MAGNETISM

1
MAGNETISM

• History of Magnetism
• Bar Magnets
• Magnetic Dipoles
• Magnetic Fields
• Magnetic Forces on Moving Charges and
  Wires
• Electric Motors
• Current Loops and Electromagnets
• Solenoids

• Sources of Magnetism
• Spin Orbital Dipole Moments
• Permanent Magnets
• Earths Magnetic Field
• Magnetic Flux
• Induced Emf and Current
• Generators
• Crossed Fields

2
History of Magnetism

• The first known magnets were naturally
  occurring lodestones, a type of iron ore
  called magnetite (Fe3O4). 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 property of lodestones 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, physician of
  Queen Elizabeth I, concluded that Earth
  itself is a giant magnet.
• In 1820 the Danish physicist Hans Christian
  Ørsted discovered an electric current flowing
  through a wire can cause a compass needle to
  deflect, showing that magnetism and electricity
  were related.

3
History (cont.)

• In 1830 Michael Faraday (British) and Joseph
  Henry (American) independently discovered
  that a changing magnetic field produced a current
  in a coil of wire. Faraday, who was perhaps
  the greatest experimentalist of all time,
  came up with the idea of electric and magnetic
  fields. He also invented the dynamo (a
  generator), made major contributions to
  chemistry, and invented one of the first
  electric motors
• In the 19th century James Clerk Maxwell, a
  Scottish physicist and one of the great
  theoreticians of all times, mathematically
  unified the electric and magnetic forces. He
  also proposed that light was electromagnetic
  radiation.
• In the late 19th century Pierre Curie
  discovered that magnets loose their magnetism
  above a certain temperature that later became
  known as the Curie point.
• In the 1900's scientists discover
  superconductivity. Superconductors are
  materials that have a zero resistance to a
  current flowing through them when they are a
  very low temperature. They also exclude magnetic
  field lines (the Meissner effect) which makes
  magnetic levitation possible.

4
Magnetic Dipoles
Recall that an electric dipole consists of two
equal but opposite charges separated by some
distance, such as in a polar molecule. Every
magnet is a magnetic dipole. A bar magnet is a
simple example. Note how the E field due an
electric dipole is just like the magnetic field
(B field) of a bar magnet. Field lines emanate
from the or N pole and reenter the - or S
pole. Although they look the same, they are
different kinds of fields. E fields affect any
charge in the vicinity, but a B field only
affects moving charges. As with charges, opposite
poles attract and like poles repel.
-

_
Electric dipole and E field
N
S
Magnetic dipole and B field
5
Magnetic Monopole Dont Exist
We have studied electric fields to due isolated
or - charges, but as far as we know, magnetic
monopole do not exist, meaning it is impossible
to isolate a N or S pole. The bar magnet on the
left is surrounded by iron filings, which orient
themselves according to the magnetic field they
are in. When we try to separate the two poles by
breaking the magnet, we only succeed in producing
two distinct dipoles (pic on right).
Bar magnet demo
6
Magnetic Fields
You have seen that electric fields and be
uniform, nonuniform and symmetric, or nonuniform
and asymmetric. The same is true for magnetic
fields. (Later well see how to produce uniform
magnetic fields with a current flowing through a
coil called a solenoid.) Regardless of symmetry
or complexity, the SI unit for any E field is the
N/C, since by definition an electric field is
force per unit charge. Because there are no
magnetic monopoles, there is no analogous
definition for B. However, regardless of symmetry
or complexity, there is only one SI unit for a B
field. It is called a tesla and its symbol is T.
The coming slides will show how to write a tesla
in terms of other SI units. The magnetic field
vector is always tangent to the magnetic field.
Unlike E fields, all magnetic field lines that
come from the N pole must land on the S pole--no
field lines go to or come from infinity.
7
Force Due to Magnetic Field
The force exerted on a charged particle by a
magnetic field is given by the vector cross
product
F q v ? B
F force (vector) q charge on the particle
(scalar) v velocity of the particle relative to
field (vector) B magnetic field (vector)
Recall that the magnitude of a cross is the
product of the magnitudes of the vectors times
the sine of the angle between them. So, the
magnitude of the magnetic force is given by
F q v B sin?
where ? is angle between q v and B vectors.
8
Cross Product Review
Let v1 ? x1, y1, z1 ?
and v2 ? x2, y2, z2 ?.
By definition, the cross product of these
vectors (pronounced v1 cross v2) is given by
the following determinant.
(y1 z2 - y2 z1) i - (x1 z2 - x2 z1) j (x1
y2 - x2 y1) k
Note that the cross product of two vectors is a
vector itself that is ? to each of the original
vectors. i, j, and k are the unit vectors
pointing, along the positive x, y, and z axes,
respectively. (See the vector presentation for a
review of determinants.)
9
Right Hand Rule Review
A quick way to determine the direction of a cross
product is to use the right hand rule. To find a
? b, place the knife edge of your right hand
(pinky side) along a and curl your hand toward
b, making a fist. Your thumb then points in the
direction of
a ? b.
a ? b
It can be proven that the magnitude of
a ? b
is given by
a b sin?
b
a ? b
?
where ? is the angle between a and b.
a
10
Magnetic Field Units
F q v B
sin?
1 N 1 C (m / s) (T)
From the formula for magnetic force we can find a
relationship between the tesla and other SI
units. The sine of an angle has no units, so
1 N
1 N
1 T

C (m / s)
A m
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. Earths magnetic field, at the surface,
varies but has the strength of about one gauss.
11
Direction of Magnetic Field Force
Near the poles, where the field lines are close
together, the field is very strong (so the field
vector are drawn longer). Anywhere in the field
the mag. field vector is always tangent to the
mag. field line there. The charge in the pic in
moving into the page. Since q is , the q v
vector is also into the page. The - charge is
moving to the right, so the q v vector is to the
left. The mag. force vector is always ? to plane
formed by the q v vector and the B vector.
B
F
The force on the - charge is into the page. If
a charge is motionless relative to the field,
there is no magnetic force on it, but if either a
magnet is moving or a charge is moving, there
could a force on the charge. If a charge moves
parallel to a magnetic field, there is no
magnetic force on it, since sin 0 0.
-

v
B
12
Magnetic Field Force Practice

• Find the direction of the magnetic force or
  velocity
• A charge at P is moving out of the page.
• A - charge at Q is moving out of the page.
• A - charge at Q is moving to the right.
• A charge at Q is moving up.
• A - charge at R is moving up and to the left.
• A charge at R is moving down and to the
  right.
• A - charge at R feels a force into the page.
• A charge at P feels a force out of the page.
• A - charge at Q feels an upward force.

P
R
Q
13
Magnetic Force Sample Problem
This magnet is similar to a parallel plate
capacitor in that there is a strong uniform field
between its poles with some fringing on the
sides. Suppose the magnetic field strength inside
is 0.07 T and a 4.3 mC charge is moving through
the field at right angle to the field lines. How
strong and which way is the magnetic force on the
charge? Answer
F q v ? B ? F q v B since sin 90 1.
S
5 m/s

N
N
So, F 0.0015 N directed out of the page.
14
Motion of a Charge in a Magnetic Field
The ?s represent field lines pointing into the
page. A positively charged particle of mass m
and charge q is shot to the right with speed
v. By the right hand rule the magnetic force on
it is up. Since v is ? to B, F FB q v B.
Because F is ? to v, it has no tangential
component it is entirely centripetal. Thus F
causes a centripetal acceleration. As the
particle turns so do v and F, and if B is
uniform the particle moves in a circle. This is
the basic idea behind a particle accelerator like
Fermilab. Since F is a centripetal force, F
FC m v2 / R. Lets see how speed, mass,
charge,
field strength, and radius of curvature are
related
FB FC
? q v B m v2 / R
m v
? R
q B
15
Magnetic Force on a Current Carrying Wire
A section of wire carrying current to the right
is shown in a uniform magnetic field. We can
imagine positive charges moving to right, each
feeling a magnetic force out of the page. This
will cause the wire to bow outwards. Shown on the
right is the view as seen when looking at the N
pole from above. The dots represent a uniform
mag. field coming out of the page. The mag. force
on the wire is proportional to the field
strength, the current, and the length of the
wire.
S
I
I
N
B
Continued
16
Magnetic Force on a Wire (cont.)
Current is the flow of positive charge. As a
certain amount of charge, q, moves with speed v
through a wire of length L, the force of this
quantity of charge is
F q v ? B
Over the time period t required for the charge
to traverse the length of the wire, we have
F (q / t ) v t ? B
Since q / t I and v t L, we can write
I
B
F I L ? B
where L is a vector of magnitude L pointing
in the direction of I.
17
Electric Motor
I
F

I
d
I
I
B
Current along with a magnetic field can produce
torque. This is the basic idea behind an electric
motor. Above is a wire loop (purple) carrying a
current provided by some power source like a
battery. The current loop is submerged in an
external field. From F I L ? B, the force
vectors in black are perpendicular to their wire
segments. The net force on the loop is zero, but
the net torque about the center is nonzero. The
forces on the left and right wires produce no
torque since the moment arm is zero for each
(they point right at the center). However, the
force F on the top wire (in the background) has
a moment arm d, so it produces a torque F d.
The bottom wire (in the foreground) produces the
same torque. These torques work together to
rotate the loop, converting electrical energy
into mechanical energy.
Continued
18
Electric Motor (cont.)
As the loop turns it eventually reaches a
vertical position (the plane of the loop parallel
to the field). This is when the moment arms of
the forces on the top and bottom wires are the
longest, so this is where the torque is at a max.
90 later the loop will be perpendicular to the
field. Here all moment arms and all torques are
zero. This is the equilibrium point. The angular
momentum of the loop, however, will allow it to
swing right through this position. Now is when
the current must change direction, otherwise the
torques will attempt to bring the loop back to
the equilibrium. This would amount to simple
harmonic motion of the loop, which is not
particularly useful. If the current changes
direction every time the loop reach equilibrium,
the loop will spin around in the same direction
indefinitely. Although a battery only pumps
current in one direction, the change in direction
of current can be accomplished with help of a
commutator, as can be seen with these animations
Animation 1 Animation 2
19
Electromagnets Straight Wire
Permanent magnets arent the only things that
produce magnetic fields. Moving charges
themselves produce magnetic fields. We just saw
that a current carrying wire feels a force when
inside an external magnetic field. It also
produces its own mag-netic field. A long straight
wire produces circular field lines centered on
the wire. To find the direction of the field, we
use another right hand rule point your thumb in
the direction of the current the way your
fingers of your right hand wrap is the direction
of the magnetic field. B diminishes with
distance from the wire. The pics at the right

show cross sections of a current
carrying wire.
I into page, B clockwise
I out of page, B counterclockwise
B
20
Straight Wire Practice
Draw some magnetic field lines (loops in this
case) along the wire.
I
Using xs and dots to represent vectors out of
and into the page, show the magnetic field for
the same wire. Note B diminishes with distance
from the wire.
I
21
Current Loops and Magnetic Fields
The magnetic field inside a current loop tends to
be strong outside, it tends to be weak. Heres
why Using the right hand rule we see that each
length of wire contributes to a B field into the
page (all lengths reinforcing one another).
Outside the loop, say at P, the field is weak
since the left side of the wire produces a field
out of the page, but the right side produces a
field into the page. Explain why the field is
weak above the top wire. The situation is the
same with a circular loop. The effect is
magnified with multiple turns of wire. Yet
another right hand rule helps with current loops
Wrap your right hand in the direction of the loop
and your thumb points in the direction of B
inside. This is reminiscent of angular momentum
for a spinning body.
I
I
strong field inside loop, directed into page
I
strong field into page
I
P
weak field outside
weak field
I
22
Current Loops and Bar Magnets
Notice how similar the magnetic field of a
current loop is to that of a simple bar magnet.
Wrap your right hand along the loop in the
direction of the current and your thumb points in
the direction of the north pole of your
electro-magnet. Note also how the field lines are
very close together inside the loop, just as they
are when they thread through a bar magnet.
I
23
Solenoids

• Solenoids are one of the most common
  electromagnets.
• Solenoids consist of a tightly wrapped coil
  of wire, sometimes around an iron core. The
  multiple loops and the iron magnify the effect
  of the single loop electromagnet.
• A solenoid behaves as just like a simple bar
  magnet but only when current is flowing.
• The greater the current and the more turns
  per unit length, the greater the field inside.
• An ideal solenoid has a perfectly uniform
  magnetic field inside and zero field outside.

24
How Solenoids Work
The cross section of a solenoid is shown. At
point P inside the solenoid, the B field is a
vector sum of the fields due to each section of
wire. Note from the table that each section of
wire produce a field vector with a component to
the right, resulting in a strong field inside.
In the ideal case the magnetic field would be
uniform inside and zero outside.
B 0
1 2 3 4 5 6 7 8
I out of the page
B
P
I into the page
9 10 11 12 13 14 15 16
25
Solenoids and Bar Magnets
A solenoid produces a magnetic field just like a
simple bar magnet. Since it consists of many
current loops, the resemblance to a bar magnets
field is much better than that of a single
current loop.
26
Sources of Magnetism
We have seen charges in motion (as in a current)
produce magnetic fields. This is one source of
magnetism. Another source is the electron
itself. Electrons behave as if they were tiny
magnets. Quantum mechanics is required to explain
fully the magnetic properties of electrons, but
it is helpful to relate these properties back to
the motion of charges. Every electron in an atom
behaves as a magnet in two ways, each having two
magnetic dipole moments Spin magnetic dipole
moment - due to the rotation of an
electron. Orbital magnetic dipole moment - due to
the revolution of an electron about the
nucleus. Note Electrons are not actually little
balls that rotate and revolve like planets, but
imagining them this way is useful when explaining
magnetism without quantum mechanics.
27
Spin Magnetic Dipole Moment
Just as electrons have the intrinsic properties
of mass and charge, they have an intrinsic
property called spin. This means that electrons,
by their very nature, possess these three
attributes. Youre already comfortable with the
notions of charge and mass. To understand spin it
will be helpful to think of an electron as a
rotating sphere or planet. However, this is no
more than a helpful visual tool. Imagine an
electron as a soccer ball smeared with negative
charge rotating about an axis. By the right hand
rule, the angular momentum of the ball due to its
rotation points down. But since its charge is
negative, the spinning ball is like a little
current loop flowing in the direction opposite
its rotation, and the ball becomes an
electromagnet with the N pole up. For an electron
we would say its spin magnetic dipole moment
vector, µs, points up. Because of its spin, an
electron is like a little bar magnet.
µs
-
N
-
-
-
-
-
I
S
-
-
28
Orbital Magnetic Dipole Moment
Imagine now a planet that not only rotates but
also revolves around its star. If the planet had
a net charge, its rotation would give it a spin
magnetic dipole moment, and its revolution would
give it an orbital magnetic dipole moment. Charge
in motion once again produces a magnetic field.
Since an electrons charge is negative, its
orbit is like a current loop in the opposite
direction. By the right hand rule, the angular
momentum vector in the pic below would point down
and the orbital magnetic dipole moment, µorb,
points up. An orbiting electron behaves like a
tiny electromagnet with its N pole in the
direction of µorb. Remember, though, that in
reality electrons are not like little planets. In
quantum mechanics, instead of circular orbits we
speak of electrons behaving like waves and we can
only talk of their positions in terms of
probabilities.
µorb
N
S
I
29
Materials and Magnetism

• Each electron in an atom has two magnetic dipole
  moments associated with it, one for spin, and
  one for orbit. Each is a vector.
• These two dipole moments combine vectorially for
  each electron.
• The resultant vectors from each electron then
  combine for the whole atom, often canceling
  each other out.
• For most materials the net dipole moment for
  each atom is about zero.
• For some materials each atom has a nonzero
  dipole moment, but because the atoms have all
  different orientations, the material as a whole
  remains nonmagnetic.
• Ferromagnetic materials, like iron, are
  comprised of atoms that each have net dipole
  moment. Furthermore, all the atoms have the same
  alignment, at least within very tiny regions
  called domains. The domains can have different
  orientations, though, leaving the iron
  nonmagnetic except when placed in an external
  field.
• Permanent magnets are produced when the domains
  in a ferromagnetic material are aligned.

30
Permanent Magnets
Each atom in a ferromagnetic material like iron
is like a little magnet, and these magnets are
all aligned in tiny regions called domains. At
high temps these domains can align in the
presence of an external field (like Earths)
leaving a permanent magnet. This happens at the
Mid-Atlantic Ridge beneath the Atlantic Ocean.
Lets melt the iron, and bring in a magnetic field.
Temp
Now, when we let the solid cool down, and take
away the external magnetic field, we have formed
a perma-nent magnet in the same direction as
external field.
Melting point
Domains
Bar Magnet
31
Earths Magnetic Field
Earths field looks similar to what wed expect
if there were a giant bar magnet imbedded inside
it, but the dipole axis of this magnet is offset
from the axis of rotation by 11.5. Also, the
south pole of this magnet is near the geographic
north pole, NG. A compass points in the direction
of the magnetic north pole, NM, around which the
field lines reenter Earths surface. (Magnetic
north is actually the south pole of Earths
magnetic dipole.) NM, which is currently located
in Greenland, drifts about over the centuries.
About every million years Earths field reverses
entirely, as we know from the orientations of
magnetic fields near the Mid-Atlantic Ridge. The
field is likely due to the motion of charged
particles in the fluid outer core, and it
protects us from an otherwise deadly solar wind.
11.5
NM
NG
µorb
32
Magnetic Fields Overview
Although the magnetic properties of electrons
must ultimately be explained with quantum
mechanics, we can think of magnetism arising
whenever we have charge in motion. This motion
can be that of an electron (either spinning or
orbiting) or it can be in the form of a current.
Remember moving charges produce magnetic fields,
and external magnetic fields exert a magnetic
force on moving charges (at least if the charge
has a component of its velocity perpendicular to
the field).
33
Magnetic Flux
Magnetic flux, informally speaking, is a measure
of the amount of magnetic field lines going
through an area. If the field is uniform, flux is
given by
?B B A B A cos?
A
The area vector in the dot product is a vector
that points perpendicular to the surface and has
a magnitude equal to the area of the surface.
Imagine youre trying to orient a window so as
to allow the maximum amount of light to pass
through it. To do this you would, of course,
align A with the light rays. With ? 0, cos?
1, and the number of light rays passing through
the window (the flux) is a max. Note with the
window oriented parallel to the rays, ? 90 and
?B 0 (no light enters the window). The SI
unit for magnetic flux is the tesla-square meter
T m2. This is also know as a weber (Wb).
?
34
Changing Magnetic Flux

• A changing magnetic flux in a wire loop induces
  an electric current.
• The induced current is always in a direction
  that opposes the change in flux.

These facts were discovered by Michael Faraday
and represent a key connection between
electricity and magnetism. One simple example of
this is a magnet moving in and out of a wire
loop. As a bar magnet approaches a wire loop
along a line perpendicular to the loop, more and
more field lines poke through the loop and the
flux increases. To oppose this change in flux a
current is induced in the direction
shown. Note that the induced current produces its
own magnetic field pointing to the right. Also
note that there is no battery in the loop! This
current will only exist when the flux inside the
loop changes. When the magnet is withdrawn the
flux decreases and current is induced in the
other direction. There is no current when the
magnet is still.
v
I
Java script
35
Induced emfs and Currents
The current induced in a loop come not from a
battery but from a changing magnetic flux. We can
think of the loop containing an imaginary battery
that gets turned on whenever flux in the loop
changes. The strength of this battery is called
the emf (electromotive force) its symbol is a
script E E, and its measured in volts. The
induced current is given by
I E / R
where R is the internal resistance in the loop.
E itself depends on the rate at which the flux
inside the loop is changing. If the flux is
changing at a constant rate,
This Faradays law. The negative sign here
indicates the emf opposes the change in flux.
E - ??B / ?t
The greater the change in flux the greater, the
greater the induced emf, and greater the induced
current.
36
Electromagnetic Induction Practice
For each scenario determine the direction of the
induced emf and current.
wire loop
. . . .
. . . .
B very large but constant
B increasing
B decreasing
B increasing
B increasing
B decreasing
37
Induction Nonuniform, Static Fields
B is static (constant in time). It is uniform in
space in the y and z directions but not in
the x direction. B decreases as x
increases. As the rectangular loop is moved in
the following directions, determine the direction
of the induced emf and current as well as the
direction of the net force on the loop by the
field.
y
x
Loop motion 1. Left 2. Right 3.
Up 4. Down 5. In 6. Out
B is uniform here but only in the region shown.
Beyond this region B is approximately zero. As
the loop is pulled out of the field determine the
direction of the induced emf and current as well
as the direction of the net force on the loop. Do
the same as the loop is pushed into the field.
38
Electric Generators
In a motor we have seen that a current loop in an
external magnetic field produces a torque on the
loop. In a generator well see that a torque on a
current loop inside a magnetic field produces a
current. In summary Motor Current
Magnetic field ? Torque Generator Torque
Magnetic field ? Current
Turbines in a power plant are usually rotated
either by a waterfall or by steam created heat
produced from nuclear power or the burning of
coal. As the turbines rotate, current loops turn
through a magnetic field to generate electricity.
This process converts mechanical energy into
electrical energy.
The simplest form of an electric generator is
called an alternating current (AC) generator. The
current produced by an AC generator switches
directions every time the wire inside of it is
rotated through a half turn. In the United
States, generator generally have a frequency of
60 Hz, 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.
Continued
39
Electric Generator (cont.)
Animation
B
Iinduced
As a turbine turns (due to some power source like
coal) a current loop (purple) is rotated inside a
magnetic field. The field is static but as the
loop turns as the number of field lines poking
through it changes. Thus we have a changing flux
and a corresponding induced emf and current. The
pic shows a loop just after it was horizontal
(perpendicular to the field). The flux is
decreasing since the loop is becoming more
vertical. Since fewer field lines are entering
the loop, the induced current is in a direction
to produce more field lines downward. Just prior
to this, as the loop was approaching horizontal,
the number of field lines inside it was
increasing, so the current was in the other
direction to oppose this change. The current
changes direction twice with each turn--whenever
the loop is horizontal. The result here is AC,
but (direct current) DC motors exist as well in
which current only flows in one direction.
40
Electric Magnetic Fields
Picture tubes in standard televisions are
basically cathode ray tubes (CRTs). In a CRT
electrons are shot from a hot filament into a
region of crossed fields in which a magnetic
field is perpendicular to an electric field. On
the other side of the crossed fields is a
fluorescent screen (not shown) where electrons
produce spots of light when they make contact
with it. J. J. Thompson used a CRT to discover
the electron in 1897. When the charge

enters the fields, FE is up and FB is down. By
adjusting B and measuring the deflection of the
electrons, Thompson determined that they were
negatively charged and calculated their mass to
charge ratio. Lets find a relationship between
q, B, and E if there is no deflection at all
? ? ? ? ? ? ?
B
? ? ? ? ? ? ?
? ? ? ? ? ? ?
? ? ? ? ? ? ?
v
? ? ? ? ? ? ?
? ? ? ? ? ? ?
q, m
E
? ? ? ? ? ? ?
? FB FE
Fnet 0
? ? ? ? ? ? ?
E
? v
- - - - - - - - - - -
? q v B q E
B
41
Credits
How speakers work http//www.geo.umn.edu/orgs/ir
m/bestiary/index.html Bestiary of magnetic
minerals http//sprott.physics.wisc.edu/demoboo
k/chapter5.htm History of magnets
http//www.webmineral.com/data/Magnetite.shtml Mag
netite http//pupgg.princeton.edu/phys104/2000
/lectures/lecture4/sld001.htm Slide show
http//www.physics.umd.edu/deptinfo/facilities/lec
dem/demolst.htm Best ever site for pictures,
simple explanations, etc. http//www.trifield.c
om/magnetic_fields.htm Another good site for how
magnets work http//bell.mma.edu/mdickins/Tech
Phys2/lectures3.html Equations and such
http//schools.moe.edu.sg/xinmin/lessons/physics/d
efault.htm
See also
http//www.micro.magnet.fsu.edu/electromag/java/in
dex.html http//www.micro.magnet.fsu.edu/electroma
g/java/detector/ 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.mag
net.fsu.edu/electromag/java/speaker/
http//hyperphysics.phy-astr.gsu.edu/hbase/magneti
c/elemag.html http//library.thinkquest.org/166
00/intermediate/magnetism.shtml http//www-geology
.ucdavis.edu/gel161/sp98_burgmann/magnetics/magne
tics.html http//www.micro.magnet.fsu.edu/electro
mag/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/d
ipole1.jpg