Physics 121: Lecture 25 Today's Agenda Announcements No more homeworks i About midterm 2 Final: Monday Dec. 12 3: 30PM Room P-38 Today' s topics Heat and energy Laws of thermodynamics Work in thermodynamics First Law of thermodynamics and applications Heat engines and Second Law of thermodynamics Reversible/irreversible processes and Entropy Physics 121: Lecture 25, Pg
Physics 121: Lecture 25, Pg 1 Physics 121: Lecture 25 Today’s Agenda Announcements No more homeworks ! About midterm 2: Final: Monday Dec. 12 @ 3:30PM Room P-38. Today’s topics Heat and energy Laws of thermodynamics Work in thermodynamics First Law of thermodynamics and applications Heat engines and Second Law of thermodynamics Reversible/irreversible processes and Entropy
Chap. 14: Energy in Thermal Processes Solids, liquids or gases have internal energy Kinetic energy from random motion of molecules translation rotation vibration At equilibrium, it is related to temperature Heat: transfer of energy from one object to another as a result of their different temperatures Thermal contact: energy can flow between objects Physics 121: Lecture 25, Pg 2
Physics 121: Lecture 25, Pg 2 Chap. 14: Energy in Thermal Processes Solids, liquids or gases have internal energy Kinetic energy from random motion of molecules translation, rotation, vibration At equilibrium, it is related to temperature Heat: transfer of energy from one object to another as a result of their different temperatures Thermal contact: energy can flow between objects T1 T2 U1 U2 >
Heat Heat:Q=C△T Q= amount of heat that must be supplied to raise the temperature by an amount AT [Q]= Joules or calories. 1 Cal=4.186 J 1 kcal= 1 cal=4186 J energy to raise 1 g of water from 14.5 to 15.5oC James prescott Joule found mechanical equivalent of heat C: Heat capacity Q=cmΔT C: specific heat(heat capacity per units of mass) amount of heat to raise T of 1 kg by 1C (( [c]=J/(kg°C) Sign convention +Q: heat gained Q: heat lost Physics 121: Lecture 25, Pg 3
Physics 121: Lecture 25, Pg 3 Heat Heat: Q = C T Q = amount of heat that must be supplied to raise the temperature by an amount T . [Q] = Joules or calories. energy to raise 1 g of water from 14.5 to 15.5 oC James Prescott Joule found mechanical equivalent of heat. C : Heat capacity 1 Cal = 4.186 J 1 kcal = 1 Cal = 4186 J Q = c m T c: specific heat (heat capacity per units of mass) amount of heat to raise T of 1 kg by 1oC [c] = J/(kg oC) Sign convention: +Q : heat gained - Q : heat lost
Specific Heat examples Substance c in J/(kg-C) alumInum 900 copper 387 iron ea 28 human body 3500 water 4186 Ice 2000 You have equal masses of aluminum and copper at the same itial temperature. You add 1000 j of heat to each of them Which one ends up at the higher final temperature? a) aluminum b)copper c)the same Physics 121: Lecture 25, Pg 4
Physics 121: Lecture 25, Pg 4 Specific Heat : examples You have equal masses of aluminum and copper at the same initial temperature. You add 1000 J of heat to each of them. Which one ends up at the higher final temperature ? a) aluminum b) copper c) the same Substance c in J/(kg-C) aluminum 900 copper 387 iron 452 lead 128 human body 3500 water 4186 ice 2000
Latent heat Latent heat: amount of energy needed to add or to remove from a substance to change the state of that substance Phase change: T remains constant but internal energy changes heat does not result in change in T(latent ="hidden) e.g.: solid o> liquid or liquid o gas heat goes to breaking chemical bonds n Heat per unit mass [L=J/kg Q=±mL stea if heat needed(boiling) if heat given(freezing) lce. Lf: Latent heat of fusion 2000 solid o liquid 30703110 nenx ade (一 Ly: Latent heat of vaporization liquid o gas Lf(J/kg) Lv(J/kg) water335×10422.6×105 Physics 121: Lecture 25, Pg 5
Physics 121: Lecture 25, Pg 5 Latent Heat L = Q / m Heat per unit mass [L] = J/kg Q = m L + if heat needed (boiling) - if heat given (freezing) Lf : Latent heat of fusion solid liquid Lv : Latent heat of vaporization liquid gas Latent heat: amount of energy needed to add or to remove from a substance to change the state of that substance. Phase change: T remains constant but internal energy changes heat does not result in change in T (latent = “hidden”) e.g. : solid liquid or liquid gas heat goes to breaking chemical bonds Lf (J/kg) Lv (J/kg) water 33.5 x 104 22.6 x 105