MAGNETISM

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MAGNETISM
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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 North-South axis

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Lodestones

• Magnetite crystals have been found in living
  organisms
• Magnetotactic bacteria!
• Migratory Bird brains!!
• Other migratory animals bees, fish
• Human brains!!!
• YOU HAVE ROCKS IN YOUR HEAD!!!!!

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Lodestones
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Permanent Magnets

• By 2nd Century AD, Chinese were able to make
  permanent magnets by repeatedly stroking an iron
  rod or needle from end to end along a lodestone,
  but always in the same direction.
• Retained strength of a magnet depends on chemical
  properties of the metal.
• Soft iron loses magnetism quickly
• Low-carbon soft steel (paper clips, nails)
  gradual loss
• Hard steel retains power for a long time and is
  referred to as a permanent magnet

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Magnetic Poles

• Magnets produce a force on other objects
• 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!

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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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Magnetic Field

• Every magnet establishes in the space surrounding
  it, a magnetic field (B-field)
• Map field with a test-compass
• Direction of field is direction in which the
  test-compass needle will point at that location.
• Draw field lines so that compass always points
  tangent to the field lines.
• Field lines point from N to S outside the magnet
• Field lines point from S to N inside the magnet
• Field lines form closed loops
• Field lines never intersect
• SI unit for B (magnetic field strength) is the
  tesla (T)

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Magnetic Field Lines
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Magnetic Field
Mapping with Test-Compass
Field Lines Form Closed Loops
Field Mapped by Iron Filings
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Earths 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
• Whats strange about this picture? ?

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Magnetism on an Atomic Level

• Charge in motion (electric current) produces
  magnetic force
• Electrons function as a subatomic dipole
• Electron spin
• Electrons existing in pairs B-fields cancel
• Electron orbit around nucleus
• Random orbits of electrons B-fields cancel

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Magnetic Field of a Moving Charge

• mo 4p x10-7 Tm/A

• There is magnetic field everywhere were charged
  particles are moving
• Moving charged particle has both electric and
  magnetic fields

B
R
V
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Diamagnetism

• Even non magnetic materials respond to an
  applied B-field
• Applied B-field changes orbital motion of
  electrons
• Produces a field that opposes applied field
• Repelled by applied field
• Diamagnetic materials have no permanent atomic
  dipoles
• Occurs for all substances, but may be swamped by
  other magnetic effects

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Paramagnetism

• Paramagnetic materials are attracted when placed
  in a strong B-field.
• Composed of atoms with permanent atomic dipoles
• Atomic dipoles do not interact w/ one another
• Atomic dipoles oriented randomly
• Material has no dipole as a whole
• A strong B-field re-orients these atomic dipoles
  in same direction as applied field

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Ferromagnetism

• Naturally magnetic magnetite, iron, nickel,
  cobalt, steel, Alnico, other alloys
• Strongly attracted to poles of a magnet
• Easily magnetized
• Atomic dipoles interact strongly with dipoles of
  adjacent atoms
• Dipoles align spontaneously, w/o an applied field
• Many atomic dipoles cooperatively align
• Creates regions of parallel orientations (domains)

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Magnetic Domains

• Domain region where many atomic dipoles are
  aligned
• Usually aligned randomly and effects cancel
• BUT
• Place ferromagnetic material in strong B-field
• Entire domains realign with applied field
• Size shape of domains remains the same
• Causes irreversible re-orientation of domains
• Creates permanent magnets

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Reorientation of Domains
Electrons in domains align with applied field
Substance is Permanently Magnetized
Domains are not aligned
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Electrodynamics The Study of Electromagnetism

• Magnetism is caused by charge in motion.
• Charges at rest have just an electric field
• But, when they move, they generate both an
  electric field and a magnetic field
• Can look at individual charges or electric
  current in a wire
• Direction of current determines direction of the
  magnetic field.
• Use right hand rules for analysis.

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Magnetic Field of a Straight Wire
B mI/2pR
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First Right Hand Rule thumb points in direction
of current, fingers curl in direction of magnetic
field- note compass readings. Use for
current-carrying wire.
Fig 19.15b, p.678
Slide 21
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2nd Right Hand Rule- Fingers curl in direction of
current, thumb points to direction of magnetic
field. Use for current-carrying loop or solenoid
coil.
B
Fig 19.20b, p.682
Slide 22
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3rd Right Hand Rule

• Gives the direction of the FORCE exerted on a
  current (or charge) by an external magnetic field
• Point fingers of RH in direction of current (or
  motion of charge)
• Curl fingers through smallest angle to direction
  of magnetic field
• Thumb indicates direction of the force

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3rd Right Hand Rule
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3rd Right Hand Rule
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3rd Right Hand Rule
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Magnetic Force on a Moving Charge

• F Bqvsin T
• B field strength in teslas (T)
• q charge in coulombs (C)
• v charge velocity in m/s
• T angle between v B

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Magnetic Force on a Current-Carrying Wire

• F BIL
• B field strength in teslas (T)
• I current in amperes (A)
• L length of current-carrying wire in meters (m)

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Magnetic field of a long straight wire

• B magnetic field strength (teslas)
• I current (amperes)
• r radius from wire (meters)
• µo permeability constant in a vacuum
• µo 4p x 10-7 Tm/A
• What is the shape of this magnetic field?

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Magnetic field of a loop of wire carrying current

• How is this equation different from the mag field
  of a straight wire?
• The strength of the field is more in a loop than
  in the straight wire.

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Magnetic field of loops of wire (or a coil)
carrying current
where n is the NUMBER of loops (in this example
n8)

• How is this equation different from the mag field
  of a single loop?
• The strength of the field is more in a coil than
  in a single loop.

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Diagramming 3-D Magnetic Fields

• Not everybody is an artist.
• Use 2-D images to draw 3-D field vectors.
• If field points perpendicularly into the page or
  board, use
• If field points perpendicularly out of the page
  or board, use
• Otherwise, draw the lines neatly.
• Dont forget, field lines are vectors!

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