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
Latent Heats of Fusion and vaporization T(°C) 120 100 60 20 Water Steam Steam -20 Water ater Ice 62.7396 815 3080 Energy added() Physics 121: Lecture 25, Pg 6
Physics 121: Lecture 25, Pg 6 Latent Heats of Fusion and Vaporization Energy added (J) T (oC) 120 100 80 60 40 20 0 -20 -40 Water Water + Ice Water + Steam Steam 62.7 396 815 3080
Energy transfer mechanisms Thermal conduction(or conduction) Energy transferred by direct contact E.g. energy enters the water through ne bottom of the pan by thermal conduction Important: home insulation, etc Rate of energy transfer through a slab of area a and thickness AX, with opposite faces at different temperatures, Tc and Tr fo=Q/At=kA(Th-TC)/4X Energy k: thermal conductivity flo Physics 121: Lecture 25, Pg 7
Physics 121: Lecture 25, Pg 7 Energy 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 through a slab of area A and thickness x, with opposite faces at different temperatures, Tc and Th k : thermal conductivity x Th Tc A Energy flow =Q/t = k A (Th - Tc ) / x
Thermal Conductivities J/s m oC U/s m oc J/s m oc Aluminum 238 0.0234 Asbestos025 Copper 397 Helium 0.138 Concrete 1.3 Gold 314 Hydrogen.172 Glass 0.84 Iron 95 Nitrogen.0234 1.6 Lead 34.7 Oxygen.0238Water 0.60 Silver 427 Rubber. Wood 0.10 Physics 121: Lecture 25, Pg 8
Physics 121: Lecture 25, Pg 8 Thermal Conductivities Aluminum 238 Air 0.0234 Asbestos 0.25 Copper 397 Helium 0.138 Concrete 1.3 Gold 314 Hydrogen 0.172 Glass 0.84 Iron 79.5 Nitrogen 0.0234 Ice 1.6 Lead 34.7 Oxygen 0.0238 Water 0.60 Silver 427 Rubber 0.2 Wood 0.10 J/s m 0C J/s m 0C J/s m 0C
Energy transfer mechanisms Convection Energy is transferred by flow of substance E.g.: heating a room (air convection E.g. warming of North Altantic by warm waters from the equatorial regions Natural convection from differences in density Forced convection from pump of fan p Radiation: Energy is transferred by photons E.g.: infrared lamps Stephans law fo oAe T4: Power o=5.7x10-8 W/m2 K4 t is in Kelvin and a is the surface area e is a constant called the emissivity Physics 121: Lecture 25, Pg 9
Physics 121: Lecture 25, Pg 9 Energy transfer mechanisms Convection: Energy is transferred by flow of substance E.g. : heating a room (air convection) E.g. : warming of North Altantic by warm waters from the equatorial regions Natural convection: from differences in density Forced convection: from pump of fan Radiation: Energy is transferred by photons E.g. : infrared lamps Stephan’s law s = 5.710-8 W/m2 K4 , T is in Kelvin, and A is the surface area e is a constant called the emissivity = sAe T4 : Power
Resisting Energy Transfer The Thermos bottle also called a Dewar flask is designed to minimize Vacuum energy transfer by conduction convection and radiation the standard flask is a double-walled Pyrex glass with silvered walls and Silvered the space between the walls is surfaces evacuated Hot or cold liquid Physics 121: Lecture 25, Pg 10
Physics 121: Lecture 25, Pg 10 Resisting Energy Transfer The Thermos bottle, also called a Dewar flask is designed to minimize energy transfer by conduction, convection, and radiation. The standard flask is a double-walled Pyrex glass with silvered walls and the space between the walls is evacuated. Vacuum Silvered surfaces Hot or cold liquid
