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;

field due to one pole obeys “Coulomb-like” law,total field of magnetic dipole =

superposition of the two fields moving charges (currents) generate magnetic

fields

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

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 Earth’s magnetic field: at Tallahassee: 49.4T at Washington, DC 53.4T at Fairbanks, Alaska: 57.0T