unlike poles attract.

The poles of two magnets will repel or attract each other. Different poles attracteach other.

MAGNETIC FORCE

Magnetic forces act at a distance.

Although two magnets may not be touching, they still interact through theirmagnetic fields. This explains the action at a distance, say of a compass.

MAGNETIZATION


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While magnetized, temporary magnets act like permanent magnets.

Methods of Magnetization:

The bar is heated while it is oriented along the earths magnetic field. The bar is hammered while it is aligned along the earths field. The bar is stroked in the same direction with the same end of another magnet. The bar is magnetized by winding a conductor around it and passing a current

through conductor.

MAGNETIC FIELD

The region around a permanent magnet is a magnetic field of force or a magneticfield because a force acts on any magnetic pole placed anywhere in that region.

The magnetic field at any given point is specified by both a directionanda magnitude as such it is a vector field.

Michael Faraday realized that a magnet has a magnetic field distributedthroughout the surrounding space.

MAGNETIC FIELD DIRECTION

Direction of magnetic field at any point is defined as the direction of motion of acharged particle on which the magnetic field would not exert a force.

Magnitude of the B-vector is proportional to the force acting on the movingcharge, magnitude of the moving charge, the magnitude of its velocity, and the

angle between v and the B-field. Unit is the Tesla or the Gauss (1 T = 10,000 G).

MAGNETIC FIELD LINES

Magnetic field lines - describe the structure of magnetic fields in three dimensions.

Field lines converge where the magnetic force is strong, and spread out where it is weak.

COULOMBS LAW OF MAGNETISM

The force of attraction between two point magnetic poles is directly proportionalto the product of their pole strengths and inversely proportional to the square of

their distance.

= k122

Where

Fis the force

m1 and m2 - are the pole strengths

ddistance between them


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kproportionality constant ( H/m)

Example:

1. The distance between a north pole of strength Am and a south pole of strength

Am is 10 cm. The poles are separated in air. Find the force between them.

= k122

=

=

2. Two magnetic poles one of which is three times stronger than the other exert on each

other a force equal to 3x10-3 N when separated by a distance of 10 cm. Find the strength

of each pole.

= k122

If m1 = m then m2 = 3m and d = 10 cm and F = 3x10 -3

=

m = 10 Am

3m = 3(10Am) = 30 Am


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TYPES OF MAGNETISM

DIAMAGNETISM

It is a very weak form of magnetism that is only exhibited in the presence of anexternal magnetic field. It is the result of changes in the orbital motion of electronsdue to the external magnetic field.

When placed between the poles of a strong electromagnet, diamagnetic materialsare attracted towards regions where the magnetic field is weak.

It occurs in nonmagnetic substances like graphite, copper, silver, goldBismuthis the substance that displays the strongest diamagnetism, which is used in

guns. Melting down bismuth and then molding it is a very efficient way of capturing the

diamagnetic properties.

Superconductors - are perfect diamagnets and when placed in an external magnetic fieldexpel the field lines from their interiors. It also has zero electrical resistance, a

consequence of their diamagnetism.


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Superconducting structures have been known to tear themselves apart with astonishing

force in their attempt to escape an external field. Superconducting magnets are the major

component of most magnetic resonance imaging systems, perhaps the only important

application of diamagnetism.

Diamagnetic materials have a relative magnetic permeability that is less than 1, and a

magnetic susceptibility that is less than 0. (i.e. r = /o = (1 + m) < 1 and m < 0).

Michael Faraday - discovered and named diamagnetism in September 1845.

paraMAGNETISM

It is the tendency of the atomic magnetic dipoles in a material that is otherwisenon-magnetic to align with an external magnetic field. The atoms or molecules ofthe substance have net orbital or spin magnetic moments that are capable of being

aligned in the direction of the applied field.

They therefore have a positive (but small) susceptibility and a relativepermeability slightly in excess of one.

It occurs in all atoms and molecules with unpaired electrons like free atoms, freeradicals, and compounds of transition metals containing ions with unfilled electron

shells.

It also occurs in metals as a result of the magnetic moments associated with thespins of the conducting electrons.

Paramagnetic materials

attract and repel like normal magnets when subject to a magnetic field exhibit magnetization according to Curie's Law. This law indicates that

paramagnetic materials tend to become increasingly magnetic as the applied

magnetic field is increased, but less magnetic as temperature is increased

have a relative magnetic permeability, r greater than unity (or, equivalently, asmall positive magnetic susceptibility greater than zero), i.e. (i.e. r = /o = (1 +

m) > 1 and m > 0).

in magnetic fields will act like magnets but when the field is removed, thermalmotion will quickly disrupt the magnetic alignment. In general paramagnetic

effects are small (magnetic susceptibility of the order ofm ~ ). It includes Aluminum, Barium, Calcium, Liquid Oxygen, Platinum, Sodium,

Strontium, Uranium

Ferromagnetic materials above the Curie temperature become paramagnetic.

FERROMAGNETISM


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It is a phenomenon by which a material can exhibit a spontaneous magnetization,and is one of the strongest forms of magnetism. It is responsible for most of the

magnetic behavior encountered in everyday life, and is the basis for all permanent

magnets.

Above the Curie temperature, the thermal motion is sufficient to offset thealigning force and the material becomes paramagnetic

Below the Curie temperature an increasing magnetic field applied to aferromagnetic substance will cause increasing magnetization to a high value called

the saturation magnetization. This is because a ferromagnetic substance consists of

small magnetized regions called domains. The total magnetic moment of a sample

of the substance is the vector sum of the magnetic moments of the component

domains.

Ferromagnetic Materials

Material Curie temp. (K)

Fe 1043

Co 1388

Ni 627

Gd 292

Dy 88

MnAs 318

MnBi 630

MnSb 587

ANTIFERROMAGNETISM

It also known as Ferrimagnetism, is a property exhibited by materials whose atomsor ions tend to assume an ordered but nonparallel arrangement in zero applied

field below a certain characteristic temperature known as the Nel temperature.

Ferromagnetic materials - it is energetically favorable for the spins atomic to align,

leading to spontaneous magnetization.

Antiferromagnetic materials - it is energetically favorable for the spins to oppose, leading

to no overall magnetization.

Above the Nel temperature, the material is typically paramagnetic.

Antiferromagnetic materials

have a negative coupling between adjacent moments and low frustration. are relatively uncommon. An example is the heavy-fermion superconductor

URu2Si2. There are also numerous examples among high nuclearity metal

clusters.

Electromagnet

It is simply a coil of wires which, when a current is passed through, generate a

magnetic field. It is another kind of magnet.


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It works when:

Electricity is running through them. An electric current makes a magnetic field. Ifyou wrap the wire into a coil, the electrons spin around the coil and make a

stronger magnetic domain.

Using a coil of wire that makes a magnetic field when there is a current in it. Inaddition to this coil of wire, a large piece of metal, usually iron, is placed inside

the coil to increase the magnetic field made

Uses of Electromagnet

Though most large electromagnets employ many solenoids to lift heavy objects,smaller solenoids are used in everyday electronics. For example, they are used tochange voltage in a transformer.

Electromagnets can also be used to make electricity. Movement of a magnet backand forth in front of the electromagnet will make an electric current.

Electromagnets are used to make many things work,like computers, televisions and radios.

Applications of magnetism

Electromagnets are utilized as key components of transformers in power suppliesthat convert electrical energy from a wall outlet into direct current energy for a

wide range of electronic devices, and in motors and generators.

High field superconducting magnets provide the magnetic field in MRI devicesthat are now used extensively in hospitals and medical centers.

Magnetic materials that are difficult to demagnetize are used to constructpermanent magnets. Permanent magnet applications are in loudspeakers,

earphones, electric meters, and small motors.

The more esoteric applications of magnetism are in the area of magnetic recordingand storage devices in computers, and in audio and video systems.

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