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WSU PHY 2140 - Lecture notes

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Lightning Review17.5 Temperature Variation of Resistance - ExamplePlatinum Resistance ThermometerA resistance thermometer, which measures temperature by measuring the change in the resistanceAppendix: Superconductivity17.7 Electrical energy and powerElectrical energyPowerExampleMini-quizDirect Current Circuits18.1 Sources of EMFEMFEMF (continued)119/22/20039/22/2003General Physics (PHY 2140)Lecture 9Lecture 9¾ Electrodynamics9Electric current9 temperature variation of resistance9 electrical energy and powerChapter 17-18http://www.physics.wayne.edu/~apetrov/PHY2140/229/22/20039/22/2003Department of Physics and Astronomyannounces the Fall 2003 opening ofThe Physics Resource CenterThe Physics Resource Centeron Monday, September 22 in Room 172Room 172of Physics Research Building.of Physics Research Building.Hours of operationHours of operation::Mondays, Tuesdays, Wednesdays 11 AM to 6 PMThursdays and Fridays 11 AM to 3 PMUndergraduate students taking PHY2130-2140 will be able to get assistance in this Center with their homework, labwork and other issues related to their physics course.The Center will be open: Monday, September 22 to Wednesday, December 10, 2003.339/22/20039/22/2003Lightning ReviewLightning ReviewLast lecture:1.1.Current and resistanceCurrent and resistance99Current and drift speedCurrent and drift speed99Resistance and Ohm’s lawResistance and Ohm’s law••I is proportional to VI is proportional to V99ResistivityResistivity••material propertymaterial propertyReview Problem:Consider two resistors wired one after another. If there is an electric current moving through the combination, thecurrent in the second resistor isa. equal tob. halfc. smaller, but not necessarily halfthe current through the first resistor. QIt∆=∆dInqv A=VIR=RAlρ=abcR1R2I449/22/20039/22/200317.4 Resistivity - Example(a) Calculate the resistance per unit length of a 22-gauge nichrome wire of radius 0.321 m.()223720.321 10 3.24 10Armmππ−−== × =×Cross section: Resistivity (Table): 1.5 x 10−6Ωm. 6721.5 104.63.24 10mRmlA mρ−Ω−×Ω== =×Resistance/unit length:559/22/20039/22/200317.4 Resistivity - Example(b) If a potential difference of 10.0 V is maintained across a1.0-m length of the nichrome wire, what is the current?10.02.24.6VVIAR∆== =Ω669/22/20039/22/200317.4 Temperature Variation of Resistance -Intro• The resistivity of a metal depends on many (environmental) factors.• The most important factor is the temperature.• For most metals, the resistivity increases with increasing temperature.• The increased resistivity arises because of larger friction caused by the more violent motion of the atoms of the metal.779/22/20039/22/2003For most metals, resistivity increases For most metals, resistivity increases approx. linearly with temperature.approx. linearly with temperature.ρ()1ooTTρρ α=+−TMetallic Conductor• ρ is the resistivity at temperature T (measured in Celsius).• ροis the reference resistivity at the reference temperature Tο(usually taken to be 20 oC).• αis a parameter called temperature coefficient of resistivity.For a conductor with fixed cross section.ρFor a conductor with fixed cross section.()1ooRRTTα=+−TSuperconductor889/22/20039/22/200317.5 Temperature Variation of Resistance 17.5 Temperature Variation of Resistance --ExampleExamplePlatinum Resistance ThermometerPlatinum Resistance ThermometerA resistance thermometer, which measures temperature by measurinA resistance thermometer, which measures temperature by measuring the g the change in the resistance of a conductor, is made of platinum andchange in the resistance of a conductor, is made of platinum andhas a has a resistance of 50.0 resistance of 50.0 ΩΩat 20at 20ooC. When the device is immersed in a vessel C. When the device is immersed in a vessel containing melting indium, its resistance increases to 76.8 containing melting indium, its resistance increases to 76.8 ΩΩ. Find the melting . Find the melting point of Indium.point of Indium.Solution:Using α=3.92x10-3(oC)-1from table 17.1.999/22/20039/22/2003Platinum Resistance ThermometerPlatinum Resistance ThermometerA resistance thermometer, which measures temperature by measurinA resistance thermometer, which measures temperature by measuring the change in the g the change in the resistance of a conductor, is made of platinum and has a resistaresistance of a conductor, is made of platinum and has a resistance of 50.0 nce of 50.0 ΩΩat 20at 20ooC. C. When the device is immersed in a vessel containing melting indiuWhen the device is immersed in a vessel containing melting indium, its resistance m, its resistance increases to 76.8 increases to 76.8 ΩΩ. Find the melting point of Indium.. Find the melting point of Indium.Solution:Solution:Using Using αα=3.92x10=3.92x10--33((ooC)C)--11from table 17.1. from table 17.1. RRoo=50.0 =50.0 ΩΩ..TToo=20=20ooC.C.R=76.8 R=76.8 ΩΩ.()[]1376.8 50.03.92 10 50.0137157ooooooRRTTRCCTCα−−−Ω−Ω−= =×Ω==.10109/22/20039/22/2003Appendix: SuperconductivityAppendix: Superconductivity19111911: H. K. : H. K. OnnesOnnes, who had figured out how to , who had figured out how to make liquid helium, used it to cool mercury to 4.2 make liquid helium, used it to cool mercury to 4.2 K and looked at its resistance:K and looked at its resistance:At low temperatures the resistance of some At low temperatures the resistance of some metalsmetalsÆÆ0, measured to be less than 0, measured to be less than 1010--1616••ρρconductorconductor(i.e., (i.e., ρρ<<1010--24 24 ΩΩmm)!)!–Current can flow, even if E=0.–Current in superconducting rings can flow for years with no decrease!19571957: : BardeenBardeen((UIUC!UIUC!), Cooper, and ), Cooper, and SchriefferSchrieffer(“BCS”) publish theoretical (“BCS”) publish theoretical explanation, for which they get the Nobel prize in 1972.explanation, for which they get the Nobel prize in 1972.It was It was Bardeen’sBardeen’ssecondsecondNobel prize (1956 Nobel prize (1956 ––transistor)transistor)11119/22/20039/22/200317.7 Electrical energy and power17.7 Electrical energy and powerIn any circuit, battery is used to induce electrical currentIn any circuit, battery is used to induce electrical currentchemical energychemical energyof the battery is transformed into of the battery is transformed into kinetic energykinetic energyof mobile charge carriers


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