Lecture 23, Nov. 19Exam III Room assignmentsWork and Ideal Gas Processes (on system)Heat and Ideal Gas Processes (on system)Exercise Identify processesSlide 6Heat and Latent HeatExercise Latent HeatSlide 9Energy transfer mechanismsSlide 11Thermal ConductivitiesExercise Thermal ConductionSlide 16Minimizing Energy TransferAnti-global warming or the nuclear winter scenarioCh. 18, Macro-micro connection Molecular Speeds and CollisionsMolecular Speeds and CollisionsSlide 21Mean Free PathMacro-micro connectionSlide 24Kinetic energy of a gasSlide 26Degrees of freedom or “modes”Slide 28The Equipartition TheoremExercisePhysics 207: Lecture 23, Pg 1Lecture 23, Nov. 19Goals:Goals:•Chapter 17Chapter 17 Apply heat and energy transfer processes Recognize adiabatic processes•Chapter 18Chapter 18 Follow the connection between temperature, thermal energy, and the average translational kinetic energy molecules Understand the molecular basis for pressure and the ideal-gas law. To predict the molar specific heats of gases and solids.•AssignmentAssignment HW10, Due Sunday (11:59 PM) For Wednesday, Read through all of Chapter 18Physics 207: Lecture 23, Pg 2Exam III Room assignments613 Room 2223 Koki601 Room 2241 Matt603 Room 2241 Heming608 Room 2241 Matt609 Room 2241 Heming607 Room 2241 KokiAnd all others in Room 2103602 604605606610611612614Physics 207: Lecture 23, Pg 3Work and Ideal Gas Processes (on system)Isothermal)/VV( nRTifnW Isobaric)V-V( pifWIsochoric0W)(12constconst2121VVPdVWVVVVVdVVFYI: Adiabatic (and reversible)Physics 207: Lecture 23, Pg 4Heat and Ideal Gas Processes (on system)Isothermal Expansion/ContractionWQ IsobaricIsochoricTnCQVTRCnTnCQVp )(Adiabatic 0QPhysics 207: Lecture 23, Pg 5Identify the nature of paths 1, 2, 3 and 4(A) Isobaric(B) Isothermal (C) Isochoric(D) Adiabatic Exercise Identify processespV1234T1T2T3T4Physics 207: Lecture 23, Pg 6Two process are shown that take an ideal gas from state 1 to state 3. Compare the work done by process A to the work done by process B.A. WA > WBB. WA < WB C. WA = WB = 0D. WA = WB but neither is zeroON BYA 1 3 W12 = 0 (isochoric)B 1 2 W12 = -½ (p1+p2)(V2-V1) < 0 -W12 > 0B 2 3 W23 = -½ (p2+p3)(V1-V2) > 0 -W23 < 0B 1 3 = ½ (p3 - p1)(V2-V1) > 0 < 0Physics 207: Lecture 23, Pg 7Heat and Latent HeatLatent heat of transformation L is the energy required for 1 kg of substance to undergo a phase change. (J / kg)Q = ±MLSpecific heat c of a substance is the energy required to raise the temperature of 1 kg by 1 K. (Units: J / K kg )Q = M c ΔTMolar specific heat C of a gas at constant volume is the energy required to raise the temperature of 1 mol by 1 K.Q = n CV ΔTPhysics 207: Lecture 23, Pg 8Most people were at least once burned by hot water or steam. Assume that water and steam, initially at 100°C, are cooled down to skin temperature, 37°C, when they come in contact with your skin. Assume that the steam condenses extremely fast, and that the specific heat c = 4190 J/ kg K is constant for both liquid water and steam.Under these conditions, which of the following statements is true?(a) Steam burns the skin worse than hot water because the thermal conductivity of steam is much higher than that of liquid water.(b) Steam burns the skin worse than hot water because the latent heat of vaporization is released as well.(c) Hot water burns the skin worse than steam because the thermal conductivity of hot water is much higher than that of steam.(d) Hot water and steam both burn skin about equally badly.Exercise Latent HeatPhysics 207: Lecture 23, Pg 9Exercise Latent HeatMost people were at least once burned by hot water or steam. Assume that water and steam, initially at 100°C, are cooled down to skin temperature, 37°C, when they come in contact with your skin. Assume that the steam condenses extremely fast, and that the specific heat c = 4190 J/ kg K is constant for both liquid water and steam.Under these conditions, which of the following statements is true?(b) Steam burns the skin worse than hot water because the latent heat of vaporization is released as well.How much heat H1 is transferred to the skin by 25.0 g of steam?The latent heat of vaporization for steam is L = 2256 kJ/kg.H1 = 0.025 kg x 2256 kJ/kg = 63.1 kJHow much heat H2 is transferred to the skin by 25.0 g of water?H2 = 0.025 kg x 63 K x 4190 J/ kg K = 6.7 kJPhysics 207: Lecture 23, Pg 10Energy transfer mechanismsThermal conduction (or conduction)ConvectionThermal RadiationFor a material of cross-section area A and length L, spanning a temperature difference ΔT = TH – TC, the rate of heat transfer iswhere k is the thermal conductivity, which characterizes whether the material is a good conductor of heat or a poor conductor.Q / t = k A T / xPhysics 207: Lecture 23, Pg 11Energy transfer mechanismsThermal conduction (or conduction): Energy transferred by direct contact. e.g.: energy enters the water through the bottom of the pan by thermal conduction. Important: home insulation, etc. Rate of energy transfer ( J / s or W ) Through a slab of area A and thickness x, with opposite faces at different temperatures, Tc and Th Q / t = k A (Th - Tc ) / x k :Thermal conductivity (J / s m °C)Physics 207: Lecture 23, Pg 12Thermal ConductivitiesAluminum 238 Air 0.0234 Asbestos 0.25Copper 397 Helium 0.138 Concrete 1.3Gold 314 Hydrogen 0.172 Glass 0.84Iron 79.5 Nitrogen 0.0234 Ice 1.6Lead 34.7 Oxygen 0.0238 Water 0.60Silver 427 Rubber 0.2 Wood 0.10J/s m °C J/s m °C J/s m °CPhysics 207: Lecture 23, Pg 15100 CTwo thermal conductors (possibly inhomogeneous) are butted together and in contact with two thermal reservoirs held at the temperatures shown. Which of the temperature vs. position plots below is most physical?Exercise Thermal Conduction300 CPositionTemperaturePositionTemperaturePositionTemperature(A)(B)(C)Physics 207: Lecture 23, Pg 16Energy transfer mechanismsConvection: Energy is transferred by flow of substance1. Heating a room (air convection) 2. Warming of North Altantic by warm waters from the equatorial regions Natural convection: from differences in density Forced convection: from pump of fanRadiation:
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