lecture10 - Resistance Is Futile! Physics 2102 Spring 2007...

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Physics 2102 Spring 2007 Physics 2102 Spring 2007 Lecture 10 Lecture 10 Current and Resistance Current and Resistance Physics 2102 Spring 2007 Jonathan Dowling Georg Simon Ohm (1789-1854) Resistance Is Futile!
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What are we going to learn? What are we going to learn? A road map A road map Electric charge Electric force on other electric charges Electric field , and electric potential Moving electric charges : current Electronic circuit components: batteries, resistors, capacitors Electric currents Magnetic field Magnetic force on moving charges Time-varying magnetic field Electric Field More circuit components: inductors. Electromagnetic waves light waves Geometrical Optics (light rays). Physical optics (light waves)
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The resistance is related to the potential we need to apply to a device to drive a given current through it. The larger the resistance, the larger the potential we need to drive the same current. ) (abbr. Ohm Ampere Volt [R] : Units = Georg Simon Ohm (1789-1854) "a professor who preaches such heresies is unworthy to teach science.” Prussian minister of education 1830 i V R iR V R V i = = ανδ τηερεφορε ανδ Ohm’s laws Devices specifically designed to have a constant value of R are called resistors, and symbolized by Electrons are not “completely free to move” in a conductor. They move erratically, colliding with the nuclei all the time: this is what we call “ resistance”. Resistance Resistance
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r : c or Ve t E r as r c o di e ti n m Sa e × = A d J i r r that uc s h E i J dA If surface is perpendicular to a constant electric field, then i=JA, or J=i/A Drift speed : v d : Velocity at which electrons move in order to establish a current. E i L A Charge q in the length L of conductor: e L A n q ) ( = n =density of electrons, e =electric charge d v L t = d d v e A n v L e L A n t q i = = = e n J e A n i v d = = d v e n J r r = Current density and drift speed Current density and drift speed
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lecture10 - Resistance Is Futile! Physics 2102 Spring 2007...

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