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current and resistance 2

# current and resistance 2 - Andre-Marie Ampere 1775-1836...

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1 1 Currents and Resistance – II This Lecture •Drift velocity •Power dissipation in resistors Andre-Marie Ampere 1775-1836 This lecture: HRW 26.5-26.9 For next time: HRW 27.1-27.6 2 Last Lecture Current The amount of charge passing through a cross section of wire per unit time Current density The current per unit area A vector that gives the local direction of current flow Resistivity The property of a material that determines the current density a given electric field will cause Resistance The property of an object that determines how much current a given potential difference across its ends will cause V =IR r r I = J•dA r r E = ρ J q I = t d d 3 When a battery is connected across a conductor, electrons drift in the direction of increasing potential. The small drift is superimposed on the very fast random motion. Current is due entirely to the small drift. E Drift Velocity (reminder) What is a typical drift velocity? Conductors conduct because some electrons are mobile. These electrons are in constant, random, very fast (~10 6 m/s!) motion, frequently colliding with vibrating nuclei. 4 Current Density and Drift Velocity Imagine a cylinder with cross-section A, length L, oriented in the direction of v d .

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current and resistance 2 - Andre-Marie Ampere 1775-1836...

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