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Magnetism

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									Magnetism
        Physics
 La Cañada High School
         Dr.E
            Magnetism1

• Magnetism is a force that attracts certain
  metals
• Magnets have two poles that act both
  similarly and differently than electrical
  charges act
• Easy to detect magnetism, although it is not
  easy to measure its strength
• Make a magnet by rubbing steel against
  another magnet
 Properties of Magnetism
• Magnetism is a force that attracts iron,
  nickel and cobalt
• Combinations of these metals as alloys can
  become permanent sources of magnetism
• A lodestone is the naturally occurring
  magnetite that has the chemical formula
  Fe3O4
       Magnetism is Safe
• Electrical charges can give a person a shock
  or even kill the person
• Magnetism doesn't seem to have any
  harmful effects
• People who claim that magnets are even
  beneficial to your health
• Some companies sell magnetic
  materials that attach to your body
  to help relieve chronic pain.
    Magnets Attract Iron
• A magnet
  – an object made of a material that attracts iron,
    nickel and cobalt, as well as alloys of these
    metals
  – both attracts and repels other magnets.
• Magnetic force attracts and repels acts at a
  distance
• A few rare-earth materials such as bismuth
  are actually repelled by a magnet
   Magnet has Two Poles
• Opposite ends of a magnet are called its
  north and south poles
  – should be called the "north seeking" and
    "south seeking" poles, because they seek
    the Earth's North Pole and South Pole,
    respectively
• Like poles repel
  – north pole of magnet will push the north
    pole of another magnet
 Magnet – Electrical Difference
• Every magnet has a
  north pole and a
  south pole
• A magnet always
  has a N and S pole
  – If you cut a magnet
    in half, each piece
    will still have a N
    and S pole
      Magnetic Field Nature
• Magnetism originates
  in the motion of the
  electrons
• Spinning electrons act
  like tiny magnets
• Cancellation of this
  effect occurs in most
  materials
• Iron, nickel, cobalt are
  exceptions
        Magnetic Domains
• The magnetic north-
  south axes of groups
  of iron atoms line up
  in the same direction
  – Magnetic domains
• Domains are
  randomly oriented
  unmagnetized iron
           Magnetization
• An external magnetic field will twist
  the domains into alignment
  – Domains are randomly oriented in
    unmagnetized iron
  – Incomplete alignment of domains in
    slightly magnetized iron
  – Virtually all of the domains are aligned in
    strongly magnetized iron
Measuring Magnetic Strength
• Not easy to measure
  the strength of a
  magnetic field
• Gauss meter is used
  to measure a
  magnetic field
  – meters uncommon
    due to limited use
    Detecting Magnetic Field
• Iron filings on a
  piece of paper
  – Spread fine iron
    filings on a piece of
    paper laid on top of
    a magnet
     • Outline of the
       magnetic lines of
       force or the
       magnetic field
  Detecting Magnetic Field
• Compass
 – Thin magnet or
   magnetized iron
   needle balanced on a
   pivot
 – needle will rotate to
   point toward the
   opposite pole of a
   magnet
 – One end marked N
   and the other S
A Compass →Magnetic Field
Electric Currents and Magnetic Field
• Spinning or rotating
  electrons are
  responsible for
  magnetism in iron
• Moving charges set up
  magnetic fields
• Compasses (bar
  magnets) line up in
  circles around a wire
  carrying current.
Iron Filings form Concentric
    Circles around Wire
Electric Currents and Magnetic Field3

• Magnetic field lines around a long wire
  which carries an electric current form
  concentric circles around the wire
• The direction of the magnetic field is in
  the direction the fingers if your right
  hand curls around the wire with your
  thumb in the direction of the current
Electric Currents and Magnetic Field
Magnetic Levitation
    • Trains float above guide
      way due to magnetic field
    • Travel at speeds of up to
      300 mph (500 kph)
Magnetic Levitation
     ‘Superman the
    Ride’ works due
      to magnetic
       levitation
   Magnetic Forces on Moving
      Charged Particles
If charge particles move in a magnetic field,
they experience a sideways force
   Magnetic Forces on Current
       Carrying Wires
• Moving electrons in wire are pushed up, or
  down, depending on their direction
       Measuring Currents
• Coils of current-carrying wires set up
  magnetic field perpendicular to plane of coil
• Compass needle aligns itself with the field
  lines
   Current-Measuring Apparatus
• Electromagnet tends to
  align its north face
  with the iron magnet's
  south face
• A spring resists this
  tendency to twist; the
  greater the current, the
  greater the deflection
  of the needle
   Earth as a Giant magnet
• Earth's magnetic field is thought to be
  generated deep inside the planet
• An inner core of solid iron is surrounded by
  an outer core of molten iron
• They rotate at different rates, and the
  interaction between the regions creates what
  scientists call a "hydromagnetic dynamo."
Earth’s Magnetosphere                 Layer 4

• Also named Van Allen Belts
• Protects the Earth from celestial bodies,
  harmful cosmic rays and particles
• Belts at thousands of kilometers above the
  earth protect the living things on the Earth
                        from the fatal energy
                        that would otherwise
                        reach it from space
Earth’s Magnetic     Field5

            • The Earth has a
              magnetic field with
              north and south poles
              – reaches 36,000 miles
                into space.
              – surrounded in a
                region called the
                magnetosphere
              – prevents most of the
                particles from the sun,
                carried in solar wind,
                from hitting the Earth
• High Speed electrons
  and protons from space
  travel along magnetic
                               Aurora
  field lines
• Field lines are nearly
  horizontal near the
  equator which protects
  the atmosphere
• The field lines are nearly
  vertical at high latitude,
  thus the high speed
  particles can enter the
  atmosphere
   Magnetic Pole Location
• Location of magnetic pole is not fixed
• Geographic north is called the Earth's
  North Magnetic Pole by convention
• The North Magnetic Pole is actually the
  south pole of the Earth's magnetic field
  – the north pole of a compass was defined
   as the pole that points to the geomagnetic
   north
       Earth’s Magnet Flips
• Magnetosphere can flip its orientation so that the
  field lines which were pointed toward the north
  pole change and point toward the south pole
  – record preserved in magnetic rocks which lie along
    the ocean floor
• Magnetism in these rocks points first in one
  direction, then in another direction, giving the
  ocean floor a stripped appearance (from a
  magnetic point of view
  – many times in the past the north pole has become the
    south pole, and vice versa
The Sun is
a Great Big
  Magnet
       Sun’s Magnet Field
• The Sun is a big magnet
• During solar minimum the Sun's magnetic
  field resembles that of an iron bar magnet,
  with great closed loops near the equator and
  open field lines near the poles
• The Sun's dipolar field is about as strong as
  a refrigerator magnet, or 50 gauss
• Earth's magnetic field is 100 times weaker
                Sunspots
• Sunspots are places where intense magnetic
  loops -- hundreds of times stronger than the
  ambient dipole field -- poke through the
  photosphere
• Sunspot magnetic fields overwhelm the
  underlying dipole
• The Sun's magnetic field isn't confined to
  the immediate vicinity of our star
• The solar wind carries it throughout the
  solar system
 Magnetic Fields on the Sun
• Plasma, like iron
  filings, follows
  magnetic field
  lines
• Coronal loops
  created by hot,
  glowing plasma
  flow along
  magnetic field
                      The Earth fits inside the loop
  lines
     Cell Phones and Pagers
• When there are many sun spots, it is called the
  solar maximum
  – lots of solar flares
  – strong solar wind
  – radiation is also extra strong



• All this solar activity can interfere with radio
  waves
  – cell phones and pagers don't work all the time
  – can hear solar static on car radio.
                       Bibliography
1. Magnetism information from School for Champions Website by Ron Kurtus (revised
   29 January 2002) @ http://www.school-for-champions.com/science/magnetism.htm,
   4/15/04
2. Magnetism: Chapter 9 by Joseph F. Alward, PhD, Department of Physics, University
   of the Pacific @ http://sol.sci.uop.edu/~jfalward/physics17/chapter9/chapter9.html,
   4/15/04
3. HyperPhysics by Carl R. (Rod) Nave Department of Physics and Astronomy Georgia
   State University @ http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magcur.html,
   4/15/04
4. Creation of the Universe The Qur’an and Life by Harun Yahya
   @ http://www.creationofuniverse.com/html/science_06.html , 4/15/04
5. Earth’s Magnetic Field by Windows to the Universe, Last modified June 3, 2003 @
   http://www.windows.ucar.edu/tour/link=/earth/Magnetosphere/overview.html ,
   4/16/04
6. Windows to Universe by Windows to the Universe, Last modified June 3, 2003 @
   http://www.windows.ucar.edu/spaceweather/location_mag_poles.html, 4/16/04
7. New clues to Earth's magnetic flip-flops by CNN News, April 7, 2004
   @http://www.cnn.com/2004/TECH/space/04/07/poles.reverse/, 4/16/04

								
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