Chapter_25 - Chapter 25 Current Resistance and Electromotive Force In this chapter end of electrostatics Now ready to study charges in motion

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44 Chapter 25: Current, Resistance, and Electromotive Force In this chapter… ± end of electro statics ! Now ready to study charges in motion ± definition of electric current ± electrical resistance of conductors ± electromotive force and circuits Current (25-1) ± electric current is motion of charge from one place to another 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 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 ± steady-state currents in circuits of conducting wires in electro static equilibrium, 0 E = G inside conductors, and no current flows but certain “driving devices” (batteries, generators, Van de Graaff) can push charges around, making a non-zero E G field inside a conductor dQ I dt =
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45 without a closed loop, eventually charge builds up, creating an electric field to cancel that of driving device, and total 0 E = G in conductor but if conductor forms a circuit (a loop), one gets a continuous flow of charges without charge build up Æ a steady current E G field also exists in surrounding air and non-conductors, but not generally useful or interesting ± motion of charge carriers in wire is complex: when no electric field, free electrons move by thermal agitation with large speeds 6 10 m/s v , but average velocity is zero frequently collide with ions in conducting material, resulting in random changes in direction of motion in an applied E G field, each free electron experiences a force eE G electrons experience accelerations in direction opposite to E G , but net effect is an average drift velocity d v G in direction opposite to E G with typical magnitude 4 10 m/s d v Æ current ! heating due to conversion of kinetic energy to random thermal motion drift velocity is very slow, but current begins to flow everywhere in a circuit immediately after switch closed Æ the driving electric field propagates along circuit elements at close to the speed of light Conductor Driving Device Conductor Conductor driving E G driving E G buildup E G buildup E G + + Driving Device driving E G Conductor driving E G + Conductor Net “drift” due to E field
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46 ± different charge carriers
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This note was uploaded on 07/14/2008 for the course PHYS 33104 taught by Professor Morningstar during the Spring '07 term at Carnegie Mellon.

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Chapter_25 - Chapter 25 Current Resistance and Electromotive Force In this chapter end of electrostatics Now ready to study charges in motion

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