MAGNETISM

1
MAGNETISM
Mole
A-Train
Kenny
Johnny Wang
2
WHAT IS MAGNETISM?
A Magnet can be compared to a electric dipole,
with field lines exiting one side and coming back
into the opposite side. All magnets have a north
end and a south end, field lines exit from the
north and enter the south. The difference
between an electric field and a magnetic field,
is that while an electric field effects all
charges, a magnetic field only effects charges
when they are in motion.
Magnetic field lines of a bar magnet
3
Force Due to Magnetic Field
The difference between an electric and a magnetic
field is that where a electric fields apply a
force on any charged particle, a magnetic field
only applies a force on a charge in motion. The
force felt by the charge in motion is given by
the formula
F q v b sin ( ? )
F Force q Charge on the particle v velocity
of the particle b magnetic field magnitude ?
angle between the velocity vector and the
magnetic field
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EFFECTS OF MAGNETISM
The force on a particle with a charge of q moving
with velocity v is given by the vector equation F
qv B, where B is the direction and magnitude
(a vector) of the magnetic field. When the
charged particle is moving parallel to the
magnetic field, the force on it is zero.
But when the particle is moving in any other
direction, there is a net force on the
particle.See next slide
No net force ?
Please Remember F qv B
?q is in coulombs, v in m/s, and B is in teslas.
5
EFFECTS OF MAGNETISM (CONTD)
As said in the previous slide, there is a net
force if the particle moves in a direction not
parallel to the magnetic field. The force on the
particle is perpendicular to both v and B.
But how do you figure out in which direction? See
next slide
?
6
EFFECTS OF MAGNETISM (CONTD)
It is by something know as the Right hand rule.
First hold out your right hand. Point your
fingers in the direction of the velocity times
the sign of the charge. Curl them towards the
direction of the magnetic field. Stick out your
thumb, and that will be the direction of the
force on the particle.
?
?
Remember (AGAIN) F qv B
Yellow line F ? Line pointing towards you
? Line pointing away from you
Red charge Blue - charge Purple
v Black line B
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EFFECTS OF MAGNETISM (CONTD)
In the last slide we mentioned the of the right
hand rule. This is also true for an wire or other
object with an electrical current flow through
it, since a current is nothing more than a flow
of positive (negative) charges. To use the right
hand rule, simply replace v with the direction of
the flow of the current. An equivalent equation
for this is F iLB, where L is the length of
the wire in meters that lies in the magnetic
field. Try solving the problem on the right (the
magnetic field is uniform).
N
i ?
S
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EFFECTS OF MAGNETISM (CONTD)
ANSWER
N
Now, lets try this. Lets say its length is 25
cm, and the current going through it is 5 A, but
we dont know the magnitude of the magnetic
field, but we measure the force on the bar to be
10 N. What is the field strength?
? ? ? ? ? ? ? ? ? ?
i ?
ANSWER 2 10 N (5A)(0.25m)B (10
N)/(5A)(0.25m) B B 8 N/Am 8 T
S
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MONOPOLE MAGNETS DONT EXIST
Magnets always have both north and south poles.
The reason why monopole magnets dont exist is
because the north and south poles are created by
the alignment of the molecules inside. They are
all aligned in the direction of the north pole,
so even if a magnet is broken in half, the
alignment of the molecules has not changed, so
there is the north and south poles still exist.
10
UNITS USED IN MAGNETISM
ampere (A or amp) The ampere is the SI base unit
of electrical currents. One ampere is the current
that would create, between two infinitely long
parallel wires with negligible cross section
place one meter apart in a perfect vacuum, a
force of 0.2 micro newtons between each other per
meter of length. All other electrical units are
all defined in terms of the ampere. The unit is
known informally as the amp, but A is its
official symbol. coulomb (C) The SI unit of
electric charge. One coulomb is the amount of
charge accumulates in one second by a current of
one ampere. Since electricity a flow of
electrons, one coulomb represents the charge of
approximately 6.241 506 x 1018 electrons. C sA
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UNITS (CONTD)
tesla (T) The tesla is the SI unit of flux
density (or field intensity) for magnetic fields.
A tesla is the field intensity required to
generate one newton of force per ampere of
current per meter of conductor. A magnetic field
of one tesla is very powerful magnetic field.
Sometimes it may be convenient to use the gauss,
which is equal to 1/10,000 of a tesla. The tesla
is probably the most important unit used in
magnetism. T N/Am kg/(As2)m
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QUANTUM MECHANICS EXPLANATION OF MAGNETISM
Magnetic fields are due to the flow of electron,
also called an electrical current. This is how
one can explain the intrinsic magnetic properties
of electrons called spin angular momentum and
orbital angular momentum.
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Orbital Angular Momentum
Orbital momentum is just as it sounds an
electron orbits the nucleus of an atom and from
this it carries orbital angular momentum.
Conceptually you can think about it this way but
this is not actually what happens. What actually
occurs is very complicated and can only be
explained with quantum mechanics.
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Spin Angular Momentum
To understand this, you must understand that
electrons have a property called spin, which is
kind of conceptually analogous to the spin of a
spinning top. This gives it spin angular
momentum. Basically, along with orbital angular
momentum, spin angular momentum gives it a vector
quantity, meaning it moves with direction.
Here is a site that shows in a more complex way
how the direction of the spin would affect the
magnetic field.Spin and Win
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Magnets
A bar magnet is made using these properties of
electrons and atoms. If all of the magnetic
poles due to these properties are lined up in a
solid, a magnet is formed. To make a magnet out
of a metal, on can metal it, and expose it to a
magnetic field as it becomes a solid again,
causing the poles to line up and form a permanent
magnet.
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PERMANENT MAGNETS
Because of its intrinsic properties, the atoms
of a metal become tiny magnets with two poles, a
dipole, and when groups of these dipoles that are
pointing in the same direction come together they
make what is known as a domain. If the power of
a magnetic field is strong enough it can align
the domains resulting in the overall magnetism of
the material.
Lets melt this down, and bring in a magnetic
field.
Temperature
Now, when we let the solid cool down, and take
away the external magnetic field, we have formed
a magnet in the same direction as the magnetic
field from earlier
Melting point
Domains
Bar Magnet
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MAGNETIC DOMAINS
Magnetic domains are groups of atoms that have
the same magnetic alignment. Think of them as
super-tiny magnets. In a non-magnetized piece of
iron, these domains have random magnetic field
alignments, and cancel each other out. But in a
magnet, they are all aligned in one direction,
producing a net magnetic field. The pictures
bellow show domains with little disbursements of
arrows pointing in various directions. Usually
only iron, nickel, or cobalt can have domains
that align.
Domain Demo
This is what happens when a magnet gets too close
to the grid of domains, it aligns the arrows with
the magnets field.
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SOURCES OF MAGNETIC FIELDS
There are many sources of magnetic fields, not
just from a bar magnet, and many of them will be
described in this presentation.
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BAR MAGNETS
Bar magnets are metal bars that have magnetic
properties. The magnetic field produced by a bar
magnet flows from the north end to the south end.
It is a permanent magnet.
The Earth can be considered a bar magnet as well.
It has a pair of geographical poles and
magnetical poles. See next slide
Bar magnet demo
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THE EARTH AS A BAR MAGNET
Therefore, Earth is a gigantic magnet. However,
the magnetic poles of the earth are offset from
the geographic ones by 11.5. Interesting enough,
the true magnetic north pole of the Earth is in
fact closer to the south pole rather than the
north pole. Although the north south poles of a
compass point to there respective directions when
used for direction finding, all magnetic poles
are attractive to the opposite pole the north
pole of a compass must point towards a south pole
and vice versa. So in truth, its more like a
168.5 offset. Scientists suggest that the
magnetic poles are moving further apart from the
geographic poles at a slow rate each year, they
also predict that in the future the magnetic flow
will be disrupted, forcing it to switch
directions.
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THE EARTH AS A BAR MAGNET (CONTD)
The fact that it has a magnetic field is very
important, because blocks out harmful solar
radiation. Also, we know that it flipped around
because of sea floor spreading. Magma escapes out
of the rift between plates and cool. Tiny bits of
magnets gets permanently aligned one way or
another due to the magnetic field of the Earth
during creation. Afterwards, when the sea floor
moves out, the bits of magnets retain their
alignment for millions of years. Scientists have
found that different strips of sea floor,
corresponding to a uniquely different time
period, have alignments opposite than, say, a
nearby strip of sea floor. This can only be
explain by that the Earths magnetic field
flips occasionally.
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Electromagnets
Anything with an electrical current running
through it has a magnetic field.You can easily
find the direction of the magnetic field produced
by a current flowing through the wire with the
Right Hand Rule.To use the Right Hand Rule
with an electromagnet, take your right hand and
make a fist. After this, point your thumb in the
direction of the current. Your thumb represents
the current, and your fingers represent the
direction of the magnetic field. If you flip your
hand over, then youre reversing the direction of
the current.
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Electromagnets (contd)

• The magnetic field lines around an electrically
  charged wire form concentric rings.

• You can also use a similar Right Hand Rule
  with solenoids.
• Take your right hand and create a fist, and
  point your thumb up. Your thumb represents the
  direction of the magnetic field in the solenoid,
  and your fingers represent the direction of the
  current in the wire loops.

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Properties of Solenoids

• Solenoids are one the most common forms of
  electromagnets.

• Solenoids consist of a tightly wrapped coil of
  wire around a core (usually iron). When a charge
  is applied to the coil, a magnetic field is
  produced.
• As the coil becomes more tightly wrapped, the
  magnetic field becomes more concentrated inside
  the coil and less concentrated outside of it.

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Solenoids (contd)

• If the direction of the current in the coil is
  reversed, the North and South poles become
  reversed as well.

• The magnetic field lines produced by a tightly
  wrapped solenoid look very similar to those
  produced by a bar magnet.

• The magnetic field produced by a current
  flowing through a wire is perpendicular to the
  direction of the current.

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Question 1
Find the direction of the magnetic field. It may
help to use the Right Hand Rule, here.
i
Answer
The easiest way to do problems like this is to
use the Right Hand Rule. Take your right hand,
and orient your thumb in the direction of the
current. Then, curl your fingers as if you were
making a first. The magnetic field follows your
fingers.
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Question 2
Find the direction of the magnetic field in the
solenoid. Hint The Right Hand Rule works here,
too.
i
Out of the page Into the page
Answer ? ? ?
Once again, a Right Hand Rule applies to these
problems as well. Open your right hand, and curl
your fingers in the direction of the current.
Then, extend your thumb outwards it should point
in the direction of the magnetic field.
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Faraday and Magnetic Flux
Magnetic flux is a measure of the amount of
magnetic field lines going through an area of a
Gaussian surface. As a bar magnet nears the
surface the flux increases, and as it goes
further away, the flux decreases

• Michael Faraday discovered that a changing
  magnetic flux in a wire can create an electric
  current. One simple example of this is a magnet
  moving in and out of a wire loop.

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Electromagnetic Induction

• Any change in magnetic flux can create a current,
  such as a wire moving in a magnetic field, or a
  magnet moving through a wire loop.

• If there is no change in magnetic flux, then no
  current can be produced, even if there is a very
  strong magnetic flux present.

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Electromagnetic Induction (contd)

• The practical application of this is in an
  electric generator, where an electric current is
  said to be induced in a wire that is experiencing
  a change in magnetic flux. In a simple
  generator, a wire loop is placed between two
  magnetic poles, and is then rotated by an
  external force. This creates a change in
  magnetic flux, which in turn creates an
  alternating electric current.

http//www.micro.magnet.fsu.edu/electromag/java/fa
raday/
(More Ahead)
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Simple Electric Generator (AC)
Electric generators transform a torque into a
current. As a rotating wire in a generator moves
from a straight angle to a 90 degree angle
relative to the magnetic field, the current
increases to a maximum. When it then moves from
a 90 degree angle to a straight angle, the
induced electric current moves to zero. When the
rotating wire continues to move after reaching a
straight angle, it begins to create a current
flowing in the opposite direction. When the wire
is once again at a 90 degree angle, this current
is at a maximum, and when the wire is back to
its starting position, the current is zero. This
cycle repeats every time the wire makes a
complete revolution, in a periodic manner.
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AC Electric Generators (contd.)
The simplest form of an electric generator is
called an alternating current (or AC)
generator. The current produced by an AC
generator switches directions every time the wire
inside of it is rotated to make a half turn.
In standard generators in the United States, the
generator has a frequency of 60Hz, which means
the current switches direction 120 times every
second! A graph of the current output from an AC
generator produces a sinusoidal curve due to the
periodic nature of the generators
rotation. Animation of an AC generator.
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Electric Motors

• The same principles that allow an electric
  generator to function also work to allow an
  electric motor to function.
• Electric motors are quite similar to electric
  generators, but work in the reverse fashion,
  generating a torque from an electric current.
• In a simple AC electric motor, a current is fed
  into a wire rotor placed within the field of a
  magnet.

• Remember that you can use the Right Hand Rule to
  determine the direction of the force due to the
  magnetic field on a current flowing through a
  wire.
• Animation of a DC motor. In a DC motor running
  from an AC current, there is a mechanism called
  the commutator that switches the contacts from
  which the rotor is getting current from when the
  current switches directions, producing direct
  current in the rotor. Second animation.

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AC Electric Motors (contd)
When a current is fed into the wire rotor of a
motor in a magnetic field, a force is felt on the
two wires that do not line up with the magnetic
field. They provide a torque on the wire loop
and turn the loop. As the loop reaches a half
turn, the current changes direction, and the
torque continues in the same direction.
This happens many times per second, causing the
rotor to constantly turn. The turning of the
rotor provides torque which can be harnessed to
do work.
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FLUX INTENSITY
Flux intensity is the number of magnetic field
lines in a given area. As in the previous slide,
tesla is SI measurement of this.
Some known magnetic fields and their flux
intensity in teslas are Earths magnetic
field 5 x 10-5 T Small bar magnet 0.01 T
Strongest laboratory electromagnetic 20
T Surface of neutron star 108 T
NEUTRON STAR
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CURIE POINT
The curie point is reached when a magnet is
heated up so much that it wants to forget its
magnetic behaviors. The loss is only temporary
the magnet will regain its characteristics as
soon as it is returned to the temperature at
which it originally had magnetic properties. Here
are a few sites which show the curie point of
iron, nickel, and
curie point of iron curie point of nickel
curie point of dysprosium
1
2
3
4
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IS LEVITATION POSSIBLE?
Yes! Through the power of diametic plate
strategically placed around an object. The reason
any living creature has the ability to be
levitated is because everything has the potential
to be magnetic. We all have domains in our body,
but ours are almost always randomly
oriented. Magnetic levitation is also sometimes
used by high speed bullet trains.
Click on the links below to see a frog being
levitated http//theory.uwinnipeg.ca/mod_tech/nod
e83.html
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APPLICATIONS IN SCIENCE
Besides being used in motors and generators,
there are many applications for magnets in
scientific or medical devices. For example,
magnets are used in MRI (magnetic resonance)
scans, which are used to help diagnose medical
conditions. Additionally, high-powered
electromagnets are used in particle accelerators.
Particle accelerators are huge machines used to
accelerate subatomic particles to nearly the
speed of light. Scientists study the
interactions of different particles being smashed
into each other at these high speeds.
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HISTORY OF MAGNETS
The first magnets were naturally occurring
lodestones, sometimes referred now as magnetite,
that were magnetized piece of iron ore. People of
ancient Greece and china discovered that a
lodestone would always align itself in a
longitudinal direction if it was allowed to
rotate freely. This ability of the lodestone
allowed for the creation of compasses two
thousand years ago, which was the first known use
of the magnet. In 1263, Pierre de Maricourt
mapped the magnetic field of a lodestone with a
compass. He discovered that a magnet had two
magnetic poles North and South poles. In the
1600's William Gilbert concluded that the earth
itself is a giant magnet.
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HISTORY CONTINUED
In 1820, Hans Christian discovered an electric
current flowing through a wire can cause a
compass needle to rotate, showing that magnetism
and electricity were related. In 1830 Michael
Faraday and Joseph Henry discovered that a
changing magnetic field produced a current in a
coil of wire. Pierre Curie discovered that
magnets loose their magnetism above a certain
temperature which became known as the Curie
point. In the 1960's and 1970's scientists
developed superconducting materials.
Superconductors are materials that have an
extremely low resistance to a current flowing
through them, usually at a very low temperature.
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Credits
How speakers work http//www.geo.umn.edu/orgs/irm/
bestiary/index.html Bestiary of magnetic
minerals http//sprott.physics.wisc.edu/demobook/c
hapter5.htm History of magnets http//www.webminer
al.com/data/Magnetite.shtml Magnetite http//pupgg
.princeton.edu/phys104/2000/lectures/lecture4/sld
001.htm Slide show http//www.physics.umd.edu/dept
info/facilities/lecdem/demolst.htm Best ever site
for pictures, simple explanations,
etc. http//www.trifield.com/magnetic_fields.htm A
nother good site for how magnets
work http//bell.mma.edu/mdickins/TechPhys2/lectu
res3.html Equations and such
http//schools.moe.edu.sg/xinmin/lessons/physics/d
efault.htm see also
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Credits

• http//www.micro.magnet.fsu.edu/electromag/java/in
  dex.html
• Main Index
• http//www.micro.magnet.fsu.edu/electromag/java/de
  tector/
• How a metal detector works
• http//www.micro.magnet.fsu.edu/electromag/java/co
  mpass/
• How a compass is oriented magnetically
• http//www.micro.magnet.fsu.edu/electromag/java/fa
  raday2/
• How Faraday did his current experiment
• http//www.micro.magnet.fsu.edu/electromag/java/ha
  rddrive/
• How a hard drive works
• http//www.micro.magnet.fsu.edu/electromag/java/ma
  gneticlines/
• How magnet lines is working
• http//www.micro.magnet.fsu.edu/electromag/java/ma
  gneticlines2/
• How two magnets repel and attract
• http//www.micro.magnet.fsu.edu/electromag/java/nm
  r/populations/index.html
• Nuclear spin up/down
• http//www.micro.magnet.fsu.edu/electromag/java/pu
  lsedmagnet/
• Pulsed magnets
• http//www.micro.magnet.fsu.edu/electromag/java/sp
  eaker/

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Credits
http//hyperphysics.phy-astr.gsu.edu/hbase/magneti
c/elemag.html http//library.thinkquest.org/16600/
intermediate/magnetism.shtml http//www-geology.uc
davis.edu/gel161/sp98_burgmann/magnetics/magnetic
s.html http//www.micro.magnet.fsu.edu/electromag
/java/index.html http//webphysics.davidson.edu/A
pplets/BField/Solenoid.html http//www.ameslab.go
v/News/Inquiry/spring96/spin.html
http//cfi.lbl.gov/budinger/medTechdocs/MRI.html
http//www.wondermagnet.com/dev/images/dipole1.j
pg