Lecture 12 Chapter 26 Capacitance Examples Capacitance Question 4 A In a are C1 and C3 in series YES B In a are C1 and C2 in parallel YES Rank the Ceq of the 4 circuits All the same Capacitance Question 9 After switches close for which circuit will the charge on the lefthand capacitor A increase 2 B decrease 3 C same 1 Charge flows until the capacitors have the same potential V q CV Lecture 12 Chapter 27 Current and Resistance Current 1 What happens when charges move Isolated conductor Random motion of conduction electrons in both directions so no net transport of charges Same potential everywhere no E field inside or on surface so no electric F on electrons No current in isolated conductor Current 2 What happens when charges move Adding a battery Bias flow of conduction electrons in one direction have net transport of charge Not a single potential have E field inside which exerts F on electrons Current in a conductor when attached to a battery Current 3 Amount of current i equals amount of q that passes in t through an area to the flow If i doesn t vary with time called steady state q is conserved i is the same for all planes which pass through conductor Orientation doesn t matter dq i dt Current 4 SI unit for current is ampere 1A 1C s Current is a scalar Use arrows to indicate charge flow along conductor q is conserved so i0 i1 i2 Current 5 Convention a current arrow is drawn in direction of charge flow Defined direction of current is opposite to direction of physical current electrons are the moving charges Current arrows are not vectors Bending or reorienting wires does not change i0 i1 i2 Current 6 Checkpoint 1 What is the magnitude and direction of the current i in the lower right hand wire q is conserved iin iout iin 11 A i 8A iout 3 A i To the right Current 7 Total current through a surface can be defined as Current density J flow of charge through a cross section If i uniform and parallel to dA SI unit for J is A m2 r r i J dA i JdA JA i J A Current 8 Represent J by streamlines q is conserved so amount of i cannot change J becomes greater in narrower conductor Streamlines closer together mean greater J i J A Current 9 No current in conductor electrons move randomly with speeds 106 m s If current present electrons also move with a drift speed vd Drift speeds are tiny vd 10 5 or 10 4 m s Why do the lights come on quickly E field moves at speed of light
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