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Magnetism Elecricity is closely related to magnetism. Magnetic forces and fields are produced by a moving electric charge. Why this happens? We do not really know. It is a relativistic effect produced by the motion of the electric charge. Page 1 January 28, 07:31 History 800 BC The Greeks discovered Lodestone, a naturally occuring magnet. This material was found in the region of Asia Minor called Magnesia. (This is where the term "Milk of Magnesia" comes from) (Ha, Ha) 12th. Century The Chinese invent the compass and begin using it for navigation. They would place a bar magnet on a piece of wood and float it in a bowl of water. Page 2 January 28, 10:25 History 1269 Petrus Peregrinus, a military engineer in Europe, introduced the idea of magnetic poles and explained how to determine north and south. 16th. Century Columbus, observed that the amount of dip of a compass needle (magnetic declination) changed in relation to your position on Earth. William Gilbert wrote the book, On the Magnet. He was the first to study magnetic lines of force. He made a large sphere of lodestone and found a change in the declination an inclination of a magnetic needle. 1820 Hans Christian Oersted discovers that an electric current will deflect a compass. He did this while demonstrating electricity to a physics class. This discovery basically started the Industrial Revolution. Page 3 January 28, 08:29 William Gilbert 1544-1603 Page 4 January 28, 09:06 Hans Christian Oersted 1777 - 1851 Page 5 January 28, 09:12 Magnetic Poles Magnetic Poles Similar, yet DIFFERENT than electric charges. Similarities 1. Can attract and repel without touching 2. The amount of attracting or repelling depends on distance 3. Like poles repel, opposite poles attract. Difference Magnetic poles can NOT be separated. They always occur in pairs. Page 6 January 28, 08:36 Magnetic Fields Magnetic Fields The space around a magnet in which a magnetic force is exerted. These are imaginary lines that scientists use to describe the effect on a compass needle when it is near a magnet. Rules for Magnetic Field lines. 1. They show the shape of the field 2. They exit the North pole and enter the South Pole. 3. If the lines are close together the field is stronger. 4. The lines may never cross. 5. Use arrows to show the direction the north pole of a compass points. Page 7 January 28, 08:40 Magnetic Field Pictures Page 8 January 28, 13:01 Magnetic Field Pictures Page 9 January 28, 13:04 Magnetic Field Pictures Page 10 January 28, 13:09 Magnetic Field Pictures Page 11 January 28, 10:50 Atomic Theory of Magnetism We now know today that magneic fields are produced by the motion of electric charges. The charges can spin or orbit. Electrons have two magnetic fields, one due to the spin and one due to its orbit about the nucleus. The field due to the spin is stronger. In most materials the spins of the electrons are in the opposite directions so the fields cancel out. Iron, Nickel, Cobalt. In these naturally occuring materials the spins do not cancel. Thus, they can be made into magnets if the conditions are right. Page 12 January 28, 08:44 Magnetic Domains Magnetic domains are microscopic clusters of aligned atoms. (The spins of the charges line up.) Magnetic Induction If a strong magnetic field is brought near a set of random domains, the domains will align with the magnetic field. The domains physically rotate. Page 13 January 28, 08:49 Page 14 January 28, 10:55 Magnetic Domain Pictures Page 15 January 28, 11:57 Magnetic Domains Destroying Magnets To destroy a magnet you must force the domains to become random in their orientation. This may be done by heating or hitting the magnet. Page 16 January 28, 08:56