Magnetism

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Magnetism
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Magnetism
Since ancient times, certain materials, called
magnets, have been known to have the property of
attracting tiny pieces of metal. This attractive
property is called magnetism.
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Magnetic Poles
The strength of a magnet is concentrated at the
ends, called north and south poles of the
magnet.
A suspended magnet N-seeking end and S-seeking
end are N and S poles.
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Magnetic Attraction-Repulsion
Magnetic Forces Like Poles Repel
Unlike Poles Attract
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Magnetic Field Lines
We can describe magnetic field lines by imagining
a tiny compass placed at nearby points.
The direction of the magnetic field B at any
point is the same as the direction indicated by
this compass.
Field B is strong where lines are dense and weak
where lines are sparse.
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Field Lines Between Magnets
Unlike poles
Attraction
Leave N and enter S
Repulsion
Like poles
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Magnetic Field of a Current Carrying Wire

• A current-carrying conductor produces a magnetic
  field

Using the right hand rule the thumb is the
direction of the current and your fingers are the
direction of the fieldclockwise or
counterclockwise
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Direction of Magnetic Force
The force is greatest when the velocity v is
perpendicular to the B field. The deflection
decreases to zero for parallel motion.
Use left hand for negative charges.
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Indicating Direction of B-fields
One way of indicating the directions of fields
perpen-dicular to a plane is to use crosses X and
dots
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Practice With Directions
What is the direction of the force F on the
charge in each of the examples described below?
negative q
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Electromagnetic Induction

• Process of inducing a current in a circuit with a
  changing magnetic field
• Suppose a bar magnet is pushed into a coil of
  wire. As the magnet moves into the coil, the
  strength of the magnetic field within the coil
  increases, and a current is induced in the
  circuit. This induced current in turn produces
  its own magnetic field, whose direction can be
  found using the right hand rule
• Ways to induce current
• 1. Circuit is moved into or out of magnetic
  field
• 2. Circuit is rotated in the magnetic field
• 3. Intensity of magnetic field is varied

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3 Ways to Induce Current

• 1. Circuit is moved into or out of magnetic
  field
• 2. Circuit is rotated in the magnetic field
• 3. Intensity of magnetic field is varied

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Magnetic Force on Moving Charge
gravitational and electric field strengths are
ratios of force to magnitude of test object (mass
or charge)
magnetic field strength B (magnitude) defined as
ratio of magnetic force to magnitude of moving
charge (? component)
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29-1,2 Magnetic Force
charge moving perpendicular to B field
experiences maximum force force proportional to ?
component of velocity vector FORCE IS ALWAYS
PERPENDICULAR TO THE MAGNETIC FIELD EVEN IF
VELOCITY IS NOT
charge moving parallel to B field experiences
zero magnetic force FB
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Magnetic Force
Note that since the force is always perpendicular
to the velocity, the magnetic force does no work
and cannot increase or decrease the speed of a
charge.
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An electron in a television tube moves towards
the front of the tube with a speed of 8.0 x 106
m/s along the x-axis. Surrounding the neck of
the tube are coils of wire that create a magnetic
field of magnitude 0.025 at an angle of 90o.
Calculate the magnetic force on the electron.
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Magnetic Force
FB BIL
Magnetic force can be calculated also through a
current carrying wire
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Magnetic Domains
Domain clusters of aligned iron atoms
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Magnetic Domains
S
S
N
N
When a magnet is broken into two pieces, each
piece is an equally strong magnet.