what is Magnetism how it works

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Magnetism
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Magnets have been known for centuries. The
Chinese and Greeks knew about the magical
properties of magnets. The ancient Greeks used a
stone substance called magnetite. They
discovered that the stone always pointed in the
same direction. Later, stones of magnetite
called lodestones were used in navigation.
William Gilbert, an English physician, first
proposed in 1600 that the earth itself is a
magnet, and he predicted that the Earth would be
found to have magnetic poles.
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What is Magnetism?
Magnetism is the force of attraction or repulsion
of a magnetic material due to the arrangement of
its atoms, particularly its electrons.
All magnetic phenomena result from forces between
electric charges in motion.
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The ends of a magnet are where the magnetic
effect is the strongest. These are called
poles. Each magnet has 2 poles 1 north, 1
south.
Like repels like
Opposites attract!
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Poles of a magnet always Come in pairs! Law of
Poles
If you cut a magnet in half,
S
N
S
N
S
N
you get 2 magnets!
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No Monopoles Allowed
It has not been shown to be possible to end up
with a single North pole or a single South pole,
which is a monopole ("mono" means one or single,
thus one pole).  Note Some theorists
believe that magnetic monopoles may have been
made in the early Universe. So far, none have
been detected.
S
N
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Magnetic Fields
The region where the magnetic forces act is
called the magnetic field
Magnetic fields are vector quantities. The
direction at any location is in the direction
that the north pole of a compass would point if
at that location
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Magnetic field lines represented by iron filings
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Field Lines Around a Bar Magnet
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Field Lines of Attracting Bars
Field Lines of Repelling Bars
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• Atoms themselves have magnetic properties due
• to the spin of the atoms electrons.

• Groups of atoms join so that their magnetic
  fields
• are all going in the same direction

• These areas of atoms are called domains

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When an unmagnetized substance is placed in a
magnetic field, the substance can become
magnetized. This happens when the spinning
electrons line up in the same direction.
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An unmagnetized substance looks like this
While a magnetized substance looks like this
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How to break a magnet
1. Drop it
2. Heat it
This causes the domains to become random again!
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Magnetic Field Vectors Due to a Bar Magnet
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N
S
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Magnetic Field Lines

• The direction of the magnetic field at any point
  is
• tangent to the magnetic field line at that point.

• the direction that the north pole of a compass
  would point if a compass were at that location

-defined as the direction of motion of a charged
particle on which the magnetic field would not
create a force.
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Electromagnetism
Up until 1820 everyone thought that magnetism and
electricity were completely separate. But in that
year, the Danish physicist Hans Oersted
(1777-1851) discovered that a compass needle was
deflected by an electric current.
Magnetic Fields are VECTOR quantities! They are
referred to as a B Field and B is the symbol
used. These fields have effects on charged
particles.
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• What does a charged particle feel in a magnetic
  field
• If the charge is NOT moving there is NO force
  acting on the particle.
• If the charge is MOVING ALONG a field line there
  is NO force acting on the particle.
• If the charge is MOVING ACROSS
• a field line, it feels a FORCE

v
F
B

N
S
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Magnetic fields produce forces on moving charged
particles. The forces are perpendicular to both
the velocity of the particle and the direction of
the magnetic field
The size of the force is proportional to the
intensity of the field and the speed with which
the particle is cutting across
F
v
F
v
b
b
Note The direction of the field and the velocity
determine a plane. The force is perpendicular to
that plane
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The RIGHT HAND RULE
Hold your right hand with your index finger
straight out, your middle finger 90o from the
index finger and your thumb straight up. Keep
this orientation!
Your index finger represents the velocity of the
positively charged particle, your middle finger
points the direction of the magnetic field (from
the north end of a magnet) and your thumbs shows
the direction of the force applied to that
positively charged particle.
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Field Vectors
Right
Up
Out of Page
x
Down
Into Page
Left
The convention of showing three dimensions on a
two dimensional page.
Examples Find the resultant force under the
given conditions
v
B
F
x
x
x
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Magnets exert forces on moving particles.and as
Oersted showed, moving charges also created
magnetic fields and thats what deflected
Oersteds compass.
To examine the simplest case, pass a current
carrying wire straight through a plane covered
with compass needles.
The needles line up in circles around the wire
The magnetic field of a current is circular
centered on the wire and lying on a plane
perpendicular to the current.
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You can find the direction of the magnetic field
in a current carrying wire by pointing your thumb
of your right hand along the direction of the
flow of positive charges. Your fingers curl in
the direction of the magnetic field.
This is known as the RIGHT HAND RULE for current
carrying wire
If you look at the negative charges flowing than
use the left hand rule.
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Two parallel currents attract each other. The
magnetic field circling each wire causes forces
on the current in the other wire, pulling it
closer.
Andrea-Marie Ampere, discovered the force between
parallel wires
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If a current carrying wire is bent into a circle,
a magnetic field is produced.
Notice that one side looks just like a NORTH POLE
the field lines are coming out.
The other side looks like a SOUTH POLE with
field lines going in.
N
S
By winding many turns, the magnetic field is made
proportionally larger. By winding turns along a
cylinder, a solenoid coil is produced, with a
magnetic field just like a bar magnet
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Inserting an iron bar into the coil concentrates
and strengthens the magnetic field, the result is
an electromagnet.
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Faraday Induction
For 12 years after Oersteds discovery
electricians looked for the complimentary
effect.
How to make a magnetic field produce a current?
In 1832 it was Michael Faraday that suggested
moving the magnet!
Thrusting a magnet through a loop of wire
connected to a sensitive ammeter, a Galvanometer,
deflects the galvanometer needle. Thus showing a
current being induced.
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When the magnet is held still, the meter
registers no current.
Faraday described this effect by saying that
ELECTROMOTIVE FORCES (EMF) are generated in the
wire whenever magnetic field lines cut across the
wire. This is actually not a force but a
potential difference measured in volts
It does not matter whether the magnetic field
moves or the wire moves with respect to the
magnet.
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When the magnet is thrust into the loop. Its
field lines cut across the wire, generating an
EMF that produces a current.
The same is true when the loop is moved over the
magnet
Although Faradays discovery was at first
received with indifference, today nearly all our
electrical power is generated by moving giant
coils of wire near magnets.
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Induction without Magnets
Another way to induce a current in a wire is to
place a second loop of wire nearby the first and
energize it with a power source.
When a current in the second loop is switched on
or off, a current pulse is induced in the first.
But when the current in the second loop is
steady, no current is induced in the first loop
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In the case of the two wire loops, when the
current is first turned on in one loop, magnetic
field lines build up, cutting across the other
loop and producing an EMF.
When the current is switched off, the field
collapses, again cutting across the loop.
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Lenzs Law
The induced emf creates a current that itself
creates a secondary magnetic field. This
secondary magnetic field also changes with time
and thus creates a changing secondary magnetic
flux. The secondary flux changes in such a way to
opposes the change in flux creating the emf.
Normally this means that the secondary magnetic
field increases or decreases in such a way as to
oppose the change in the magnetic field creating
the induced emf.
Induced current flows in a direction to oppose
the charge that produced it
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ElectroMagnetic Equations
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Magnetic Field
F q v B sinq
F gt Force (N)
q gt Charge (c)
v gt Velocity of Charge particle (m/s)
B gt Magnetic Field (N/Am T Tesla)
q gt Angle between v and B
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Magnetic Forces on Current Carrying Wire
F I L B sinq
F
F
B
I
F gt Force (N)
v
v
L
I gt Current (A)
L vt (m) gt Length of wire a charge would
move in a given time
B gt Magnetic Field (T Tesla)
qgt Angle between v and B
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Magnetic Field of an Ideal Solenoid
B m0 n I
B gt Magnetic Field (T Tesla)
m gt 4p x 10 -7 (Tm/A)
n gt Linear Turn Density (N/L) of turns per
meter
I gt Current (A)
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Induced Electromotive Forces - EMFs
Magnetic Flux A relative measure of the number
of field lines passing through an area
F B A cosq
Axis of Rotation
F gt Magnetic Flux (Tm2 wb (weber))
B gt Magnetic Field (T Tesla)
A gt Area Vector (m2)
q gt Angle between A and B
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I
II
III
IV
B
A
A
q
A
A
I II If B and A are parallel, q 0o or 180o
then the Magnetic Flux is at maximum. F B A
cos 0 B A gt maximum of lines through the
loop
III If B and A are perpendicular, q 90o than
the Magnetic Flux is at minimum or zero. F B
A cos 90 0 gt No lines pass through the loop
IV If B and A are between parallel and
perpendicular then there is a partial Magnetic
Flux. F B A cos q gt maximum of lines gt F gt
0
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Faradays Law of Induction
The EMF induced in a coil of N loops depends on
the time rate change of the number of filed lines
through the loop.
Time Rate Change of the Magnetic Flux
x -N DF/D t
x gt Electromotive Force (v)
N gt Number of loops in the wire
DF gt Change in the flux through one loop (Tm2
wb)
D t gt Time (s)
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Faradays Law of Induction
The induced electromotive force (EMF) in any
closed circuit is equal to the time rate change
of the magnetic flux through the circuit.
Or alternatively,
The EMF generated is proportional to the rate of
change of the magnetic flux.
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Faradays Law of Induction
The induced electromotive force (EMF) is not
actually a force but a measure of potential
difference. It is measured in volts (V)
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Generators

• Generator converts mechanical energy into
  electrical energy (AC current)
• A loop of wire is rotated between the poles of a
  magnet by a power source (in this case by the
  water) and the loop moves through the field of
  the magnet
• Thus there is a change in the magnetic field
  resulting in an induced current through the wire

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Transformers
A transformer is a static device that transfers
electrical energy from one circuit to another
through inductively coupled conductorsthe
transformer's coils. A varying current in the
first or primary winding creates a varying
magnetic flux in the transformer's core and thus
a varying magnetic field through the secondary
winding. This varying magnetic field induces a
varying electromotive force (EMF) or "voltage" in
the secondary winding. This effect is called
mutual induction.
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Step-Up Transformers

• If there are more loops in the secondary coil the
  voltage in the second coil is greater
• This increases the voltage, making it a step-up
  transformer
• Used by power companies to transmit high-voltage
  electricity as well as fluorescent light and
  X-rays

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Step-Down Transformers

• In a step-down transformer, there are more loops
  in the first coil than the second coil
• The voltage in the second coil is less than the
  first coil
• Used to lower the voltage of electricity before
  it can be used in homes or offices as well as
  doorbells, small radios, and calculators

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Electric Motor
An electric motor, is a machine which converts
electrical energy into mechanical (rotational or
kinetic) energy.    A current is passed through
a loop which is immersed in a magnetic field. A
force exists on the top leg of the loop which
pulls the loop, while a force on the bottom leg
of the loop pushes the loop.
The net effect of these forces is to rotate the
loop.
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DC Motor
DC motors are in many ways the simples electric
motors. All DC "brushed" motors operate in the
same way. There is a stator (a larger stationary
part) and a rotor (a smaller part spinning on an
axis within the stator). There are magnets on the
stator and a coil on the rotor which is
magnetically charged by supplying current to it.
Brushes are responsible for transferring current
from the stationary DC voltage source to the
spinning rotor. Depending on the position of the
rotor its magnetic charge will change and produce
motion in the motor. The animation below further
explains the basic operation of a DC motor.
Utilizing a DC power source, very few controls
are needed. To control speed an inline variable
resistance can be utilized to change the amount
of current reaching the coils.
The animation to the shows a DC motor in
operation. The motor shown is a simplified
"two-pole" motor which uses just two magnets in
the stator. In this case the magnets in the
stator are permanent magnets for the sake of
simplicity. The brushes deliver current from a DC
voltage source which supplies a magnetic field to
that end of the rotor. The polarity of the field
depends on the flow of the current. As the rotor
turns the brushes make contact with one side of
the DC source, then briefly do not make contact
with anything, then continue making contact with
the other side of the DC source effectively
changing the polarity of the rotor. The timing of
this change is determined by the geometrical
setup of the brushes and leads to the DC source.
The animation helps to illustrate how at the
moment of maximum attraction the current will
change direction and thus change the polarity of
the rotor. At this moment the maximum attraction
suddenly shifts to maximum repulsion which puts a
torque on the rotor's shaft and causes the motor
to spin.
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Top Ten List
1. Magnets make modern life possible. 2. There
are North Poles and South Poles. 3. Like poles
repel, unlike poles attract. 4. Magnetic forces
attract or repel only magnetic materials. 5.
Magnetic forces act at a distance. 6. While
magnetized, temporary magnets act like permanent
magnets. 7. A charged particle experiences no
magnetic force when moving parallel to a magnetic
field, but when it is moving across a field it
experiences a force perpendicular to both the
field and the direction of motion.
8. A current-carrying wire in a perpendicular
magnetic field experiences a force in a direction
perpendicular to both the wire and the field. 9.
Magnetic Flux is the relative number of magnetic
field lines passing through an area. 10. The EMF
induced in a coil of N loops depends on the time
rate change of the number of filed lines through
the loop.
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Mr. McMullen, may I be excused? My brain is
full