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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.