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MAGNETISM

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;

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MAGNETISM

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  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; • 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

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

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