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SMU PHYS 1304 - Current Resistance And Resistor

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Chapter 27Electric Current, the definitionElectric Current, definition and unitDirection of CurrentCurrent in a conductor and the drift speedCurrent linearly proportional to drift speedA typical value of the drift speedMotion of the charge carrier, the electrons, in a conductorMotion of Charge Carriers, final discussionCurrent Density J, the definitionConductivity, the defintionChapter 27CurrentResistance And ResistorElectric Current, the definitionAssume charges are moving perpendicular to a surface of area AIf ΔQ is the amount of charge that passes through A in time Δt, then the average current istQIavgΔΔordtdQI +++ +++IAElectric Current, definition and unitElectric current is the rate of flow of charge through some region of spaceThe SI unit of current is the ampere (A)1 A = 1 C / sAnd ampere is a base unit. The unit for time, second, is also a base unit. The unit for charge is then defined as 1C=1A/1sThe symbol for electric current is IDirection of CurrentThe charges passing through the area could be positive or negative or both. This area may or may not be an actual physical surface and the charges can flow in a conductor or in vacuum.It is conventional to assign to the current the same direction as the flow of positive chargesThe direction of current flow is opposite the direction of the flow of electronsIt is common to refer to any moving charge as a charge carrierCurrent in a conductor and the drift speedCharged particles move through a conductor under the drive of an electric field inside.Assume a cross-sectional area A that is perpendicular to and the number of charge carriers per unit volume n.Then nAΔx is the total number of charge carriersEdVCurrent linearly proportional to drift speedThe total charge is the number of carriers times the charge per carrier, qΔQ = (nAΔx)qThe drift speed, vd, is the speed at which the carriers movevd = Δx / Δt and Δx = vd ΔtRewritten: ΔQ = (nAvd Δt)qFinally, current, Iave = ΔQ/Δt = nqvdAThis is to say that current is linearly proportional to the drift speed vdA typical value of the drift speedAssume a copper wire, with one free electron per atom contributed to the currentThe drift speed for a 12-gauge copper wire carrying a current of 10.0 A is 2.23 x 10-4 m/sThis is a typical order of magnitude for drift speedsThis is very slow, how come when a switch is close, the light comes on from a light bulb ~ 10m away? It should take ~10/ 2.23 x 10-4 sec which is about 12 hours.What is wrong?Motion of the charge carrier, the electrons, in a conductorThe actual charge carrier in conductor is the electronThe zigzag black lines represents the motion of a charge carrier in a conductorThe net drift speed is smallThe sharp changes in direction are due to collisionsThe net motion of electrons is opposite the direction of the electric fieldThe current direction is conventionally defined to be the positive carrier motion direction, or the electric field direction.Use the active figure to change the field and observe the effectPLAYACTIVE FIGUREMotion of Charge Carriers, final discussionThe electric field exerts forces on the electrons in the wire at the same time (almost). These forces cause the electrons to move in the wire and create a current. So the current starts to flow anywhere in the circuit when the switch is closed. In the presence of an electric field, like the one set up by a battery, in spite of all the collisions, the charge carriers slowly move along the conductor with a drift velocity,The battery does not supply the electrons, it only establishes the electric fielddvrCurrent Density J, the definitionCurrent density is a vector and is in the direction of the positive charge carriersdnqVJThe current density J of the current I a conductor is defined as the current per unit areaJ has SI units of A/m2 AdI JThe relationship to the current isConductivity, the defintionA current density and an electric field are established in a conductor whenever a potential difference is maintained across the conductorFor some materials, the current density is directly proportional to the field, that is The constant of proportionality, σ, is called the conductivity of the


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SMU PHYS 1304 - Current Resistance And Resistor

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