MAGNETISM Urbana School District #116（磁性厄巴纳学区116）.pdf

MAGNETISM • History of Magnetism • Sources of Magnetism • Bar Magnets • Spin Orbital Dipole Moments • Magnetic Dipoles • Permanent Magnets • Magnetic Fields • Earth’s Magnetic Field • Magnetic Forces on Moving Charges and Wires • Magnetic Flux • Electric Motors • Induced Emf and Current • Current Loops and • Generators Electromagnets • Crossed Fields • Solenoids History of Magnetism • The first known magnets were naturally occurring lodestones, a type of iron ore called magnetite (Fe O ). People of ancient Greece and 3 4 China discovered that a lodestone would always align itself in a longitudinal direction if it was allowed to rotate freely. This property of lodestones allowed for the creation of compasses two thousand years ago, which was the first known use of the magnet. • In 1263 Pierre de Maricourt mapped the magnetic field of a lodestone with a compass. He discovered that a magnet had two magnetic poles North and South poles. • In the 1600s William Gilbert, physician of Queen Elizabeth I, concluded that Earth itself is a giant magnet. • In 1820 the Danish physicist Hans Christian Ørsted discovered an electric current flowing through a wire can cause a compass needle to deflect, showing that magnetism and electricity were related. History (cont.) • In 1830 Michael Faraday (British) and Joseph Henry (American) independently discovered that a changing magnetic field produced a current in a coil of wire. Faraday, who was perhaps the greatest experimentalist of all time,