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Magnetism
Single moving charge and the magnetic field created.
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http://www.falstad.com/vector3dm/
The magnetic field created by a loop of wire with a
current flowing through it.
http://www.falstad.com/vector3dm/
What Causes Magnetism?
The movement of any charged
particle creates a magnetic field.
Even the spinning of a single electron as it
orbits a nucleus produces a magnetic field.
Magnetic fields of different moving
charged particles then interact to produce
forces of attraction or repulsion.
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Lodestones
• Natural Magnets
• Magnetite, Fe3O4 (an oxide of iron)
• Ancient civilizations (Greek 590 BCE, Chinese 2600 BCE) realized that these stones would cling to iron tools.
• A suspended, pivoting lodestone always pointed along the North­South geographical axis on a map.
North Side of the magnet is
the North seeking End.
Lodestones
• Magnetite crystals have been found in living organisms
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Magnetotactic bacteria!
Migratory Bird brains!!
Other migratory animals: bees, fish
Human brains!!!
YOU HAVE ROCKS IN YOUR HEAD!!!!!
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Magnetic Poles
• The force exerted by a magnet is strongest at two
points on the object.
• Poles are regions where the magnetic force is the
strongest
Like magnetic poles repel.
Opposite magnetic poles attract.
Most magnets have two poles (dipole), but can have
three or more!
Monopole? (No, not Monopoly!)
• Monopole: piece of a
magnet that is simply a
north pole or a south pole
• Many have tried to
isolate a monopole by
breaking magnets in half.
• No matter how we break
a magnet, the pieces are
always dipoles!
• A monopole cannot be
isolated.
• Do not pass GO.
• Do not collect $200.
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Every Magnet creates a magnetic field
around it.
The field lines travel from the North Pole to the
South pole outside of the magnet.
A compass is used to identify
which end of the magnet is North
and which is South.
North end is the North Seeking end.
Magnetic field lines are continuous.
The field lines then continue from the South Pole to the
North Pole inside the magnet. Lines are then continuous.
Magnetic Field Lines never cross and always form
closed loops
Drawing the Magnetic field.
N
S
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N
S
S
N
N
S
N
S
Earth’s Magnetism
• Magnetic field has reversed direction ~300 times in the past 170 million years
• Magnetic poles wander!
• Magnetic & geographic poles not the same.
• Magnetic declination: 11.5°
• What’s strange about this picture? à
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When More than one charged particle moves in
a common direction, what is formed?
The motion of the charges then create a magnetic
field. The direction of the field that is formed is
found using the 1st Right Hand rule.
First Right Hand Rule: thumb points in direction of current, fingers curl in direction of magnetic field­ note compass readings. Use for current­carrying wire.
Fig 19.15b, p.678
Slide 19
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Now that the field have gained dimension, we need a way to show them in 3D.
• Not everybody is an artist.
• Use 2­D images to draw 3­D field vectors.
• If field points perpendicularly into the page or board, use X
• If field points perpendicularly out of the page or board, use Magnetic field created in loops of wire
• How is this field different from the mag field of a straight wire?
Coil of Wire
• Why is the strength of the field more in a loop than in the straight wire?
• How is the magnetic field in a coil
different from a single loop?
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Does a charged particle at rest create a magnetic field?
Can a constant magnetic field cause a charged particle to start moving?
Does a moving charged particle experience a Magnetic Force when in a magnetic field? If so, what will that magnetic force cause the particle to do?
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A current in motion in a wire can be used for more than just making a magnetic field. The Field can be put to work.
An electric motor uses the magnetic force resulting from a charged particle moving through a magnetic field to produce motion.
The Lorentz Force in action.
http://www.walter­fendt.de/ph14e/electricmotor.htm
A charged particle moving through a magnetic field will experience a magnetic force that will cause a change in direction.
The direction of the force is found through the 3rd right hand rule.
Extend the right hand so the fingers point along the direction of the magnetic field and the thumb points along the velocity of the charge. The palm of the hand then faces in the direction of the magnetic force that acts on a positive charge. 10
B
+
v
What is the direction of the force on the proton above?
+
v
B
What is the direction of the force on the proton above?
Bx
x
x
x
x
+
x
v
x
What is the direction of the magnetic force?
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Magnetic Fields can do more than produce a force on a moving charge, as long as the field is willing to change.
If a magnet is moved through a loop of wire, the changing strength of the field will cause charges in the wire to move. A current is produced.
Faraday's Law describes this and the voltage(EMF) produced.
The voltage produced depends on the number of loops, Area of the loops, strength of the field and how quickly the field changes.
A generator uses the principles of Faraday's Law to generate a current. The current can be either DC or AC depending on the construction of the generator.
However, Faraday's Law does not determine the direction of the current produced. That is determined by Lenz's Law.
Lenz's Law states that when a current is induced by Faraday's Law, the induced current will travel in a direction as to oppose the changing external magnetic field. 12