Magnetism

Document Sample
Magnetism Powered By Docstoc
					 Good morning!
   You will need:
     Your Notes
Worksheet by the door
•   Notes – Magnetism
•   Video clips
•   Magnetism Wkst
•   Homework – Finish W/S
    – Magnetism Quiz Next Class
    – Test on March 31 – April 1
           History of Magnets
• More than 2000 years ago, rocks called
  lodestones were found in the region of
  Magnesia in Greece.
• In the 12th century, the Chinese used them for
  navigating ships.
What are magnets?


• Most materials
  – Have paired up electrons moving in opposite
    directions.
  – The field created by one moving charge is
    canceled by the other.
  – No magnetic field is created.
What are magnets?

• Any charges in motion
  produce a magnetic field.

• Some materials like Iron, Nickel, or Cobalt
  – Have a single electron or paired electrons spinning
    in the same directions.
  – The magnetic field created by one electron is not
    canceled by the other.
  – An atomic sized magnet is created.
   Why is Fe magnetic and Al not?
• What makes a good magnet?
  – Every spinning electron is a tiny magnet.
  – A pair of electrons spinning in the same direction is a stronger
    magnet.
  – A pair of electrons spinning in opposite directions work against
    one another; the magnetic fields cancel.
• Fe has two unpaired electrons spinning in the
  same direction.
  – Cobalt has 3.
  – Nickel has 4.
  – Aluminum has one unpaired electron.
          Temporary Magnets
• What happens when you place a magnet next
  to a nail?
• This is because the magnet causes the nail to
  become polarized; the nail becomes a magnet.
• This is temporary; if you pull the
  magnet away, the nail loses its
  magnetism.
         Permanent Magnets
• Permanent magnets are produced in the same
  manner as the nail; however, due to the
  microscopic structure of the material, the
  magnetism becomes more permanent.
• Most permanent magnets are made of ALNICO,
  an iron alloy containing 8% Aluminum, 14%
  Nickel, and 3% Cobalt.
• Some rare earth elements, such as neodymium
  and gadolinium, produce strong permanent
  magnets.
           Magnetic Domains
• In magnetic materials, neighboring atoms pair
  up to form large groups of atoms whose net
  spins are aligned.
• These groups are called domains.
• When a piece of iron is not a magnet, the
  domains point in random directions.
              Magnetic Domains
• If the non-magnetized iron is placed in a strong
  magnetic field, the domains will line up in the
  direction of the field.
• In temporary magnets, the domains will return to
  their random orientation after the field is removed.
• In permanent magnets, the domains will remain
  aligned.
• 1 domain = 1 quadrillion (1015) atoms
                 Magnetic Poles
• All magnets have two regions “poles” that produce
  magnetic forces.
• They are named like this because if you take a
  magnet and suspend it from the middle (so that it
  can swing freely), it will rotate until the north pole
  of the magnet points north and the south pole
  points south.
• Like poles repel.
• Opposite poles attract.
            No less than two.
• North and South cannot be separated.
• If a magnet is broken, poles aren’t separated;
  two smaller magnets are formed.
Magnetic Fields

• You have probably noticed that forces
  between the magnets (both attraction and
  repulsion), are felt not only when the magnets
  are touching each other, but also when they
  are held apart.
• In the same way that gravity can be described
  by a gravitational field, magnetic forces can be
  described by the magnetic fields around
  magnets.
         Magnetic Field Demo
• What kinds of magnetic fields are produced by
  pairs of bar magnets?
                       Magnetic
                      Field Lines



• The shape of the
  magnetic field is
       revealed by
    magnetic field
             lines.
           Magnetic Field Lines
• Magnetic field lines are the same as
  electric field lines in that both are stronger
  when lines are drawn closer together.
  – So the magnetic field is stronger at the poles
• The magnetic field lines have arrows going
  from north to south.
• Magnetic field lines do not cross because the
  magnetic field cannot go in two directions at
  once.
           Common Uses of Magnets
• Magnetic recording media: VHS tapes, audio cassettes, floppy disks, hard
  disks.
• Credit, debit, and ATM cards
• Common television and computer monitors
• Speakers and Microphones
• Electric motors and generators
• Compasses
• Magnets can pick up magnetic items (iron nails, staples, tacks, paper clips)
  that are either too small, too hard to reach, or too thin for fingers to hold.
  Some screwdrivers are magnetized for this purpose.
• Magnets can be used in scrap and salvage operations to separate
  magnetic metals (iron, steel, and nickel) from non-magnetic metals
  (aluminum, non-ferrous alloys, etc.).
• Magnetic levitation transport, or maglev, is a form of transportation that
  suspends, guides and propels vehicles (especially trains). The maximum
  recorded speed of a maglev train is 361 mph.
   How to demagnetize a magnet
• Heating a magnet past its Curie temperature -
  the molecular motion destroys the alignment
  of the magnetic domains.
  – 768°C for Iron
• Hammering or jarring –
  the mechanical disturbance tends to
  randomize the magnetic domains.
• Placing the magnet in an alternating magnetic
  field.
                Ferrofluid
  • a mixture of tiny iron particles covered with a
liquid coating that are then added to water or oil.
      • Used in car suspensions, cancer detection,
                                      loud speakers
                                            • video
    Ferrofluid in Acura:

http://www.youtube.com/watch?v=r
           nZ4EyKrLAI
           Earth’s Magnetic Field
• Earth is a huge
  magnet.
• This is possibly
  due to the
  molten Iron core.
• The magnetic
  field around Earth
  is called the
  Magnetosphere
            Earth’s Magnetic Field
• Magnetic north pole is different than geographical north pole.
• There is about a 25 ̊difference from geographic north pole to
  magnetic north pole, this is called magnetic declination
• In addition, the north pole
  of a magnet is attracted to
  earth’s north pole because that
  is the magnetic south pole.
• The south pole of a magnet
  is attracted to the earth’s
  south pole because that
  is the magnetic north pole.
               Magnetosphere
• Extends several tens of thousands of km into space.
• Protects Earth from solar winds.
             Dynamo Theory
• The dynamo theory proposes a mechanism by
  which a celestial body such as the Earth
  generates a magnetic field.
• In the case of the Earth, the
  magnetic field is induced and
  constantly maintained by the
  convection of liquid iron in the
  outer core.
  Magnetic field of Earth is not stable
• The magnetic poles of Earth wander up to 15 km
  every year.
• Based upon the study of lava flows throughout
  the world, Earth's magnetic field reverses at
  intervals, ranging from tens of thousands to
  many millions of years, with an average interval
  of approximately 250,000 years.
• The last reversal is theorized to
  have occurred 780,000 years ago.
    Other Planets’ Magnetic Fields
• The sun also has a strong magnetic field.
  Evidence seen in sun spots.
  – They occur in pairs.
  – They also peak at an 11-year cycle, which
    coincides with the flipping of the sun’s magnetic
    field.
• Jupiter’s magnetic field is 10 times the
  strength of Earth’s.
• The moon has no magnetic core, and hence,
  no magnetic field.
                       Auroras
• Charged particles from the sun become
  trapped in Earth’s magnetic field.
• This occurs near the magnetic poles.
• These charged particles collide with electrons of
  the atoms in our atmosphere and transfer their
  energy.
• The colors of the lights are determined by the type
  of gases in the atmosphere.
  – O2 releases green light; N2 releases red light
• aurora borealis (northern lights); aurora australis
  (southern lights)
             Neutron Stars

• The most intense magnetic field ever found in
  the universe has been observed around a
  neutron star 40,000 light years from Earth.
• Neutron stars are compact objects that are
  created during supernova explosions.
• The magnetic field of a Neutron Star is
  estimated to be one thousand trillion times
  the strength of Earth's magnetic field.
                 Animal Migration
• Some animal species do have the
  ability to detect the magnetic field,
  & they use it to make their migrations.
• Bats and sea turtles use magnetic
  information to find their way.
• We're not 100 percent sure how animals detect the
  magnetic field, but small particles of magnetite have
  been found in the brains of some species. Those
  particles may be reacting to the magnetic field and
  activating nerves in such a way as to send orientation
  information to the animal's brain.
           Bacteria & Magnets
• Some bacteria have a chain of magnetite as part of
  their internal structure
• They use this magnetite to find their way in swamps
• Bacteria in the northern hemisphere have magnetite
  that are opposite in polarity than the bacteria with
  magnetite in the southern hemisphere.
• Animal Migration Video

				
DOCUMENT INFO