热力学原理翻译

The factors that cause a process to be irreversible are called irreversibilities. They include friction, unrestrained expansion, mixing of two fluids, heat transfer across a finite temperature difference, electric resistance, inelastic deformation of solids, and chemical reactions. The presence of any of these effects renders a process irreversible.

A reversible process involves none of these. Some of the frequently encountered irreversibilities are discussed briefly below.

Friction is a familiar form of irreversibility associated with bodies in motion. When two bodies in contact are forced to move relative to each other (a piston in a cylinder, for example, as shown in Fig. 6–32), a friction force that opposes the motion develops at the interface of these two bodies, and some work is needed to overcome this friction force. The energy supplied as work is eventually converted to heat during the process and is transferred to the bodies in contact, as evidenced by a temperature rise at the interface. When the direction of the motion is reversed, the bodies are restored to their original position, but the interface does not cool, and heat is not converted back to work. Instead, more of the work is converted to heat while overcoming the friction forces that also oppose the reverse motion. Since the system (the moving bodies) and the surroundings cannot be returned to their original states, this process is irreversible. Therefore, any process that involves friction is irreversible. The larger the friction forces involved, the more irreversible the process is.

Friction does not always involve two solid bodies in contact. It is also encountered between a fluid and solid and even between the layers of a fluid moving at different velocities. A considerable fraction of the power produced by a car engine is used to overcome the friction (the drag force) between the air and the external surfaces of the car, and it eventually becomes part of the internal energy of the air. It is not possible to reversethis process and recover that lost power, even though doing so would not violate the conservation of energy principle.

Another example of irreversibility is the unrestrained expansion of a gas separated from a vacuum by a membrane, as shown in Fig. 6–33. When the membrane is ruptured, the gas fills the entire tank. The only way to restore the system to its original state is to compress it to its initial volume, while transferring heat from the gas until it reaches its initial temperature. From the conservation of energy considerations, it can easily be shown that the amount of heat transferred from the gas equals the amount of work done on the gas by the surroundings. The restoration of the surroundings involves conversion of this heat completely to work, which would violate the second law. Therefore, unrestrained expansion of a gas is an irreversible process.

A third form of irreversibility familiar to us all is heat transfer through a finite temperature difference. Consider a can of cold soda left in a warm room (Fig. 6–34). Heat is transferred from the warmer room air to the cooler soda. The only way this process can be reversed and the soda restored to its original temperature is to provide refrigeration, which requires some work input. At the end of the reverse process, the soda will be restored to its initial state, but the surroundings will not be. The internal energy of the surroundings will increase by an amount equal in magnitude to the work supplied to the refrigerator. The restoration of the surroundings to the initial state can be done only by converting this excess internal energy completely to work, which is impossible to do without violating the second law. Since only the system, not both the system and the surroundings, can be restored to its initial condition, heat transfer through a finite temperature difference is an irreversible process. Heat transfer can occur only when there is a temperature difference between a system and its surroundings. Therefore, it is physically impossible to have a reversible heat transfer process. But a heat transfer process becomes less and less irreversible as the temperature difference between the two bodies approaches zero. Then heat transfer through a differential temperature difference dT can be considered to be reversible. As dT approaches zero, the process can be reversed in direction (at least theoretically) without requiring any refrigeration. Notice that reversible heat transfer is a conceptual process and cannot be duplicated in the real world.

The smaller the temperature difference between two bodies, the smaller the heat transfer rate will be. Any significant heat transfer through a small temperature difference requires a very large surface area and a very long time. Therefore, even though approaching reversible heat transfer is desirable from a thermodynamic point of view, it is impractical and not economically feasible.

不可逆性

导致一个过程不可逆转的因素被称为不可逆转。它们包括摩擦、不受约束的膨胀、两种液体的混合、在有限的温度差异的传热、电阻、固体的非弹性变形和化学反应。任何这些效应的存在都使过程不可逆转。可逆过程不涉及这些。下面简要讨论一些经常遇到的不可逆的问题。

摩擦力是一种常见的不可逆性形式，与物体运动有关。当两个接触的物体被迫移动相对的时候(例如，一个汽缸中的活塞，如图6 - 32所示)，一个反对运动的摩擦力在这两个物体的界面上发展，需要一些工作来克服摩擦力。当工作过程中提供的能量最终转化为热量，并转移到接触的物体上，这可以从界面的温度上升中得到证明。当运动方向相反时，身体会恢复到原来的位置，但界面不会冷却，热量不会转换回工作。相反，更多的工作转化为热量，同时克服了同样反对反向运动的摩擦力。由于系统(移动的物体)和环境不能返回到原来的状态，这个过程是不可逆转的。因此，任何涉及摩擦的过程都是不可逆转的。摩擦力越大，过程越不可逆。摩擦并不总是与两个固体接触。它也会在流体和固体之间，甚至在流体在不同速度运动的层之间遇到。汽车发动机产生的相当一部分功率是用来克服空气和汽车外部表面之间的摩擦力(阻力)，最终成为空气内部能量的一部分。不可能逆转这个过程，恢复失去的能量，即使这样做不会违反能量守恒定律。另一个不可逆转的例子是气体从真空中分离出来的无限制膨胀，如图6 - 33所示。当隔膜破裂时，整个油箱都充满了气体。将系统还原到原始状态的唯一方法是将其压缩到初始体积，同时将热量从气体中传递到初始温度。从能量守恒的角度，可以很容易地看出，气体从气体中传递的热量等于周围环境对气体的处理量。环境的恢复需要将这种热量完全转化为工作，这将违反第二定律。因此，气体的无限制膨胀是一个不可逆过程。

我们所熟悉的第三种不可逆性形式是通过有限的温差传递热量。考虑一下在温暖的房间里留下的一罐冷饮(图6 - 34)。热量从温暖的室内空气转移到较冷的汽水上。这一过程可以逆转，而苏打水恢复到原来的温度就是提供制冷，这需要一些工作投入。在反过程的最后，苏打水会恢复到初始状态，但环境不会。环境的内部能量将会增加，与提供给冰箱的工作相等。只有把这种过剩的内部能量完全转化为工作，才能使环境恢复到初始状态，这在不违反第二定律的情况下是不可能做到的。因为只有系统，系统和周围环境，才能恢复到初始状态，通过有限温差的热传递是一个不可逆的过程。只有当系统和环境之间存在温差时，才可以进行传热。因此，在物理上不可能有一个可逆的传热过程。但随着两体之间的温差接近于零，传热过程变得越来越不可逆。然后通过微分温差dT进行传热，可以认为是可逆的。随着dT接近于零，这个过程可以在不需要任何制冷的情况下朝方向(至少理论上)反转。注意，可逆热传导是一个概念过程，不能在现实世界中复制。两个物体的温差越小，传热率就越小。任何一个微小温差的重要的热量传递都需要一个非常大的表面积和很长的时间。因此，尽管从热力学的角度来看，可逆热交换是可取的，但它是不切实际的，在经济上是行不通的。

Internally and Externally Reversible Processes

A typical process involves interactions between a system and its surroundings,and a reversible process involves no irreversibilities associated witheither of them.

A process is called internally reversible if no irreversibilities occurwithin the boundaries of the system during the process. During an internal lyreversible process, a system proceeds through a series of equilibrium states, and when the process is reversed, the system passes through exactly the same equilibrium states while returning to its initial state. That is, the paths of the forward and reverse processes coincide for an internally reversible process. The quasi-equilibrium process is an example of an internally

reversible process.

A process is called externally reversible if no irreversibilities occur outside the system boundaries during the process. Heat transfer between a reservoir and a system is an externally reversible process if the outer surface of the system is at the temperature of the reservoir.

A process is called totally reversible, or simply reversible, if it involves no irreversibilities within the system or its surroundings (Fig. 6–35). A totally reversible process involves no heat transfer through a finite temperature difference, no nonquasi-equilibrium changes, and no friction or other dissipative effects.

As an example, consider the transfer of heat to two identical systems that are undergoing a constant-pressure (thus constant-temperature) phasechange process, as shown in Fig. 6–36. Both processes are internally reversible, since both take place isothermally and both pass through exactly the same equilibrium states. The first process shown is externally reversible also, since heat transfer for this process takes place through an infinitesimal temperature difference dT. The second process, however, is externally irreversible, since it involves heat transfer through a finite temperature difference _T.

内部和外部可逆过程

一个典型的过程包括一个系统和它周围的环境之间的交互，一个可逆过程不包括与它们相关的不可逆过程。一个过程被称为内部可逆过程，如果在此过程中系统的边界内不发生任何不可逆过程。在一个内部的可逆过程中，系统通过一系列平衡态进行，当过程被逆转时，系统通过完全相同的平衡状态，同时回归到初始状态。也就是说，正向和反向过程的路径是在内部可逆过程中发生的。准平衡过程是内部的一个例子

可逆过程。一个过程被称为外部可逆，如果在过程中不存在系统边界之外的不可逆过程。如果系统的外表面处于热源的温度，则热源与系统之间的传热是一个外部可逆过程。一个过程被称为完全可逆，或者简单的可逆，如果它不涉及系统内部或其周围的不可逆过程(图6 - 35)。一个完全可逆过程不涉及到通过有限温差的热传导，没有非准平衡的变化，没有摩擦或其他耗散的影响。

举个例子，考虑将热传递给两个相同的系统，它们正处于恒压(因此恒定温度)的相变过程，如图6 - 36所示。这两个过程都是内部可逆的，因为它们都是等温的，而且都经过了完全相同的平衡态。第一个过程是外部可逆的，因为这个过程的热量传递是通过一个无穷小的温差dT进行的。然而，第二个过程是不可逆转的，因为它涉及到通过有限温差来传递热量。