2 Thermal systems Classical mechanics Thermal systems: - deals with many individual objects- conceptually different from mechanical systems- don’t know the position, velocity, and energy ofany molecules or atoms or objects- can’t perform any calculation on themSacrifice microscopic knowledge of the system,using macroscopic parameters instead- volume (V)- temperature (T)- pressure (P)- number of particles (N)- energy (E), etc.Macroscopic systems with many individual objects:- processes are often irreversible- arrow of time does exist- energy conservation is not enough to describethe thermal statesThermal system
3 Thermodynamic systems Isolated systems can exchange neither energy nor matter with the environment.reservoirHeatWorkreservoirHeatWorkOpen systems can exchangeboth matter and energy withthe environment.Closed systems exchange energybut not matter with the environment.
4 The ideal gas equation of state: Idea gas modelLattice model for solid state materialsThe ideal gas modelall the particles are identicalthe particles number N is hugethe particles can be treated as point massesthe particles do not interact with each otherthe particles obey Newton’s laws of motion, but their motion is randomcollisions between the particles are elasticThe ideal gas equation of state:kB = 1.38 J/K
5 Internal energy The internal energy of a system of N particles, U, is all the energy of the system that isassociated with its microscopic componentswhen view from a reference frame at restwith respect to the object.Internal energy includes:- kinetic energy of translation, rotation, andvibration of particles- potential energy within the particles- potential energy between particlesInternal energy is a state function – it dependsonly on the values of macroparameters (thestate of a system)For a non-ideal gas:For an ideal gas (no interactions):Monatomic:Diatomic:
6 HeatHeat and work are both defined to describe energy transfer across a system boundary.Heat (Q): the transfer of energy across the boundary of a system due to a temperature difference between the system and its surroundings.- Q > 0: temperature increases; heating process- Q < 0: temperature decreases; cooling process(C: heat capacity)Heat transfer mechanisms- conduction: exchange of kinetic energy betweenmicroscopic particles (molecules, atoms, andelectrons) through collisions- convection: energy transfer by the movement of aheated substance such as air- radiation: energy transfer in the form of electromagneticwavesWork (W): any other kind of energy transfer across boundaryheat
7 Quasi-static processes Quasi-static (quasi-equilibrium) processes:Sufficiently slow processes, and any intermediate state can be considered as at thermal equilibrium. The macro parameters are well-defined for all intermediate states.The state of a system that participates in a quasi-equilibrium process can be described with the same number of macro parameters as for a system in equilibrium.Examples of quasi-static processes:- isothermal: T = constant- isovolumetric: V = constant- isobaric: P = constant- adiabatic: Q = 0
8 Work done during volume changes Quasi-static processat each infinitesimalmovementWork done by thegas as its volumechanges from Vi to Vf
9 Work done during volume changes (cont.) dV > 0: the work done on the gas is negativedV < 0: the work done on the gas is positiveIn thermodynamics, positive work represents a transfer of energy out of thesystem, and negative work represents a transfer of energy into the system.ifPVPiPfViVfP-V diagramThe work done by a gas in the expansionis the area under the curve connectingthe initial and final states
10 Work and heat are not state functions bca. isovolumetricb. isobarica. isobaricb. isovolumetricisothermalBecause the work done by a system depends on the initial and final states andon the path followed by the systems between the states, it is not a state function.Energy transfer by heat also depends on the initial, final, and intermediate statesof the system, it is not a state function either.
11 When heat enters a system, will it increase the system’s internal energy? When work is done on a system, will it increase the system’s internal energy?It depends on the path!
12 The first law of thermodynamics HeatWorkreservoirTwo ways to exchange energy between a systemand its surroundings (reservoir):heat and workSuch exchanges only modify the internal energy ofthe systemThe first law of thermodynamics: conservation of energyQ > 0: energy enters the systemQ < 0: energy leaves the systemW > 0: work done on the system is negative;energy leaves the systemW < 0: work done on the system is positive;energy enters the systemFor infinitesimal processes:
13 Several examples Isolated systems: Cyclic processes Adiabatic processesPi, fVInsulatingwallinitial state = final stateExpansion: U decreasesCompression: U increasesThe internal energyof an isolated systemsremains constantEnergy exchange between“heat” and “work”
14 Idea gas isovolumetric process Isovolumetric process: V = constantP21(CV: heat capacityat constant volume)V1,2VHeatreservoirDuring an isovolumetric process, heat enters(leaves) the system and increases (decreases)the internal energy.
15 Idea gas isobaric process Isobaric process: P = constant21(CP: heat capacityat constant pressure)V1V2VDuring an isobaric expansion process,heat enters the system. Part of the heat isused by the system to do work on theenvironment; the rest of the heat is usedto increase the internal energy.HeatWorkreservoir
16 Idea gas isothermal process 1Isothermal process: T = constant2V1V2VDuring an isothermal expansion process,heat enters the system and all of the heatis used by the system to do work on theenvironment.During an isothermal compression process,energy enters the system by the work doneon the system, but all of the energy leavesthe system at the same time as the heatis removed.
17 Idea gas adiabatic process Adiabatic process: Q = 0P21V2V1VIdea gas:Adiabatic process:
18 Idea gas adiabatic process 21let, and divided byV2V1V
19 Idea gas adiabatic process 21V2V1VFor monatomic gas,
21 Idea gas adiabatic process 21V2V1VDuring an adiabatic expansion process, the reduction of the internal energy isused by the system to do work on the environment.During an adiabatic compression process, the environment does work on the system and increases the internal energy.
22 Summary Internal energy, heat, and work: - internal energy is the energy of the system; a state function- heat and work are two ways to exchange energy between the systemand the environment. They are not state functions and depend on the pathThe first law of thermodynamics connects the internal energy with heat andwork:Quasi-staticprocessCharacterisovolumetricV = constantisobaricP = constantisothermalT = constantadiabatic