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MAGNETISM
Magnetism
A Strangely Attractive Topic
History #1
à Term comes from the ancient Greek city of Magnesia, at
which many natural magnets were found. We now refer to
these natural magnets as lodestones (also spelled
loadstone; lode means to lead or to attract) which contain
magnetite, a natural magnetic material Fe3O4.
à Pliny the Elder (23-79 AD Roman) wrote of a hill near the
river Indus that was made entirely of a stone that attracted
iron.
History #2
à Chinese as early as 121 AD knew that an iron rod
which had been brought near one of these natural
magnets would acquire and retain the magnetic
property…and that such a rod when suspended from a
string would align itself in a north-south direction.
à Use of magnets to assist in direction-finding can be
traced back to at least the eleventh century.
Basically, we knew the phenomenon existed and
we learned useful applications for it.
We did not understand it.
Scientists Can Be Famous, Too!
Tesla
Famous, continued
Gauss
The Concept of “Fields”
Michael Faraday
realized that ...
A magnet has a
‘magnetic field’
distributed
throughout
the
surrounding
Finally, the Science
à Not until 1819 was a connection between electrical and
magnetic phenomena shown. Danish scientist Hans
Christian Oersted observed that a compass needle in the
vicinity of a wire carrying electrical current was deflected!
à In 1831, Michael Faraday discovered that a momentary
current existed in a circuit when the current in a nearby
circuit was started or stopped
à Shortly thereafter, he discovered that motion of a magnet
toward or away from a circuit could produce the same
effect.
Let This Be a Lesson!
à Joseph Henry (first Director
of the Smithsonian Institution)
failed to publish what he had
discovered 6-12 months before
Faraday
The Connection is Made
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
A Sheep in a Cow Costume?
All magnetic
phenomena result
from forces between
electric charges in
motion.
Looking in More Detail
à Ampere first suggested in 1820 that
magnetic properties of matter were due to tiny
atomic currents
à All atoms exhibit magnetic effects
à Medium in which charges are moving has
profound effects on observed magnetic forces
For most of our discussions, we
will assume the medium is empty
space, which is a reasonable
approximation of air in this
context.
MAGNETISM –THE ABILITY OF CERTAIN
MATERIALS TO ATTRACT IRON, COBALT
OR NICKEL.
CLASSIFICATION OF MAGNETS
 NATURAL MAGNETS
 ARTIFICIAL PERMANENT MAGNETS
 ELECTROMAGNET
NATURAL MAGNET -EARTH
The Earth’s Magnetic Field
We know Earth has a core of
metal that rotates.
This
generates the so-called “Main
Field” of the earth.
Still some debate about exactly
how this happens (e.g. why
currents
don’t
dissipate).
Movement of metal & charge in
geodynamo creates field - more
about this in a moment.
NATURAL MAGNETS-LODESTONES
LODESTONES BECAME
MAGNETIZED BY LYING IN
THE EARTH’S MAGNETIC
FIELD FOR THOUSANDS OF
YEARS
THE ANCIENT GREEKS RECOGNIZED
THE MAGNETISM OF LODESTONES
MORE THAN 2500 YEARS AGO
THE SPHERE OF ARCHIMEDES
ARTIFICIAL PERMANENT
MAGNETS
 HARD STEEL
 MAGNETIC COMPASS
 ALNICO
HARD STEEL—HORSHOE
MAGNET
MAGNETIC COMPASS
The magnetic compass
was brought to Europe
in the Middle Ages from
the Chinese who had
been using the compass
for over 1500 years
ALNICO—ALLOY OF:
 AL—ALUMINIUM
 NI—NICKEL
 CO—COBALT
ALUMINUM CAN NOT BE
MAGNETIZED
ALNICO—SUPER STRONG
MAGNETS
Creating Magnets
Methods used to create a magnet
 The
picture
shows that a
screwdriver does
not pick up paper
clips.
Creating Magnets
Stroking using a permanent magnet

However, after
stroking the
screwdriver with a
magnet, the blade
itself becomes
magnetic and
attracts the paper
clips.
Removing Magnetism
Heat is one way to remove magnetism.
Flux
 Magnetic lines of
force
 Strongest at the
poles
LAWS OF MAGNETISM
 EVERY MAGNET HAS TWO POLES
 LIKE MAGNETIC POLES REPEL:
UNLIKE POLES ATTRACT
 THE FORCE OF ATTRACTION OR
REPULSION
FOLLOWS
INVERSE
SQUARE LAW
EVERY MAGNET HAS TWO
POLESNORTH & SOUTH
LIKE MAGNETIC POLES REPEL EACH
OTHER
UNLIKE POLES ATTRACT EACH OTHER
THE FORCE OF ATTRACTION
OR REPULSION BETWEEN
TWO MAGNETS
Pp
F= k
d2
THE FORCE IS PROPORTIONAL TO THE
PRODUCT OF THE MAGNETIC POLE
STRENGTHS DIVIDED BY THE SQUARE
OF THE DISTANCE BETWEEN THEM
NATURE OF MAGNETISM
BREAKING A MAGNET WILL RESULT IN
EACH FRAGMENT BECOMING A WHOLE
MAGNET
HEATING OR HAMERING STEEL IN
PROXIMITY TO
MAGNETIZE IT.
A
MAGNET
–WILL
STROKING OF IRON WITH MAGNET
WILL MAGNETIZE IT
GENTLY RUBBING A TEST TUBE OF
IRON FILINGS NEAR A MAGNET WILL
MAGNETIZE IT
MAGNETIZATION
AN ELECTRIC CHARGE
SPINNING ON ITS AXIS WILL
CREATE MAGNETIC FIELD
MOVING CHARGED PARTICLE
WILL INDUCE MAGNETIC
FIELD
THE SMALL MAGNET
CREATED BY ELECTRON
ORBIT IS CALLED A
MAGNETIC DIPOLE
ACCUMULATION OF MANY ATOMIC
MAGNETS WITH THEIR DIPOLES
ALIGNED CREATES A MAGNETIC
DOMAIN
THE MAGNETIC DIPOLES IN
BAR MAGNET GENERATE
IMAGINARY LINES OF THE
MAGNETIC FIELD
MAGNETIC LINES OF FORCE
ARE CLOSE LOOPS
MAGNETIC LINES OF FORCE
EARTH
MAGNETIC LINES OF FORCE
BAR MAGNET
THE STRENGTH OF
MAGNETIC FIELD IS
EXPRESSED IN SI UNIT
TESLA
1 TESLA = 10,000 GAUSSES
MAGNETIC ELEMENT TWO OR
MORE ELECTRONS SPINNING
IN THE SAME DIRECTION
NUCLEUS
A MAGNETIC ELEMENT CAN
EXISTS EITHER IN
MAGNETIZED OR NONMAGNETIZED STATE
MAGNETIZED MAGNETIC
MATERIAL
NON-MAGNETIZED MAGNETIC
MATERIAL
IN NONMAGNETIC MATERIALS JUST AS
MANY ELECTRONS SPIN IN ONE
DIRECTION AS IN OTHER.
THEIR OPPOSING MAGNETIC FIELDS
CANCEL OUT
NUCLEUS
The Earth’s Field & Solar Wind
The earth is continuously blasted by ionized particles blown off of the
sun - called the “solar wind.” These fast particles generate their own
field and interfere with that of the earth.
Variations in both
Main and External
Fields
cause
the
change in magnetic
declination.
Not to worry - near
the earth, the Main
Field dominates.
CHARACTERISTICS OF THE
LINES OF FORCE
 FROM NORTH TO SOUTH OUSIDE THE
MAGNET IN THE SPACE
 FROM SOUTH TO NORTH INSIDE THE
MAGNET
 IF IN THE SAME DIRECTION THEY WILL
REPEL
 MAGNETIC FIELD IS DISTORTED BY
MAGNETIC MATERIALS BUT NOT AFFECTED
BY NON-MAGNETIC MATERIALS
LINES OF FORCE
LINES OF FORCE
REPEL
N
N
S
S
LINES OF FORCE
ATTRACT
S
N
N
S
NONMAGNETIC MATERIALS DO
NOT AFFECT THE LINES OF
FORCE OF MAGNETIC FIELD
MAGNETIC MATERIALS LIKE
IRON AFFECT THE LINES OF
FORCE
MAGNETIC INDUCTION
(MAGNETIZATION)
MAGNETIZATION –DOMAINS
MOVE
MAGNETIC
PERMEABILITY
EASE WITH WHICH A GIVEN
MATERIAL CAN BE
MAGNETIZED
MAGNETIC
RETENTIVITY
MATERIAL RESISTANCE TO
MAGNETIZATION
SOFT IRON HIGH
PERMEABILITY LOW
RETENTIVITY
HARD STEEL LOW
PERMEABILITY HIGH
RETENTIVITY
“EASY COME—EASY GO”
MAGNETIC CLASSIFICATION OF
MATTER
 FERROMAGNETIC
 PARAMAGNETIC MATERIALS
 NONMAGNETIC MATERIALS
 DIAMAGNETIC MATERIALS
FERROMAGNETIC
MATERIALS –IRON,
COBALT, NICKEL.
STRONGLY ATTRACTED
BY MAGNETS
PARAMAGNETIC
MATERIALS—PLATINUM.
weakly ATTRACTED BY
MAGNETS
NONMAGNETIC
MATERIALS—COPPER,
WOOD, GLASS. THEY ARE
NOT ATTRACTED TO
MAGNETS.
DIAMAGNETIC
MATERIALS—BERYLLIUM
& BISMUTH. weakly
REPELLED and
ATRRACTED BY
MAGNETS
DETECTION OF
MAGNETISM ---COMPASS
GEOGRAPHIC NORTH POLE IS
MAGNETIC SOUTH POLE?????
S. MAGNETIC
N. GEOGRAPHIC
N. MAGNETIC
S. GEOGRAPHIC
SMALL MAGNETS
ELECTROMAGNETS
TEMPORARY MAGNETS
PRODUCED BY MEANS OF
ELECTRIC CURRENT.
ELECTROMAGNET
MRI ELECTROMAGNET
Electromagnetism
Electromagnets
A large electromagnet
used to lift scrap metal
A 30-foot diameter electromagnet
used in research for a superconductor
Electromagnets
The source of electromagnetism is electricity
(current) flowing through a coil of wire.
Electromagnets
A coil of wire can be made into a magnet by passing an electric current through it.
Without electricity, there
is no magnetic force
Electric current applied to a coil creates a
magnetic field
Electromagnetic Strength
How can an electromagnet be made stronger?
 Increase the number of coils.
 Increase the electric current flowing
through the coil.
 Add an iron core in the center of the coil.
Electromagnetic Strength
The left hand rule to
determine North Pole
NORTH POLE
SOUTH POLE
Making a Magnet
An example of a very strong magnet
Making a Magnet
The coil of wire has been attached to a
12 volt DC power source making an
extremely strong magnet.
The magnet is strong enough to
lift this 50 pound engine cylinder
head.
Saturation
Saturation of a magnetic field occurs when an increase of electric current flow
does not increase the strength of the magnetic field as shown by the graph. The
knee of the curve is the point that saturation occurs.
Y
Saturation Knee
Y- Axis
Increasing
Magnetic
Force
X
X- Axis Increasing Electric
Current
MAGNETIC COIL LOSSES
 Copper losses - resistance of copper
coils.
 Hysteresis - magnetic flux inducing a
current in the core.
 Eddy currents - magnetic polarity of the
iron core is rapidly changed by an AC
current causing friction and heat
between the molecules
ELECTROMAGNETIC
DEVICES
Electromagnetic Devices
Electromagnetic Devices