PHYA22 Chapter 31.doc

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Physics and Astrophysics
Brian Wilson

Chapter 31 Current and ResistanceThe Electron CurrentCurrent the motion of chargesA current is present in a wire when it makes the wire warm and deflects a compass needleCharge Carriers the charges that move in a conductorWhen a metal bar accelerates to the right inertia causes the charge carriers to be displaced to the rear surface and the front surface becomes oppositely chargedThe charge carriers in metals are electrons as found when negatively charged particles move to the rear surface as it acceleratesConduction electrons in metals undergo random thermal motions but there is no net motionDrift Speed v the net motion causing the entire sea of electrons to move in done directionElectron Current i the number of electrons per second that pass through a e1cross section of a wire or other conduction and unit are in sNumber of Electrons that pass through the cross section during the time interval Dt NiDtnADxnAvDteeeedIncreasing the drift speed will increase the number of electrons passing through a wire each secondElectric current in the wire inAveedYou can increase the electric current by making them move faster by having more of them per cubic meter or by increasing the size of the pipe theyre flowing throughDischarging a wire occurs instantaneously because the wire is already full of electrons and as soon as the excess electrons move from the negative capacitor plate into the wire they immediately push an equal number of electrons out the other end of the wire and onto the positive plate thus neutralizing itCreating a CurrentThe sea of electrons will quickly slow down and stop unless you continue pushing it with an electric fieldAn electron current is a nonequilibrium motion of charges sustained by an internal electric field so E0An electric field creates a currentThe nonuniform surface charge density creates an electric field inside the wire it is positive at the positive capacitor plate zero at the midpoint and negative at the negative plateThe onaxis field of a ring of chargeoPoints away from a positive ring and toward a negative ringoIs proportional to the amount of charge on the ringoDecreases with distance away from the ringA nonuniform distribution of surface charges along a wire creates a net electric field inside the wire that points from the more positive end of the wire toward the more negative end of the wire This is the internal electric field E that pushes electron current through the wireWithout an electric field an electron bounces back and forth between collisions with ions in the lattice of the metal but its average velocity is zero and it undergoes no net displacement
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