MAGNETISM l 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 ( ) (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 ( )  electric current generates magnetic field (1820) l 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: l 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 l 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 13 o (i.e. about 1500km) from geographic N “declination’ = angle between geographic (true) N and magnetic N;   15 o E in Los Angeles, Las Vegas, Salt Lake City;   0 o in Houston, Tulsa, Omaha   3 o W in Tallahassee;   15 o W in Boston, Montreal “inclination” = magnetic dip = angle between horizontal plane and magnetic field vector; magnitude of Earth’s magnetic field:  at Tallahassee:  49.4  T  at Washington, DC  53.4  T  at Fairbanks, Alaska:  57.0  T