Download MAGNETISM - Floyd County High School

MAGNETISM
Mole
A-Train
Kenny
Johnny Wang
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
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
)
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
Please Remember:
F = qv * B
No net force 
q is in coulombs, v in
m/s, and B is in teslas.
EFFECTS OF MAGNETISM (CONT’D)
As said in the
But how do you
previous slide, there figure out in which
is a net force if the
direction? See next
slide
particle moves in a
direction not parallel
to the magnetic field.
The force on the
particle is
perpendicular to both
v and B.
?
EFFECTS OF MAGNETISM (CONT’D)
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

EFFECTS OF MAGNETISM (CONT’D)
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
= iL*B, 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
EFFECTS OF MAGNETISM (CONT’D)
ANSWER:
Now, let’s try this. Let’s say
its length is 25 cm, and the
current going through it is 5
A, but we don’t know the
magnitude of the magnetic
field, but we measure the
force on the bar to be 10 N.
What is the field strength?
N
    
i     
ANSWER #2:
10 N = (5A)(0.25m)*B
(10 N)/[(5A)(0.25m)] = B
B = 8 N/A*m = 8 T
S
MONOPOLE MAGNETS DON’T EXIST
Magnets always have both north and south poles. The reason
why monopole magnets don’t 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.
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 = s·A
UNITS (CONT’D)
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/A·m = kg/(A·s2)·m
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.
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.
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
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.
PERMANENT MAGNETS
Because of its intrinsic properties,
Lets melt this down, and Temperature
the atoms of a metal become tiny
bring in a magnetic field.
magnets with two poles, a dipole,
and when groups of these dipoles
Now, when we let the solid
Melting
that are pointing in the same
cool down, and take away
point
direction come together they make
the external magnetic field,
what is known as a domain. If the
power of a magnetic field is strong
we have formed a magnet
enough it can align the domains
in the same direction as the
resulting in the overall magnetism of
magnetic field from earlier
the material.
Bar
Domains
Magnet
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.
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.
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
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, it’s 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.
THE EARTH AS A BAR MAGNET (CONT’D)
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
Earth’s magnetic field “flips” occasionally.
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 you’re reversing the direction of
the current.
Electromagnets (cont’d)
•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.
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.
Solenoids (cont’d)
•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.
Question #1
Find the direction of the magnetic field. It
may help to use the Right Hand Rule, here.
Answer:
i
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.
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
Answer:   
+ + + + + + +
Into the page
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.
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.
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.
Electromagnetic Induction (cont’d)
•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/faraday/
(More Ahead)
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 it’s starting
position, the current is zero. This cycle repeats every time
the wire makes a complete revolution, in a periodic
manner.
AC Electric Generators (cont’d.)
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 generator’s rotation.
Animation of an AC generator.
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.
AC Electric Motors (cont’d)
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.
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:
Earth’s 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
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 dysprosium
curie point of nickel
1
2
3
4
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/node83.html
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.
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.
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.
Credits
How speakers work
http://www.geo.umn.edu/orgs/irm/bestiary/index.html
Bestiary of magnetic minerals
http://sprott.physics.wisc.edu/demobook/chapter5.htm
History of magnets
http://www.webmineral.com/data/Magnetite.shtml
Magnetite
http://pupgg.princeton.edu/~phys104/2000/lectures/lecture4/sld001.htm
Slide show
http://www.physics.umd.edu/deptinfo/facilities/lecdem/demolst.htm
Best ever site for pictures, simple explanations, etc.
http://www.trifield.com/magnetic_fields.htm
Another good site for how magnets work
http://bell.mma.edu/~mdickins/TechPhys2/lectures3.html
Equations and such
http://schools.moe.edu.sg/xinmin/lessons/physics/default.htm
see also
Credits
http://www.micro.magnet.fsu.edu/electromag/java/index.html
Main Index
http://www.micro.magnet.fsu.edu/electromag/java/detector/
How a metal detector works
http://www.micro.magnet.fsu.edu/electromag/java/compass/
How a compass is oriented magnetically
http://www.micro.magnet.fsu.edu/electromag/java/faraday2/
How Faraday did his current experiment
http://www.micro.magnet.fsu.edu/electromag/java/harddrive/
How a hard drive works
http://www.micro.magnet.fsu.edu/electromag/java/magneticlines/
How magnet lines is working
http://www.micro.magnet.fsu.edu/electromag/java/magneticlines2/
How two magnets repel and attract
http://www.micro.magnet.fsu.edu/electromag/java/nmr/populations/index.html
Nuclear spin up/down
http://www.micro.magnet.fsu.edu/electromag/java/pulsedmagnet/
Pulsed magnets
http://www.micro.magnet.fsu.edu/electromag/java/speaker/
Credits
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/elemag.html
http://library.thinkquest.org/16600/intermediate/magnetism.shtml
http://www-geology.ucdavis.edu/~gel161/sp98_burgmann/magnetics/magnetics.html
http://www.micro.magnet.fsu.edu/electromag/java/index.html
http://webphysics.davidson.edu/Applets/BField/Solenoid.html
http://www.ameslab.gov/News/Inquiry/spring96/spin.html
http://cfi.lbl.gov/~budinger/medTechdocs/MRI.html
http://www.wondermagnet.com/dev/images/dipole1.jpg