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Historical background
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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)
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 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:
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 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
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 magnetic field lines describe direction, density 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

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 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

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 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;
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 15 o
City;
E in Los Angeles, Las Vegas, Salt Lake
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 0 o
 3 o
 15 o in Houston, Tulsa, Omaha
W in Tallahassee;
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
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 at Washington, DC  53.4
 T at Fairbanks, Alaska:  57.0
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