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