mstat1 - Physics 54 Magnetostatics 1 The Creation of the...

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Physics 54 Magnetostatics 1 The Creation of the Universe was made possible by a grant from Texas Instruments. — Credit on a PBS Program Overview Like electricity, magnetism as a scientifc study dates back to antiquity. The earliest reFerence is to stones Found in the Greek province oF Magnesia (whence the term "magnet") which were observed to attract iron. At some point it was noticed that these stones also lined themselves up with the north-south direction on the earth. This led to the invention oF the magnetic compass, but where and when this was done is in doubt. In 1600 William Gilbert published an important book giving most oF what was then known about both electricity and magnetism, and putting Forth the idea that the earth itselF was a large magnet, which accounted For the action oF the compass. During the 18th century the Force between magnetic "poles" was studied and Found to be much like that between electric charges. But the simplest magnetic structure is a dipole. Despite careFul searches, no one has ever Found an isolated magnetic pole. A breakthrough came in 1820 when Oersted Found that electric currents produce aligning eFFects on magnetic compass needles, showing that electric currents act as sources oF magnetic felds. Within a Few years the experiments oF Ampere, Biot and Savart developed Fully the relationship between steady electric currents and magnetism. In 1830 ±araday and Henry (independently) discovered that time-varying magnetic felds produce emFs, much like batteries. This provided another connection between electricity and magnetism. ±inally in 1860 Maxwell proposed that the converse also occurs, that changing electric felds act like currents in producing magnetic eFFects. That changing felds could create each other led Maxwell to conjecture that energy could be transmitted by waves consisting oF nothing but the felds. He determined (theoretically) the speed oF these waves, fnding that it was the speed oF light. This theory — which ranks with the theory oF evolution by natural selection and the general law oF conservation oF energy among the greatest scientifc accomplishments oF the 19 th century — completed the unifcation oF electricity, magnetism and optics. One major problem remained. The laws oF electromagnetism (embodied in Maxwell's equations) do not have the same Form in all inertial reFerence Frames, iF the rules oF Galileo are used to transForm From one Frame to another. This seemed to imply that PHY 54 1 Magnetostatics 1
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nature has a preferred reference frame (the one in which the "aether" is at rest). Efforts to determine what frame that is were fruitless, however. This problem was resolved in 1905 by the theory of special relativity, largely due to Einstein. Maxwell's equations are indeed the same in all inertial frames, if the correct transformation rules are used. The Galilean rules are approximations to these, valid when the relative velocity of the two frames is small compared to the speed of light.
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mstat1 - Physics 54 Magnetostatics 1 The Creation of the...

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