MAGNETISM

1
MAGNETISM

• Historical background
• lodestone (magnetite) known for 1000s of years
• Thales of Miletus studied lodestones (590 BC)
• magnetic compass invented by Chinese around 200
  AD
• Pierre de Maricourt a.k.a. Petrus Peregrinus
  (1269) studied magnets, Earth's magnetism
  concept of poles, tried to isolate single pole
• William Gilbert (1544-1603) (court physician of
  Elizabeth I and James I)
• first serious studies of magnets
• two poles of magnets
• Earth is a magnet
• iron can be magnetized
• magnetism destroyed by heating
• Hans Christian Oersted (1777-1851)
• electric current generates magnetic field (1820)
• Essentials of magnetism
• every magnet has two poles - dipole''-- there
  are no magnetic monopoles
• like poles repel each other, unlike poles attract
  
• magnetic field
• magnetic forces due to magnetic field
  (Faraday), caused by magnet in its surrounding
• magnetic field lines describe direction, density
  of lines represents magnitude of field

2
MAGNETISM OF MATERIALS

• origin of magnetism
• atoms can have magnetic dipole field, partly due
  to effects of orbital motion of electrons, but
  mainly due to electron spin (intrinsic angular
  momentum of electrons)
• in most materials, atoms have no net dipole
  field, or directions of elementary dipoles random
  ?? effects cancel
• in some materials (ferromagnetic materials),
  many atomic dipoles aligned ?? magnetic
  domains
• if domains not aligned, material is not magnetic
  if domains aligned, material is magnetic,
• strong magnetic field can align domains -
  magnetization
• if domains stay aligned after magnetizing field
  turned off permanent magnet
• magnetically soft materials do not retain
  magnetization used for electromagnets

3
EARTH'S MAGNETIC FIELD

• Earth is a magnet
• north-seeking pole of compass needle called (by
  arbitrary definition) a northpole
• ? the Earth's northern magnetic pole is actually
  a magnetic south pole
• Earth's geomagnetic poles are not at geographic
  poles,
• positions change in time presently, magnetic N
  is about 13o (i.e. about 1500km) from geographic
  N
• declination angle between geographic (true) N
  and magnetic N
• ? 15o E in Los Angeles, Las Vegas, Salt Lake
  City
• ? 0o in Houston, Tulsa, Omaha
• ? 3o W in Tallahassee
• ? 15o W in Boston, Montreal
• inclination magnetic dip angle between
  horizontal plane and magnetic field vector
• magnitude of Earths magnetic field
• at Tallahassee ? 49.4?T
• at Washington, DC ? 53.4?T
• at Fairbanks, Alaska ? 57.0?T