Magnetism

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Chapter 19

• Magnetism

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19.1 Magnets

• Applications meters, motors, loudspeakers,
  sound and video equipment, picking up heavy
  loads, and MRI equipment.
• Poles end of a bar magnet (north seeking and
  south seeking)
• Like poles repel opposites attract

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Magnetic poles

• Magnetic poles can not be isolated, electric
  charges can!
• No matter how many times you cut a magnet, there
  will always be a north and a south pole.
• Magnetic poles always occur in pairs.

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• Naturally occurring magnetic material such as
  magnetite obtains its magnetism through prolonged
  exposure to the earths magnetic field.
• Unmagnetized iron can be magnetized by rubbing it
  with magnetized iron.

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Soft and Hard Magnets

• Soft magnetic materials materials such as iron
  are easily magnetized but lose their magnetism
  easily.
• Hard magnetic materials materials such as
  nickel and cobalt are difficult to magnetize but
  retain their magnetism over longer periods.

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Magnetic fields

• A magnetic field surrounds any magnetized
  material.
• The direction of the magnetic field, B, at any
  location is the direction in which the north pole
  of a compass needle points at that location.
• Dusting for fingerprints (p. 588 figure 19.3)

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19.2 Magnetic field of the Earth

• North pole of a magnet is north seeking
• South pole of a magnet is south seeking
• The geographic north pole corresponds to the
  magnetic south pole, and the geographic south
  pole corresponds to a magnetic north pole.
• Source of earths magnetic field is most likely
  do to electric current in the liquid part of the
  earths core.

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19.3 Magnetic Fields

• When moving through a magnetic field, a charged
  particle experiences a magnetic force.
• Maximum force when moving perpendicular to the
  field lines.
• F qvBsin ?
• F force, v velocity of charge, q magnitude
  of charge, B magnetic field, ? angle between
  v and B.
• Unit for B is the tesla (T)
• Ex. 19.2

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Right Hand Rule

• Hold your hand open, then place your fingers in
  the direction of B with your thumb pointing in
  the direction of v. The force F on a positive
  charge is directed out of the palm of your hand.

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19.4 Magnetic Force on a Current Carrying
Conductor

• The resultant force on the wire is due to the sum
  of the individual forces on the charged particles
  within the wire.
• F BIl sinq
• Figure 19.12
• I is current
• l is the length of wire

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19.6 Motion of a charged particle in a magnetic
field

• The magnetic force is always directed towards the
  center of the circular path.
• FqvB mv2/r