Magnetism

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Magnetism
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Magnetism

• Known since antiquity
• Pieces of Magnetite, also called lodestone
  (Fe3O4) known by Greeks to exert both forces of
  attraction and repulsion on each other
• Chinese invented compass for navigation
• The earth exerts a force on magnetite.

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Bar Magnet
Unlike electrical charges, which can
occur individually, we always find the
magnetic poles in pairs.
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Basic model of Magnetic Materials

• All magnetic materials have two poles
• Labeled North and South poles
• Just as in electrostatics,
• Like repels like and opposites attract.
• N repels N, S repels S, N attracts S

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Magnetic Forces

• Yet another kind of force
• Known since prehistory lodestone compasses used
  in China thousands of years ago
• Bar Magnets Two poles (North South)
• Force description Like poles repel Unlike poles
  attract.
• Explanation?
• Is there a magnetic charge (analogous to electric
  charge) ?
• Try to isolate the charges cut the magnet in
  half
• In fact, no experiment to date (and there have
  been many attempts) has shown evidence for the
  existence of magnetic charge. We believe the
  source of the magnetic force is not a new kind of
  charge, but is due to motion of electric charge.

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Magnetic Monopoles

• Unlike electrostatics
• Magnetic monopoles have never been detected.
• But magnetic monopoles would resolve many puzzles
  in particle physics and cosmology

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Modern View of Magnetism(Öersted, Faraday,
Maxwell, 19th Centuryplus Quantum Mechanics
20th Century)
G.Öersted

• Magnetism is associated with charges in motion
• These currents can be microscopic currents in the
  atoms of magnetic materials.
• These currents can be macroscopic currents in the
  windings of an electromagnet.

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Magnetic Fields

• The region around a moving charge is disturbed
  by the charges motion. We call this disturbance
  a magnetic field B.
• If an isolated charge is moving, the space
  contains both an electric field and a magnetic
  field. If the charge is stationary, only an
  electric field is present.

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Magnetic Field Lines

• We can plot the magnetic field lines
  surrounding a magnetic object.
• Magnetic field lines (outside the object)
  always go from the N pole to the S pole

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The Earths Magnetic Field

• The spinning iron core of the earth produces a
  magnetic field.
• The magnetic north pole corresponds to the
  geographic south pole...

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Electric Motors
Electric Motors
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Electromagnets and Motors
http//electronics.howstuffworks.com/motor4.htm
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Armature and Commutator
http//electronics.howstuffworks.com/motor3.htm
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Armature, Commutator and Brushes
http//electronics.howstuffworks.com/motor5.htm
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Inside an Electric Motor
http//electronics.howstuffworks.com/motor1.htm
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Putting it All Together
http//electronics.howstuffworks.com/motor6.htm
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Very Simple Motor
http//www.scitoys.com/scitoys/scitoys/electro/ele
ctro.html