大连理工大学：《工程热力学》课程教学资源（PPT课件）2 热力学定律 First Law of Thermodynamics

First Law of ThermodynamicsObjective· First Law of Thermodynamics Energy Forms and Energy Transfer..Work, Heat and Mass· Energy Balance for Closed System·EnergyBalance forOpen System· Energy Balance for Steady-Flow System

First Law of Thermodynamics Objective • First Law of Thermodynamics • Energy Forms and Energy Transfer .Work, Heat and Mass • Energy Balance for Closed System • Energy Balance for Open System • Energy Balance for Steady-Flow System

First Law of ThermodynamicsConservationof Energy PrincipleEnergy can be neither created nordestroyed ;lt can only change formsEnergy can change many forms, but its totalvalue keeps constant.The perpetual-motion machine of the first kindcan never be made

First Law of Thermodynamics Conservation of Energy Principle • Energy can be neither created nor destroyed ;It can only change forms. • Energy can change many forms, but its total value keeps constant. • The perpetual-motion machine of the first kind can never be made

亨内考的“魔轮”（13世纪）达.芬奇的“永动机”（15世纪）斯特尔的“永动机”（16世纪）浮力“永动机

First Law of ThermodynamicsIn 1843, at the age of 25, JamesPrescott Joule did a series ofcareful experiments to provetheClEnguinequivalence of heat and work

First Law of Thermodynamics In 1843, at the age of 25, James Prescott Joule did a series of careful experiments to prove the equivalence of heat and work

Forms of Energy Transfer(EnergyEnteringCV}-(EnergyLeavingCV)={EnergyAccumulatinginCV)There are three forms of energy transfer:Work:caused by a force acting through a distancevolume work: moving boundary work.Heat: caused by temperature difference between thesystemandsurroundings?Mass flow: mass flow in or out of the system serves as anenergy transfer

Forms of Energy Transfer There are three forms of energy transfer: •Work:caused by a force acting through a distance volume work: moving boundary work •Heat: caused by temperature difference between the system and surroundings. •Mass flow: mass flow in or out of the system serves as an energy transfer. {Energy Entering CV } － {Energy Leaving CV} ={Energy Accumulating in CV}

Energy Transfer by Work·Definitions- Energy transfer associated with force acting through a distance- Energy crossing the boundary of a closed system that is not heatmust bework系统与外界相互作用而传递的能量，其全部效果为使外界物体改变宏观运动状态。包含两个必要条件：1)确定有力作用在边界上，即系统与外界有相互作用，且力的不平衡势差无限小：2）系统边界发生位移，即外界物体改变宏观运动状态。二者缺一不可

Energy Transfer by Work • Definitions – Energy transfer associated with force acting through a distance – Energy crossing the boundary of a closed system that is not heat must be work 系统与外界相互作用而传递的能量，其全部效果为使外界 物体改变宏观运动状态。 包含两个必要条件： 1) 确定有力作用在边界上，即系统与外界有相互作用，且力 的不平衡势差无限小； 2) 系统边界发生位移，即外界物体改变宏观运动状态。 二者缺一不可

Energy Transfer by Work例1:拔掉销钉后，系统向真空膨胀VacuumO系统是否做功？例2:刚性容器，加热后系统内压力升高，系统是否向外做功？DQ

Energy Transfer by Work 例1： p Vacuum 拔掉销钉后，系统向真空膨胀， 系统是否做功？ 例2： p Q 刚性容器，加热后系统内压力 升高，系统是否向外做功？

Energy Transfer by Work.Units- N·m or J; usually kJ in Sl.Workexamples- Boundary work (e.g., a gas working against amoving piston)closedsystem- Shaft work (e.g., a rotating crankshaft on a motor)opensystem

Energy Transfer by Work • Units – or J; usually kJ in SI • Work examples – Boundary work (e.g., a gas working against a moving piston) closed system – Shaft work (e.g., a rotating crankshaft on a motor) open system N m

Energy Transfer by WorkSW = Fdx = p·Adx = pdVProcess pathsW=("pdVWdA=PdV示功图IVdvW8W =W12 +W2 -WWorkProcess function功是过程量

Energy Transfer by Work δW F x p A x p V = =  = d d d 2 2 1 1 W W p V = = δ d   2 12 2 1 1 W W W W W = =  − δ  Work－Process function 功是过程量 示功图

Energy Transferby Work. Work per unit massWW=m? Sign ConventionW > 0: work done by the systemW < o: work done on the systemW=0: no work

Energy Transfer by Work • Work per unit mass • Sign Convention W > 0: work done by the system W < 0: work done on the system W = 0: no work  = = 2 1 p v m W w d