Page 1Physics 207 – Lecture 23Physics 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 assignments 613 Room 2223 Koki 601 Room 2241 Matt603 Room 2241 Heming608 Room 2241 Matt609 Room 2241 Heming607 Room 2241 Koki And all others in Room 2103602 604605606610611612614Page 2Physics 207 – Lecture 23Physics 207: Lecture 23, Pg 3Work and Ideal Gas Processes (on system) Isothermal)/VV( nRTifnWl−= Isobaric)V-V( pif−=W Isochoric0=W)(12constconst2121γγγγ−−−=−=−=∫∫VVPdVWVVVVVdVV FYI: Adiabatic (and reversible)Physics 207: Lecture 23, Pg 4Heat and Ideal Gas Processes (on system) Isothermal Expansion/ContractionWQ−= Isobaric IsochoricTnCQV∆=TRCnTnCQVp∆+=∆=)( Adiabatic 0=QPage 3Physics 207 – Lecture 23Physics 207: Lecture 23, Pg 5 Identify 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 byprocess B.A. WA> WBB. WA< WBC. WA= WB= 0D. WA= WBbut neither is zeroON BYA 1 3 W1 2= 0 (isochoric)B 1 2 W1 2= -½ (p1+p2)(V2-V1) < 0 -W1 2> 0B 2 3 W2 3= -½ (p2+p3)(V1-V2) > 0 -W2 3< 0B 1 3 = ½ (p3 - p1)(V2-V1) > 0 < 0Page 4Physics 207 – Lecture 23Physics 207: Lecture 23, Pg 7Heat and Latent Heat Latent heat of transformation L is the energy required for 1 kg of substance to undergo a phase change. (J / kg)Q = ±ML Specific 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 T Molar 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 CVTPhysics 207: Lecture 23, Pg 8 Most 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 w ith 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 latentheat 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 HeatPage 5Physics 207 – Lecture 23Physics 207: Lecture 23, Pg 9Exercise Latent Heat Most 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 latentheat of vaporization is released as well. How much heat H1is 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 kJ How much heat H2is 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 mechanisms Thermal conduction (or conduction) Convection Thermal RadiationFor a material of cross-section area A and length L, spanning a temperature difference T = 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 / ∆xPage 6Physics 207 – Lecture 23Physics 207: Lecture 23, Pg 11Energy transfer mechanisms Thermal 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, Tcand ThQ / ∆t = k A (Th- Tc) / ∆x k :Thermal conductivity (J / s m °C) Physics 207: Lecture 23, Pg 12Thermal Conductivities0.10Wood0.2Rubber427Silver0.60Water0.0238Oxygen34.7Lead1.6Ice0.0234Nitrogen79.5Iron0.84Glass0.172Hydrogen314Gold1.3Concrete0.138Helium397Copper0.25Asbestos0.0234Air238AluminumJ/s m °CJ/s m °CJ/s m °CPage 7Physics 207 – Lecture 23Physics 207: Lecture 23, Pg 13(B) Ttop= Tbottom(A) Ttop> Tbottom(C) Ttop< Tbottom100 CTjoint Two identically shaped bars (one blue and one green) are placed between two different thermal reservoirs . The thermal conductivity coefficient k is twice as large for the blue as the green. You measure the temperature at the joint between the green and blue bars. Which of the following is true?Home Exercise Thermal Conduction300 C(D) need to know kPhysics 207: Lecture 23, Pg 14 Two identically shaped bars (one blue and one green) are placed between two different thermal reservoirs . The thermal conductivity coefficient k is twice as large for the blue as the green.Home Exercise Thermal Conduction100 CTjoint300 CTop: Pgreen= Pblue= Q / ∆t = 2 k A (Thigh- Tj) / ∆x= k A (Tj- Tlow) / ∆x2 (Thigh- Tj) = (Tj- Tlow) 3 Tj(top)= 2 Thigh– TlowBy analogy for the bottom: 3 Tj(bottom)= 2 Tlow– Thigh 3 (Tj(top)- Tj(bottom) = 3 Thigh– 3 Tlow> 0(A) Ttop> TbottomPage 8Physics 207 – Lecture 23Physics 207: Lecture 23, Pg 15100 C Two thermal conductors (possibly inhomogeneous) are butted together and in contact with two thermal
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