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Unformatted text preview: Lecture 39 Current. Resistance. Ohms law. Electric current Electric current = charges in motion. Units: A (Ampere) 1 A = 1 C/ s dQ I dt = Current f or unif or m cur r ent A dI I J dA = = Current density (current per cross sectional area) A = crosssectional area (Example: crosssection of a wire of radius r is r 2 ) I n terms of microscopic quantities I n terms of microscopic quantities dA current Current density = charge that goes though a crosssection of area dA in a time interval dt n = density of carriers (~ 10 29 m 3 ) car r ier d J q nv = char ge J dA dt = (char ge densit y)(volume) dA dt = ( 29 ( 29 car r ier d q n v dt dA dA dt = v d dt v d = drift velocity (average velocity of carriers) = charge in the box Historical parenthesis Historical parenthesis For historical reasons, even though we now know that the carriers are usually negative (electrons), current I is usually defined as positive from higher to lower potential . This means, opposite to the actual motion of the electrons, or as if the positive charges were the ones...
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 Fall '08
 HerreraSiklody
 Charge, Current, Resistance

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