Physics II chapter 25 notes

# Physics II chapter 25 notes - Chapter 25 Current Resistance...

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V6.02 RS/BQ F’10 56 Chapter 25: Current, Resistance, and Electromotive Force end of electro statics ! Now ready to study charges in motion In this chapter… definition of electric current electrical resistance of conductors electromotive force and circuits electric current is motion of charge from one place to another Current (25-1) rate of charge flowing across some cross-sectional area if charge dQ passes through an area A during a small time interval dt , the current I flowing through that area is defined as: SI unit: the ampere 1A = 1 C/s (André-Marie Ampère ~1820) current is a scalar quantity, but it can be positive or negative , depending on direction of flow defined as flow of positive charge, so electrons move in direction opposite to direction of current (due to Ben Franklin’s unfortunate choice of “positive”) examples: flow of electrons in a copper wire electron beam in an old-style video monitor or TV swarm of charged particles from sun analog: water flow in a pipe, measured in, say, gallons per minute dQ I dt = + + + + I I I I=0 ! A dQ = I dt

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V6.02 RS/BQ F’10 57 steady-state currents in circuits of conducting wires in electro static equilibrium, 0 E = inside conductors, and no current flows but certain “driving devices” (batteries, generators, Van de Graaff) can push charges around, making a non-zero E field inside a conductor without a closed loop, eventually charge builds up, creating an electric field to cancel that of driving device, and total 0 E = in conductor but if conductor forms a circuit (a loop), one gets a continuous flow of charges without charge build up a steady current E field also exists in surrounding air and non-conductors, but not generally useful or interesting current density – an areal density in analogy with a velocity flow field of a fluid, can define a vector field () Jr known as a current density (units of charge per unit time per unit cross- sectional area, A/m 2 ) current through an open surface S is then Conductor Driving Device Conductor Conductor driving E driving E buildup E buildup E + + Driving Device driving E Conductor driving E + Conductor J S S I J dA = ( J is to I as E is to E Φ )
V6.02 RS/BQ F’10 58 motion of charge carriers in wire is complex: when no electric field, free electrons move with large speeds 6 10 m/s v , but average velocity is zero frequently collide with atoms (or really phonons) in conducting material, resulting in random changes in direction of motion in an applied E field, each free electron experiences a force eE electrons begin to accelerate in direction opposite to E , but collisions limit net effect to an average drift velocity d v in direction opposite to E with typical magnitude of only 4 10 m/s d v current !

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Physics II chapter 25 notes - Chapter 25 Current Resistance...

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