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