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© Copyright Dean P. Neikirk 2004-2009 Mikhail Belkin, EE 325, ECE Dept., UT Austin 1 9.Steady magnetic field EE325 Mikhail Belkin

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© Copyright Dean P. Neikirk 2004-2009 Mikhail Belkin, EE 325, ECE Dept., UT Austin 2 Historical introduction 1 st cent BCE: Chinese fortune tellers begin using loadstone to construct divining boards, eventually leading to the first compasses. 3 rd century: True compasses come into use by this date in China. 1190-1199: Alexander Neckam's “On the Natures of Things” contains the first western reference to compasses used for navigation. 1820: Oersted notes deflection of a magnetic compass needle caused by electric current; demonstrates effect is reciprocal; initiates unification of electricity and magnetism. 1820: Biot and Savart deduce the formula for the strength of the magnetic effect produced by a short segment of current carrying wire. [1827: Ohm formulates the relationship between current to electromotive force and electrical resistance] [1835: Gauss formulates separate electrostatic and electrodynamical laws, including Gauss's law] 1831-1838: Faraday discovers and explains electromagnetic induction and formulates his notion of lines of force
© Copyright Dean P. Neikirk 2004-2009 Mikhail Belkin, EE 325, ECE Dept., UT Austin 3 Ørsted (Oersted) experiment No current: compass needle is rotated to point south-north Send current through the wire and the compass needle will rotate perpendicular to current direction This effect was discovered in 1820 by Oersted during lecture demonstration of electric current (this being said, the discovery was not due to mere chance, since he had been looking for a relation between electricity and magnetism for several years)

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© Copyright Dean P. Neikirk 2004-2009 Mikhail Belkin, EE 325, ECE Dept., UT Austin 4 Previous observations on magnetic fields there are no “magnetic point charges” bar magnets, for instance, always have force “loops” that start and end on the bar You cannot separate ‘south pole’ and ‘north pole’ of a magnet no matter how you break it. if there are no permanent magnets present if there is no current flowing in a circuit, there is no magnetic field if there is current flow, there is always a magnetic field produced
© Copyright Dean P. Neikirk 2004-2009 Mikhail Belkin, EE 325, ECE Dept., UT Austin 5 Origin of magnetic fields: currents simple experiment: two parallel dc-current carrying wires, carrying current in the same direction, are attracted to one another this is NOT due to Coloumbic attraction the wires are space-charge neutral, and hence there is no electric field outside the wires a sheet of “floating” metal placed between the wires has no effect on the force the magnitude of the force between the wires is proportional to the product of the two currents the inverse of the square of the distance between the wires if the directions of current flow are opposite, the force is repulsive

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© Copyright Dean P. Neikirk 2004-2009 Mikhail Belkin, EE 325, ECE Dept., UT Austin 6
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