# Lect_39 - Electric current Lecture 39 Current Resistance...

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Lecture 39 Current. Resistance. Ohm’s law. Electric current = charges in motion. Units: A (Ampere) 1 A = 1 C/s dQ I dt = Current for uniform current A dI J dA  ==   Current density (current per cross- sectional area) A = cross-sectional area (Example: cross-section of a wire of radius r is π 2 ) In terms of microscopic quantities In terms of microscopic quantities current Current density = charge that goes though a cross-section of area in a time interval n = density of carriers (~ 10 29 m 3 ) carrier d Jq nv = charge dA dt = (charge density)(volume) = ( )( ) carrier q v dt dA = 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 that are really moving.

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## This note was uploaded on 04/16/2008 for the course PHYSICS 221 taught by Professor Johnson during the Fall '06 term at Iowa State.

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Lect_39 - Electric current Lecture 39 Current Resistance...

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