Magnetism

Summary

• A magnet is an object that exibits magnetic properties such as…
  • exerting an attractive force on iron or other ferromagnetic materials
  • exerting both attractive and repulsive forces on other magnets
  • deflecting the path of a moving charged particle
• Magnetic Poles
  • Regions on a magnet where the forces are the strongest.
  • Come in two types — north pole and south pole, often shortened to north and south or abbreviated N and S.
  • Magnetic poles always occur in north-south pairs called dipoles.
    • Combinations of of dipoles are called multipoles.
    • Some multipole arrangements have special names
      • two dipoles form a quadrupole
      • three dipoles form a sextupole
      • four dipoles form an octupole
  • Magnetic monopoles do not seem to exist.
    • When a dipole magnet is broken, all of its pieces are also dipoles.
    • This is true down to the subatomic level. Electrons, protons, and neutrons are dipoles.
    • Some theoretical physicsts have predicted the existance of particles with a single magnetc pole, but these claims have not been verified experimentally.
• Rule of Action
  • Like poles repel.
  • Opposite poles attract.
• Compass
  • A compass is any dipole magnet that is free to rotate.
• Magnetic Field
  • The direction of the magnetic field is determined by following the north pole of a compass.
    • Magnetic field lines diverge from the north pole of a magnet and converge on its south pole.
  • The strength of the magnetic field at any location is proportional to the density of the lines drawn.
  • Magnetic field lines never intersect (since the field can only point in one direction at any location).
  • Magnetic field lines form closed loops.
    • Magnetic field lines reconnect inside a magnet.
  • The symbol for magnetic field is an uppercase, bold B (vector notation) or an uppercase, italic B (for the magnitude only).
    • The reason for this choice of symbol is unknown.
    • The magnetic field is also known as the B field, magnetic flux density, and magnetic induction.
  • The SI unit of magnetic field is the tesla [T].
    • In terms of other units, the tesla is also a newton per ampere meter (from the Lorentz force law), a weber per meter square (from Faraday's law), or a kilogram per ampere second square (in fundamental units)

      ⎡ ⎣	T =	N	=	Wb	=	kg	⎤ ⎦
      		Am		m2		As2

    • 1 T is a very large magnetic field.
• Geomagnetism
  • The earth is basically a dipole magnet.
  • A compass will rotate until it aligns with the Earth's magnetic field.
    • The north pole of a compass tends to point north (toward the Earth's north magnetic pole).
    • The south pole of a compass tends to point south (toward the Earth's south magnetic pole).
  • The magnetic poles of the Earth (locations on the surface of the Earth where the magnetic field is the strongest) are near the geographic poles (locations where the spin axis of the Earth intersects its surface).
    • The north magnetic pole of the Earth is currently in the Arctic Ocean near the north geographic pole. A compass would point straight down at this location making it a south magnetic pole.
    • The south magnetic pole of the Earth is currently in the Indian Ocean near Antarctica. A compass would point straight up at this location making it a south magnetic pole.
  • The magnetic axis of the Earth differs from its rotational axis by about 10°.
  • The magnetic field strength on the surface of the Earth is around 45±15 µT.
• Materials
  • Ferromagnetic
    • strongly magnetic, attractive
    • neighboring atoms align their magnetic axes
    • capable of forming permanent magnets
    • elements
      • iron, cobalt, nickel
      • dysprosium, gadolinium
    • alloys, blends of metals or metals and nonmetals
    • Some iron minerals are magnetic (e.g. magnetite, Fe₃O₄), some are not (e.g. pyrite, FeS₂)
    • Some steels are magnetic (e.g. ferritic steels), some are not (e.g. austenitic steels)
    • Some alloys of nonferromagnetic metals are ferromagnetic (e.g. Heusler alloy, Cu₂MnSn)
  • Paramagnetic
    • weakly magnetic, attractive
    • atoms align their magnetic axes somewhat with an external magnetic field
    • a temporary effect
    • most materials are paramagnetic (e.g. molecular oxygen, O₂)
  • Diamagnetic
    • weakly magnetic, repulsive
    • atoms align their magnetic axes opposite to an external magnetic field
    • a temporary effect
    • few materials are diamagnetic (e.g. carbon, mercury, water)
  • Superdiamagnetic
    • strongly magnetic, repulsive
      • Meissner effect, magnetic levitation
    • magnetic fields cannot penetrate below a thin surface layer
    • a temporary effect
    • superdiamagnetism is a property of superconductors