Magnetism and Electromagnetism

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Magnetism and Electromagnetism

• SPH4C

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Permanent Magnets

• A permanent magnet has two poles North and South.

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Permanent Magnets

• A permanent magnet has two poles North and
  South. Like poles repel. Unlike poles attract.

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Permanent Magnets

• These repulsive or attractive forces can act at a
  distance (no contact is required). The region in
  space over which these forces can act is called a
  magnetic field.

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Magnetic Fields

• The iron filings in the picture below show the
  magnetic field lines.

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Magnetic Fields Are Different

• Magnetic lines are drawn out of the North Pole
  and into the South Pole but they dont stop and
  start there the magnetic field lines are drawn
  through the poles.

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No Monopoles

• Every magnet is a dipole it must have two poles.
  If a dipole magnet is broken in two,

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No Monopoles

• Every magnet is a dipole it must have two poles.
  If a dipole magnet is broken in two, it becomes
  two dipoles

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Many Dipoles

• Why does this happen?

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Many Dipoles

• Why does this happen?
• A bar magnet is made up of many smaller dipoles,
  each with North and South poles, all aligned.

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Many Dipoles

• Why does this happen?
• A bar magnet is made up of many smaller dipoles,
  each with North and South poles, all aligned.
• The dipoles may be knocked out of alignment by
  heating or otherwise abusing the material.

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Repairing Magnets

• A bar magnet may be re-magnetized by placing it
  in a magnetic field. This is induced magnetism.

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Ferromagnetic

• Not all materials may be easily magnetized.
  Those that can are called ferromagnetic. They
  include iron ore (lodestone), cobalt, zinc, and
  nickel.

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Rare-Earth Magnets

• The strongest permanent magnets are made from
  rare earth (lanthanoid) elements, the strongest
  of these being neodymium-iron-boron (NIB) magnets,

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Rare-Earth Magnets

• The strongest permanent magnets are made from
  rare earth (lanthanoid) elements, the strongest
  of these being neodymium-iron-boron (NIB)
  magnets, now greatly reduced in price and used in
  childrens toys.

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Electromagnets

• Current (moving charge) will also produce a
  magnetic field. This is called Oersteds
  Principle.

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Showing Directions

• To show that a current, field line, or force is
  directed out of the page (towards us), we draw
• To show that a current or field line is directed
  into the page, we draw

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Right-Hand Rule 1

• When the thumb is pointed in the direction of
  conventional current flow, the fingers curl in
  the direction of the magnetic field.

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Solenoids

• To strengthen and straighten the magnetic field,
  we coil the current-carrying wire into a solenoid.

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Right-Hand Rule 2

• When the fingers are curled in the direction of
  conventional current flow, the thumb indicates
  the direction of magnetic North.

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Magnetic Forces

• Since current will produce a magnetic field, the
  interaction of this field with an external
  magnetic field will result in a force acting on
  the moving charge. This is the Motor Principle.

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Magnitude of Force

• The magnitude of the magnetic force FM on a
  current-carrying wire is directly proportional to
  the current I and length L of the wire and to the
  magnitude of the external magnetic field B.

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B

• Note that B, the magnetic field, is measured in
  Tesla (T).

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B

• Note that B, the magnetic field, is measured in
  Tesla (T). 1 T is a very strong magnetic field.

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Magnitude of Force

• The magnitude also depends on the angle q between
  the magnetic field vector and the current vector.
  
• When q 90o, (the current is moving
  perpendicular to the field), the force is
  maximum.
• When q 0o or 180o (the current is moving
  parallel to the field), the force is zero.

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Direction of Force Right-Hand Hule 3

• The direction of the force is given by another
  right-hand rule if the right thumb follows the
  direction of conventional current flow and the
  extended fingers point in the direction of the
  magnetic field, the force is in the direction the
  palm would push.

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Quick Quiz

• What is the direction of the force (if any) on
  the particle in each of the following cases?

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More Practice

• Electromagnets Lab Activity