chapter29

chapter29 - Chapter 29 Magnetic Fields A Brief History of...

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Chapter 29 Magnetic Fields
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A Brief History of Magnetism z 13 th century BC z Chinese used a compass z Uses a magnetic needle z Probably an invention of Arabic or Indian origin z 800 BC z Greeks z Discovered magnetite (Fe 3 O 4 ) attracts pieces of iron
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A Brief History of Magnetism, 2 z 1269 z Pierre de Maricourt found that the direction of a needle near a spherical natural magnet formed lines that encircled the sphere z The lines also passed through two points diametrically opposed to each other z He called the points poles
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A Brief History of Magnetism, 3 z 1600 z William Gilbert z Expanded experiments with magnetism to a variety of materials z Suggested the Earth itself was a large permanent magnet
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A Brief History of Magnetism, 4 z 1819 z Hans Christian Oersted z Discovered the relationship between electricity and magnetism z An electric current in a wire deflected a nearby compass needle
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A Brief History of Magnetism, final z 1820’s z Faraday and Henry z Further connections between electricity and magnetism z A changing magnetic field creates an electric field z Maxwell z A changing electric field produces a magnetic field
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Magnetic Poles z Every magnet, regardless of its shape, has two poles z Called north and south poles z Poles exert forces on one another z Similar to the way electric charges exert forces on each other z Like poles repel each other ± N-N or S-S z Unlike poles attract each other ± N-S
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Magnetic Poles, cont. z The poles received their names due to the way a magnet behaves in the Earth’s magnetic field z If a bar magnet is suspended so that it can move freely, it will rotate z The magnetic north pole points toward the Earth’s north geographic pole z This means the Earth’s north geographic pole is a magnetic south pole z Similarly, the Earth’s south geographic pole is a magnetic north pole
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Magnetic Poles, final z The force between two poles varies as the inverse square of the distance between them z A single magnetic pole has never been isolated z In other words, magnetic poles are always found in pairs z All attempts so far to detect an isolated magnetic pole has been unsuccessful z No matter how many times a permanent magnetic is cut in two, each piece always has a north and south pole
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Magnetic Fields z Reminder: an electric field surrounds any electric charge z The region of space surrounding any moving electric charge also contains a magnetic field z A magnetic field also surrounds a magnetic substance making up a permanent magnet
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Magnetic Fields, cont. z A vector quantity z Symbolized by z Direction is given by the direction a north pole of a compass needle points in that location z Magnetic field lines can be used to show how the field lines, as traced out by a compass, would look B r
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Magnetic Field Lines, Bar Magnet Example z The compass can be used to trace the field lines z The lines outside the magnet point from the North pole to the South pole
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Magnetic Field Lines, Bar Magnet z Iron filings are used to show the pattern of the electric field lines z The direction of the field is the direction a north pole would point
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chapter29 - Chapter 29 Magnetic Fields A Brief History of...

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