化工专业英语Lesson-3

0 Lesson 3 The Second Law of Thermodynamics 热力学第二定律热力学第二定律 A1 Thermodynamics is concerned with transformation of energy and the laws of thermodynamics describe the bounds within which these transformation are observed to occur 热力学关心的是能量的转化 热力学定律描述了这些转化能够发生的边界条件 2 The first law stating that energy must be conserved in all ordinary processes has been the underlying principle of the preceding chapters 第一定律指出了在所有普遍的过程中 能量是守恒的 它是前面章节遵循的根本原则 3 The first law imposes no restriction on the direction of energy transformation 第一定律对能量转化的方向性没有限制 4 Yet all our experience indicates the existence of such a restriction 然而 经验表明这种限制的存在 5 To complete the foundation for the science of thermodynamics it is necessary to formulate this second limitation Its concise statement constitutes the second law 为了完善热力学科学的基础 有必要明确表达第二个限制条件 它简洁的陈述构成了第二定律 B1 The differences between the two forms of energy heat and work provide some insight into the second law 两种形式的能量 热和功之间的不同 为第二定律提供了一些见解 2 These differences are not implied by the first law 这些不同是第一定律没有提到的 3 In an energy balance both work and heat are included as simple additive terms implying that one unit of heat such as a joule or BTU is equivalent to the same unit of work 在能量平衡中 功和热作为简单的加和项 表明了 一个单位的热 如 1J 或 1BTU 等于相同单位 的功 4 Although this is true with respect to an energy balance 虽然 对于能量平衡来讲 这是正确的 5 Experience teaches that there is a difference in quality between heat and work 经验告诉我们 热和功之间有一个本质的差异 6 This experience can be summarized by the following facts 这方面的经验可以从以下事实中总结出来 C1 First the efficiency of the transformation from one form of work to another such as electrical to mechanical as accomplished in an electric motor can be made to approach 100 percent as closely as is desired 首先 从一种形式的功向另一种形式的功的转换效率接近 100 是我们所向往的 比如 从电能 到机械能的转换能够通过电动机来实现 2 One needs merely to exert more and more care in eliminating irreversibilities in the apparatus 一方面 人们仅仅越来越关心设备中不可逆性的消除 3 On the other hand efforts to convert energy transferred to a system as heat into any of the forms of work show this regardless of improvements in the machines employed 另一方面 人们努力的将转移到系统中的热量转换成任何一种形式的功表明 这与所使用机器的改 进无关 4 The conversion is limited to low values 40 percent is an approximate maximum 这种转化被限制在一个很低的值 最大值接近 40 5 These efficiencies are so low in comparison with these obtained for the transformation of work from form to another that there can be no escape from the conclusion that there is an intrinsic difference between heat and work in the reverse direction the conversion of work into heat with 100 percent efficiency is very common 1 与不同形式的功之间转化率相比 这些效率是太低了 以至于得出了一个不可逃避的结论 热与功 之间有一个本质的区别 相反地 功向热转化率为 100 这是非常普通的 6 Indeed efforts are made in nearly every machine to eliminate this conversion which decrease efficiency of operation 甚至 为了消除这种降低操作效率的转变 人们在努力改进几乎每台机器 7 These facts lead to the conclusion that heat is a less versatile or more degraded form of energy than work 从这些事实中可以得出一个结论 与功相比 热没有那么多变 或者说更退化的能量形式 8 Work might be termed energy of a higher quality than heat 功被称为是比热具有更高质量的能量 D1 To draw further upon our experience we know that heat always flows from a higher temperature level to a lower one and never in the reverse direction 为了进一步的总结我们的经验 我们知道热量经常从高温流向低温 绝不会沿相反的方向流动 2 This suggests that heat itself may be assigned a characteristic quality as well as quantity and that this quality depends upon temperature the relation of temperature to the quality of heat is evident from the increased in efficiency with which heat may be converted into work as the temperature of the source is raised 这表明 热本身可能被分配了一个和数量一样的特有的性质 这个性质取决于温度 从增加的效率 来看 温度和热的性质之间的关系是明显的 在这样的效率下 随着原始材料温度的升高 热也许 能够转化成功 3 For example the efficiency or work output per unit of fuel burned of a stationary power plant increases as the temperature of the steam in the boiler and superheater rises 例如 一个固定式发电站的效率 或者燃烧每单位燃料的输出功 随着锅炉或过热器中蒸汽温度的 升高而增加 E1 Statements of the Second Law 第二定律的陈述 2 The observations just described are results of the restriction imposed by the second law on the direction of actual processes 刚刚描述的观察结果是第二定律对实际过程在方向上的限制的结果 3 Many general statements may be made which described this restriction and hence serve as statements of the second law Two of the most common are 许多通用的语句能够描述这个限制 因此 这些描述可以作为第二定律的陈述 最常见的两个是 F1 No apparatus can operate in such a way that its only effect in system and surroundings is to convert heat absorbed by a system completely into work 没有设备操作能够仅仅产生这样一种影响 在系统内和环境中 将系统吸收的热量全部转化为功 G2 Any process which consists solely in the transfer of heat from one temperature to a higher one is impossible 任何一个从低温到高温的过程仅仅包含热量的转移是不可能的 H1 Statement 1 does not imply that heat cannot be converted into work but does mean that changes other than those resulting directly from the conversion of heat into work must occur in either the system or surroundings 陈述 1 并不意味着热量不能转化为功 但是意味着 除了由热转化为功直接产生的变化之外 在系 统或环境中一定有其他的变化发生 2 Consider the case of an ideal gas in a cylinder and piston assembly expanding reversibly at constant temperature 考虑理想气体恒温下在活塞气缸内可逆膨胀 2 3 Work is produced in the surroundings consider the gas as the system equal to the integral of the pressure times the change in volume 将气体考虑为系统 对环境做的功等于压力乘以体积变化的积分 4 Since the gas is ideal u 0 then according to the first law the heat absorbed by the gas from the surrounding is equal to the work produced in the surroundings because of the reversible expansion in the gas 由于气体是理想气体 所以 u 0 根据第一定律 气体从周围环境吸收的热量等于气体对周围环境 所做的功 因为气体膨胀是可逆的 5 At first this might seem to be a contradiction of statement 1 since in surroundings the only result has been the complete conversion of heat into work 乍一看 这好像与陈述 1 相矛盾 因为在环境中 唯一的结果是热完全转化为功 6 However the second law statement requires that there also be no change in the system a requirement which has not been met in this example 然而 第二定律要求在系统中也没有任何变化 在这个例子中 并没有满足这个要求 7 Since the pressure of the gas decreased this process cannot be continued indefinitely 由于气体压力下降 这个过程不能无限地继续下去 8 The pressure of the gas would soon reach that of the surroundings and further expansion would be impossible 气体的压力很快就会达到环境的压力 进一步的膨胀是不可能的 9 Therefore a method of continuously producing work from heat by this method would fail if the original state of the system were restored in order to comply with the requirements of statements 1 it would be necessary to take energy from the surroundings in the form of work in order to compress the gas back to its original pressure 因此 用这种方法由热来连续产生功的方法是失败的 为了符合陈述 1 的要求 如果系统的初始状 态被恢复 必须要从环境中以功的形式获得能量 用来把气体压缩到原始压强 10 At the same time energy as heat would be transferred to the surroundings in order to maintain constant temperature 同时 为了保持恒温 能量以热量的形式将会转移到环境中 11 This reverse process would require just the amount of work gained from the expansion hence the net work produced would be zero 这个相反的过程仅仅需要膨胀时获得的功的量 因此 产生的净功为 0 12 From this discussion it is evident that statement 1 might be expressed in an alternative way viz la it is impossible to convert the heat absorbed completely into work in a cyclical process 从这个讨论中 可以很明显地看出 陈述 1 可能有另外一种表达方式 即 1a 在一个循环过程中 不可能将吸收的热量全部转化为功 I1 The term cyclical requires that the system be restored periodically to its original state 循环这个术语 要求系统周期性的恢复到原始状态 2 In the previous example the expansion and compression back to the original state constitute a complete cycle 在前面的例子中 膨胀和压缩回到原