Magnetism

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Magnetism
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What are magnets?

• Lets first start off with what causes an
  magnetic field
• A magnetic field is created around any moving
  charged object.

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What is charged that is moving within an atom?

• Electrons (e-)
• The atoms within most materials have paired up
  electrons spinning in opposite directions so the
  magnetic field that is created by one is
  cancelled out by the other.

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• Some materials like iron, nickel, and colbalt
  have a single electron or pair of electrons that
  spin in the same direction creating a magnetic
  field or a small atomic magnet.

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• The atomic sized magnets line up to create
  domains within the material.

The individual domains line up to form a magnet.
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A magnet has two ends called poles (dipoles),
where the magnetic force is the strongest.

• No matter how many times a magnet is broken, each
  piece always has a north pole and a south pole.
• There is no such thing as a monopole.

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A magnetic field exists around a magnet or any
moving charged object.

• Magnetic fields are like electric fields or
  gravitational fields in that they allow magnets
  to interact without touching.

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Imaginary lines that map out the magnetic field
(B) around a magnet are known as magnetic field
lines or magnetic flux lines.
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Rules for drawing magnetic field lines (same as
an electric field)

• 1. Magnetic field lines always go from the north
  pole to the south pole (outside a magnet).

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The direction of the magnetic field is determined
by using a compass. The needle of the compass
points in the direction of the magnetic field.
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Rules for drawing magnetic field lines
2. Magnetic field lines are closed loops and
never cross or intersect.
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Rules for drawing magnetic field lines
3. Where the magnetic field lines are closer the
magnetic field is stronger.
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Draw magnetic flux lines around the magnets below
S
N
S
N
S
N
S
N
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Magnetic Field Strength (B)
B F qv
The number of magnetic lines of flux per unit
area passing through a plane perpendicular to
the direction of the lines is called the
magnetic field strength (B).
The magnetic field strength is a vector like
electric field strength.
The weber (Wb) is the unit used for measuring
the number of lines of flux.
1 Wb 1 Tesla (T) m2
Teslas are used as the unit for magnetic field
strength (magnetic flux density).
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Feeling the strongest magnetic force
An object that enters this magnetic field will
feel a maximum force only if it is moving
perpendicular to the flux lines.
F 0
Fmax
It will feel no magnetic force if is is moving
in line, or parallel, with the flux lines.
B
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Magnetism

• Magnetism is the force of attraction or repulsion
  between magnetic poles.

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Magnetic Field around the Earth
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What we do know
The north magnetic pole and the geographic North
Pole do not coincide. The magnetic pole is about
1500 km (930 mi) south of the geographic North
Pole and it wanders.
A compass actually indicates the direction of
magnetic north, not true north. Therefore a
navigator must need to know the magnetic
declination for a specific area. This is the
angular difference between magnetic and true
north.
The details and mechanisms of how and why the
Earth has a magnetic field are unclear. They
do believe that it may have something to do with
motions in the liquid outer core.
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Charges moving through a wire
Remember what creates magnetic fields- a
charged object in motion.
Current is the flow of electrons. A moving
charged object.
Current in a wire produces a magnetic field
around the wire.
A compass can be used to detect a magnetic field
around a wire that had current flowing through
it.
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LEFT HAND RULE Helps us to analyze the path of
a charged object in a magnetic field.
Which way will the magnetic field below push this
electron traveling through the field?
S
N
B
e-
Thumb is aligned with the direction of current
(I) or movement.
We use the LHR for electron flow.
Fingers are aligned with the direction of the
magnetic field (B).
Your palm indicates the direction the object
would move do to the magnetic force.
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LHR Symbols
Sometimes the magnetic field or current is going
into or out of the page. For this we us the
following symbols
Coming out of page
Going into page
WIRE
FIELD
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LHR with wires
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Effects of two wires
1. Two wires next to each other that both have
currents in the same direction.
WIRES WILL ATTRACT EACH OTHER
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Effects of two wires
2. Two wires next to each other that have
currents in opposite directions.
WIRES WILL REPEL EACH OTHER
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Effects of two wires
3. Two wires with currents flowing perpendicular
to each other.
NO INTERACTION, F0
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Practice
The diagram below shows an end view of a current
carrying wire between the poles of a magnet.
The wire is perpendicular to the magnetic field.
F
If the direction of the electron flow is into the
page, what is the direction of the magnetic
force (F) acting on the wire?
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Practice
The wire below is moved toward the right through
a magnetic field. In which direction will the
magnetic force push the wire?
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Looped wire
Whats the magnetic field around a loop of wire?
Wheres the north pole created?
The north pole is located where your finger
tips end up.
e-
Behind the wire.

-
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Whats the magnetic field around a loop of wire?
Wheres the north pole created?

-
e-
Above the wire.
Looping a wire around several times into a coil
will produce an
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Electromagnet
An electromagnet is a type of magnet whose
magnetic field is produced by the flow of
electric current. The magnetic field disappears
when the current ceases.
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• An electromagnet is most commonly made by coiling
  wire around a piece of iron. This electromagnet
  is called a solenoid. The shape of the magnetic
  field is the same as a bar magnet.

As electrons move through the coil of wire, the
magnetic field of one electron adds to the field
of any others moving in the same direction.
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Factors affecting the magnetic field of a
solenoid

• The magnetic field around a solenoid is directly
• related to the current through the coil.

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Factors affecting the magnetic field of a
solenoid
2. The magnetic field around a solenoid is
directly related to the number of turns or
coils around the solenoid.
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Factors affecting the magnetic field of a
solenoid
3. The magnetic field around a solenoid is
directly related to the material around
which the wire is coiled. The soft iron is
more permeable to the magnetic field than
the air is.
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Determining the poles of a solenoid

• LHR Modified
• Have palm away from you.
• Point fingers of left hand in
• the direction of the current
• (many coils, many fingers).
• 3. Stick thumb out and thats
• where the north pole is.

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Determine the poles of this solenoid
S
N
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Electromagnetic Induction
This is the process of generating a potential
difference (voltage) in a conductor (wire) due
to the motion of the conductor in a magnetic
field.
Generating a voltage in a wire would mean
generating areas of uneven charge within a wire.
How can this be done?
What can be forced to move within a wire that
will create a potential difference?
Moving a wire through a magnetic field will cause
a magnetic force which will move electrons
towards one end of the wire creating a potential
difference.
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Lets take a closer look
Motion of wire
B
-
-
-
-
-
wire
-
If this wire is moved up or down (perpendicular
to the field), a voltage will be generated and
if the wire is part of a complete circuit then
current is induced.
If this wire is moved left or right (parallel to
the field), no current will be generated because
no force is generated to move electrons and
create a voltage.
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Other ways to induce voltage
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This is how Niagara Falls was used to produce
electricity
Water flow is used to turn a turbine through a
magnetic field inducing a potential difference
and producing electrical current.
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This is how an electric motor works
An electric motor utilizes the property of
electromagnetic induction to convert electricity
into mechanical energy to make things move. The
conductor itself, a coiled wire, will move to
oppose the magnetic field. Just when it gets
into position the current is reversed, and the
coil spins round and round and round full of
mechanical energy.
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A Generator
A generator is simply the same process in
reverse, converting mechanical energy into
electricity. Almost all of the electrical energy
we use in our daily lives is supplied by
electric generators.
http//www.stmary.ws/highschool/physics/home/java/
fendt/ph11e/generator_e.htm
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• http//www.physics.sjsu.edu/becker/physics51/mag_f
  ield.htm

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• http//www.regentsprep.org/Regents/physics/phys03/
  cintromag/sld001.htm