热力学第一定律TheFirstLawofThermodynamics.ppt
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1、热力学第一定律 (The First Law of Thermodynamics),能量守恒与转换定律,能量之间数量的关系,回顾(Review),是否满足能量守恒与转换定律的过程一定都能自动发生?,A process must satisfy the first law of Thermodynamics to occur. (一个过程必须满足热力学第一定律才能发生),Satisfying the first law alone does not ensure that the process will actually take place. (但是,满足热力学第一定律的过程未必都能发生),
2、5.1自发过程的方向性) The Direction of Spontaneous Process 5.2热力学第二定律的表述 Statement of the Second Law of Thermodynamics 5.3卡诺循环与卡诺定律 Carnot cycle and Carnot Theorem 5.4熵、熵增原理及熵方程 Entropy, The increase principle of Entropy and Entropy Equation 5.5熵的意义及应用 Significance of Entropy and its application,Chapter 5.Th
3、e Second Law of Thermodynamics (第五章 热力学第二定律),过程的方向性,温差传热 (Heat Transfer under temperature difference) 自由膨胀 (Free Expansion),A,B,TA TB,A B,QA,QB,U1=U2,QA= QB,QA= QB,U1=U2,5.1 Spontaneous Process (自发过程),温差传热和自由膨胀均为不平衡势差推动下的非准静态过程,也即不可逆过程,摩擦生热 电容电阻电路,U2=U1+mc2/2,U1=U2+mc2/2,自发过程的方向性 (Direction of Spont
4、aneous Process),自发过程:不需要任何外界作用而自动进行的过程。 Such as (例如:) a. heat transfer driven by finite temperature difference (有限温差传热) b. work is converted into heat by friction (通过摩擦功转变为热) c. Free or unrestrained expansion (空气向真空自由膨胀)。 d. Mixing process of different substances (不同的流体混合),自发过程的方向性 (Direction of Spo
5、ntaneous Process),功量,自发过程具有方向性、条件、限度,摩擦生热,热量,100%,热量,发电厂,功量,40%,放热,自发过程只能沿某一方向进行,而不能反向自动发生,自然界自发过程都具有方向性,3.热力学第二定律的任务 Essence of the Second Law of Thermodynamics,能不能找出共同的规律性? 能不能找到一个判据?,自然界过程的方向性表现在不同的方面,热力学第二定律,研究过程的方向性和补偿限,5.2 Statement and Essence of the Second Law of Thermodynamics (热力学第二定律的表述和实
6、质),热二律的表述有 60-70 种,传 热 热功转换,1851年 开尔文普朗克表述 热功转换的角度,1850年 克劳修斯表述 热量传递的角度,1. 克劳修斯表述,不可能将热从低温物体传至高温物体而不引起其它变化。,It is impossible to construct a device that operates in a cycle and produces no effect other than the transfer of heat from a lower-temperature body to a higher-temperature body.,Clausius stat
7、ement,克劳修斯表述,Clausius statement,Transfer heat from high temperature reservoir to low temperature ones is a spontaneous process. However it is irreversible. (从高温物体向低温物体的传热是一个自发过程,但是不可逆) Transfer heat from low temperature reservoir to high temperature ones is possible. However it will leave influence
8、on environment. (从低温物体向高温物体的传热过程是可以实现的,但会给环境造成一定影响),空调,制冷 (Air-Conditioning, Refrigerating),代价:耗功 (Cost: Energy Consumption),热量不可能自发地、不付代价地从低温物体传至高温物体。,2. 开尔文普朗克表述,不可能从单一热源取热,并使之完全转变为有用功而不产生其它变化。,KelvinPlanck Statement,It is impossible for any device that operates on a cycle to receive heat from a s
9、ingle reservoir and produce a net amount of work.,No heat engine can achieve a 100 percent thermal efficiency. (热机的效率不可能达到100%) For a heat engine, the working fluid must exchange heat with the environment. (对于热机来说,工质必然和环境交换热量),热机不可能将从热源吸收的热量全部转变为有用功,而必须将某一部分传给冷源。,开尔文普朗克表述,KelvinPlanck Statement,Heat
10、 reservoirs,Thermal Energy Source,冷热源:容量无限大,取、放热其温度不变,但违反了热 力学第二定律,第二类永动机 perpetual-motion machine of the second kind,第二类永动机:设想的从单一热源取热并 使之完全变为功的热机。,这类永动机 并不违反热力 学第一定律,第二类永动机是不可能制造成功的,环境是个大热源,Perpetual motion machine of the second kind,锅 炉,汽轮机,发电机,给水泵,凝汽器,Wnet,Qout,Q,Suppose we can construct a heat
11、pump which transfers heat from a low temperature reservoir to a high temperature one without using external work. Then, we can couple it with a heat engine in such a way that the heat removed by the heat pump from the low temperature reservoir is the same as the heat rejected by the heat engine, so
12、that the combined system is now a heat engine which converts heat to work without any external effect. This is thus in violation of the Kelvin-Planck statement of the second law.,3.克劳修斯说法与开尔文-普朗克说法等价 Equivalence of Clausius and Kelvin-Planck Statements,Now suppose we have a heat engine which can con
13、vert heat into work without rejecting heat anywhere else. We can combine it with a heat pump so that the work produced by the engine is used by the pump. Now the combined system is a heat pump which uses no external work, violating the Clausius statement of the second law. Thus, we see that the Clau
14、sius and Kelvin-Planck statements are equivalent, and one necessarily implies the other.,两种表述的关系,开尔文普朗克 表述,完全等效!,克劳修斯表述:,违反一种表述,必违反另一种表述!,热一律否定第一类永动机,热机的热效率最大能达到多少? 又与哪些因素有关?,?,热一律与热二律,t 100不可能,热二律否定第二类永动机,t =100不可能,热二律的实质, 自发过程都是具有方向性的 表述之间等价不是偶然,说明共同本质 若想逆向进行,必付出代价,代价为多少方可进行?,1.卡诺循环 Carnot Cycle,法
15、国工程师卡诺 (S. Carnot), 1824年提出 卡诺循环,热二律奠基人,Carnot Cycle is a a particular cycle that has the best possible efficiency, which is important in practice. It sets an upper limit on what is possible for real engines.,5.3 Carnot Cycle and Carnot Theorem (卡诺循环及卡诺定理),p,V,Heat is transferred to the working flui
16、d during 1-2 (Qh) and heat is rejected during 3-4 (Ql).,an isothermal expansion at high temperature Th (在高温h 下的等温吸热过程) (2) an adiabatic expansion (可逆绝热膨胀过程) (3) an isothermal compression at low temperature Tl (在低温l下的等温放热过程) (4) an adiabatic compression (可逆绝热压缩过程),卡诺循环 Carnot Cycle,卡诺循环热机效率,卡诺循环热机效率,
17、q1,q2,w,Thermal efficiency of Carnot Cycle,Applying first law,For Isothermal Process of Ideal gas: (对理想气体的等温过程),Thermal efficiency of Carnot Cycle (卡诺循环的热效率),For ideal gas,在相同的高温热源和相同的低温热源间工作的可逆热机的热效率恒高于不可逆热机的热效率; The efficiency of an irreversible heat engine is always less than that of a reversible
18、 one operating between the same two thermal reservoirs.,2.Carnots Theorems (卡诺定理),在相同的高温热源和相同的低温热源间工作的一切可逆热机有相同的热效率,而与工质无关。 The efficiencies of all reversible heat engines operating between the same two thermal reservoirs are the same.,2.Carnots Theorems (卡诺定理),Assumed that the work done by these tw
19、o heat engines is the same. (假定两个热机所做的功相等),W,W,reversible,irreversible,卡诺定理的证明 (Investigation on the Carnot Theorem),For a Carnot engine, the efficiency simplifies to This is the highest efficiency a heat engine operation between the two thermal energy reservoirs at temperatures and can have. The th
20、ermal efficiencies of actual and reversible heat engines operating between the same temperature limits compares as follows,(1)卡诺定理指明了热变功的最高效率 The Carnot Theorem indicates the maximum thermal efficiency of heat engine, which converts heat into work.,3. The significance of the Carnot Theorems (卡诺定理的意义
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