Magnetism

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Magnetism
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Magnetism

• Magnetic effects from natural magnets have been
  known for a long time. Recorded observations
  from the Greeks more than 2500 years ago.
• The word magnetism comes from the Greek word for
  a certain type of stone (lodestone) containing
  iron oxide found in Magnesia, a district in
  northern Greece.
• Properties of lodestones could exert forces on
  similar stones and could impart this property
  (magnetize) to a piece of iron it touched.
• Small sliver of lodestone suspended with a string
  will always align itself in a north-south
  directionit detects the earths magnetic field.

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Bar Magnet

• Bar magnet ... two poles N and S
• Like poles repel Unlike poles attract.
• Magnetic Field lines (defined in same way as
  electric field lines, direction and density)

• Does this remind you of a similar case in
  electrostatics?

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Magnetic Monopoles

• Perhaps there exist magnetic charges, just like
  electric charges. Such an entity would be called
  a magnetic monopole (having or - magnetic
  charge).
• How can you isolate this magnetic charge?
• Try cutting a bar magnet in half

Even an individual electron has a magnetic
dipole!

• Many searches for magnetic monopolesthe
  existence of which would explain (within
  framework of QM) the quantization of electric
  charge (argument of Dirac)
• No monopoles have ever been found!

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Source of Magnetic Fields?

• What is the source of magnetic fields, if not
  magnetic charge?
• Answer electric charge in motion!
• e.g., current in wire surrounding cylinder
  (solenoid) produces very similar field to that of
  bar magnet.
• Therefore, understanding source of field
  generated by bar magnet lies in understanding
  currents at atomic level within bulk matter.

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Magnetic Fields in analogy with Electric Fields

• Electric Field
• Distribution of charge creates an electric field
  E(r) in the surrounding space.
• Field exerts a force Fq E(r) on a charge q at r
• Magnetic Field
• Moving charge or current creates a magnetic field
  B(r) in the surrounding space.
• Field exerts a force F on a charge moving q at r
• (emphasis this chapter is on force law)

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Magnetic Materials(a simple look at an advanced
topic)

• Materials can be classified by how they respond
  to an applied magnetic field, Bapp.
• Paramagnetic (aluminum, tungsten, oxygen,)
• Atomic magnetic dipoles (atomic bar magnets)
  tend to line up with the field, increasing it.
  But thermal motion randomizes their directions,
  so only a small effect persists Bind Bapp
  10-5
• Diamagnetic (gold, copper, water,)
• The applied field induces an opposing field
  again, this is usually very weak Bind -Bapp
  10-5 Exception Superconductors exhibit
  perfect diamagnetism ? they exclude all magnetic
  fields
• Ferromagnetic (iron, cobalt, nickel,)
• Somewhat like paramagnetic, the dipoles prefer to
  line up with the applied field. But there is a
  complicated collective effect due to strong
  interactions between neighboring dipoles ? they
  tend to all line up the same way.
• Very strong enhancement. Bind Bapp 105

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Ferromagnets, cont.

• Even in the absence of an applied B, the dipoles
  tend to strongly align over small patches
  domains. Applying an external field, the
  domains align to produce a large net
  magnetization.
• Soft ferromagnets
• The domains re-randomize when the field is
  removed
• Hard ferromagnets
• The domains persist even when the field is
  removed
• Permanent magnets
• Domains may be aligned in a different direction
  by applying a new field
• Domains may be re-randomized by sudden physical
  shock
• If the temperature is raised above the Curie
  point (770 for iron), the domains will also
  randomize ? paramagnet

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Mini-quiz
1A

• Which kind of material would you use in a video
  tape?

(a) diamagnetic
(c) soft ferromagnetic
(d) hard ferromagnetic
(b) paramagnetic
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Mini-quiz
1A

• Which kind of material would you use in a video
  tape?

(a) diamagnetic
(c) soft ferromagnetic
(d) hard ferromagnetic
(b) paramagnetic
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Mini-quiz
The materials are all soft ferromagnets. The
external field temporarily aligns the domains so
there is a net dipole, which is then attracted to
the bar magnet. - The effect vanishes with no
applied B field - It does not matter which pole
is used.
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Magnetic Field Direction

• The magnetic field direction (of a magnet bar)
  can studied with a small compass.

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S
N
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Magnetic Field Lines
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S
N
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Applications A bit of history

• IBM introduced the first hard disk in 1957, when
  data usually was stored on tapes. It consisted of
  50 platters, 24 inch diameter, and was twice the
  size of a refrigerator.

It cost 35,000 annually in leasing fees (IBM
would not sell it outright). Its total storage
capacity was 5 MB, a huge number for its time!
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Magnetic Field of the Earth

• A small magnetic bar should be said to have north
  and south seeking poles. The north of the bar
  points towards the North of the Earth.
• The geographic north corresponds to a south
  magnetic pole and the geographic south
  corresponds to a magnetic north.
• The configuration of the Earth magnetic resemble
  that of a (big) magnetic bar one would put in its
  center.

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Magnetic Field of the Earth
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Magnetic Field of the Earth

• Near the ground, the field is NOT parallel to the
  surface of the Earth.
• The angle between the direction of the magnetic
  field and the horizontal is called dip angle.
• The north and south magnetic pole do not exactly
  correspond to the south and north geographic
  north.
• South magnetic pole found (in 1832) to be just
  north of Hudson bay in Canada 1300 miles from
  the north geographical pole.

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More on the Magnetic Field of the Earth

• The difference between the geographical north and
  the direction pointed at by a compass changes
  from point to point and is called the magnetic
  declination.
• Source of the field charge-carrying convection
  currents in the core of the earth.
• In part related to the rotation of the earth
• The orientation of the field flips and changes
  over time every few million years
• Basalt rocks
• Other planets (e.G. Jupiter) are found to have a
  magnetic field.

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Mini-quiz

• You travel to Australia for a business trip and
  bring along your American-made compass. Does the
  compass work correctly in Australia???

• No problem using the compass in Australia.
• North pole of the compass will be attracted to
  the South geographic pole
• The vertical component of the field is different
  (opposite) but that cannot be detected with
  normal operation of the compass.