说明工程热力学

1、ME 200 Thermodynamics ILecture 5: Mechanical concepts of energy,Moving Boundary WorkYong LiShanghai Jiao Tong University Institute of Refrigeration and Cryogenics800 Dong Chuan Road Shanghai, 200240, P. R. China: liyoPhone: 86-21-34206056; Fax: 86-21-342060561.1In the Chapter 1l Fundamental Thermody

2、namic Concepts»»»»Closed/Open Systems State & equilibrium ProcessesPropertiesPressurel absolute pressure Specific volume Temperaturel constant-volume gas thermometerl absolute thermodynamic temperature scalel different temperature scale1.2This Lecturel Work and Kinetic Energy

3、l Potential Energyl Conservation of Energy in Mechanicsl Moving Boundary Work1.3Forms of EnergyKinetic energyPotential energy(PE) 动能(KE) 势能MacroscopicCan be seen“hidden energy” Related to molecular structure andactivityMicroscopic Internal energy(U) 热力学能lTotal energy:总能E = KE+PE + UMACROSCOPIC FORMS

4、+ MICROSCOPIC FORMSTOTAL ENERGYOF ENERGYOF ENERGYE is a propertyApropertyisacharacteristicofasystem,anditsvalue isindependent of the history of the systemA quantity is a property if, and only if, its change in value between two states is independent of the process.1.4Macroscopic Energy and WorklMech

5、anical Work: Energy transfer to or from a system due to a force acting through a distanceForce causes object to accelerate or balance frictiondW = FdslConstant force:(Force in direction of movement)2òlVariable force:W =Fds1= F ds = F gVdtW&(V = velocity)llInstantaneous power:Sign Convention

6、:Is work a property?No»»W > 0 work done by the system (work output)W < 0 work done on the system (work input)1.5Macroscopic Energy (KE) and Workl Kinetic Energy (KE): Work required to accelerate system tovelocity (no frictNewtonrelative to reference frame, in the absence of an opposi

7、ng forcen or Gravity);lsecond law of motionIs KE a property?Yesl Extensive property, scalar quantityl KE does not depend on path WorkN·mWorkJKE1.6Kinetic Energy Examplel What is the power required to accelerate a 2000 kg mass from 0 to 100 km/h in 10 s. Assume that thesource of power provides c

8、onstant power.1.7Macroscopic Energy (PE) and Workl Potential Energy (PE): Work required to raise a system to aheight h above a reference plane in a gravitational field (no drag)l Gravitational Potential Energy (PE)Is PE a property?Yesl PE is extensive property of the systeml PE does not depend on pa

9、th1.8Conservation of Energy in Mechanicsl If a body is affected only by a gravitational field(no drag)1.9Microscopic Forms of Energy-Internal Energyl Accounts for all forms of microscopic energies(molecular structure of the system and degree ofmolecular activity)Ul Disorganized form of energyl exten

10、sive property1.10Important Notes about Energyl Energy is always measured relative to a reference»»»Reference plane for potential energy (PE)Reference frame for kinetic energy (KE) Reference state for internal energy (U)l In Thermodynamics we are interested in energy changes:1.11UnitTh

11、ermodynamic definition of workl Work in mechanics: forces and displacementl Work in thermodynamics: done by a system on its surrounding if the sole effect on everything external to the system could have been the raising of a weightl Energy crossing the boundary of a closed system that is not heatl S

12、ome important characteristics»»»»Work enters or leaves the system through the boundaries Work is not a propertyWork is associated with a process (change of states)In general work is a function of the path followed throughout the process.1.12Sign Convention for workl Sign Conventi

13、on and Notation in Thermodynamics»»»W > 0 : work done by the system W < 0 : work done on the systemSign convention is the opposite of that in mechanicsl Used throughout the bookl work is not a propertyNo meaningl Power:The rate of energy transfer by workJ/swatt1.13Moving boundar

14、y workl Form of mechanical workl Constant or Variable force: l Interaction between system and surroundings:»»»Must have a force acting on boundaryBoundary must move (e.g. force must act through a distance) If either is absent, then no boundary work1.14Moving Boundary Workl Area under

15、process path on PV diagram represents QEboundary workl Gas can follow several different paths as it expands from State 1 to State 2l Different areas so different workWORK IS A PATH FUNCTION1.15Boundary Workl Expansion or compression of a gas in a piston cylinder devicel Can only evaluate work analyt

16、ically if we have a QE process (slow)l BECAUSE: work is a path function and we need to know the path1.16Boundary Work QE Processl Expansion or compression work: Movingboundary workl If the process is too fast, equilibrium is not reached at every pointl the process is slow, near equilibrium condition

17、s can be achieved:»QuasiEquilibrium Process or QuasiStaticProcess1.17Boundary WorkQuasiequilibriumprocess1.18Boundary Workl Moving boundary work: Quasiequilibrium processl The process is considered a series of equilibrium states, i.e., “almost static”!l The system can be returned to the initial

18、 state by reversing the process, i.e., process is reversible!l The work can be evaluated in terms of changes in system state!1.19Boundary Work: Example1.20forn = 1.5forn ¹ 1Boundary Work: Example1.21forn = 1In Summaryl Work is the form of energy when energy crosses the system boundary without t

19、emperature differencel Work is observed as boundary phenomenon and involves force acting through a distancel System does not possess work i.e. work is not a property of a given systeml Work is relevant to a process not a state (similar to heat, Q)1.22quizl Separate the list P, F, Vo, v, , T, a, m, L, t and Ve into intensive, extensive and non-properties.Intensive properties are independent upon mass: P, v, , T, Ve Extensive properties scales with mass: Vo, m, VeNon-properties: F, a, L, t,1.23l What pressure d