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Often the surroundings of a thermodynamic system may also be regarded as another thermodynamic system. In this view, one may consider the system and its surroundings as two systems in mutual contact, with long-range forces also linking them. The enclosure of the system is the surface of contiguity or boundary between the two systems. In the thermodynamic formalism, that surface is regarded as having specific properties of permeability. For example, the surface of contiguity may be supposed to be permeable only to heat, allowing energy to transfer only as heat. Then the two systems are said to be in thermal equilibrium when the long-range forces are unchanging in time and the transfer of energy as heat between them has slowed and eventually stopped permanently; this is an example of a contact equilibrium. Other kinds of contact equilibrium are defined by other kinds of specific permeability. When two systems are in contact equilibrium with respect to a particular kind of permeability, they have common values of the intensive variable that belongs to that particular kind of permeability. Examples of such intensive variables are temperature, pressure, chemical potential.

Often the surroundings of a thermodynamic system may also be regarded as another thermodynamic system. In this view, one may consider the system and its surroundings as two systems in mutual contact, with long-range forces also linking them. The enclosure of the system is the surface of contiguity or boundary between the two systems. In the thermodynamic formalism, that surface is regarded as having specific properties of permeability. For example, the surface of contiguity may be supposed to be permeable only to heat, allowing energy to transfer only as heat. Then the two systems are said to be in thermal equilibrium when the long-range forces are unchanging in time and the transfer of energy as heat between them has slowed and eventually stopped permanently; this is an example of a contact equilibrium. Other kinds of contact equilibrium are defined by other kinds of specific permeability. When two systems are in contact equilibrium with respect to a particular kind of permeability, they have common values of the intensive variable that belongs to that particular kind of permeability. Examples of such intensive variables are temperature, pressure, chemical potential.

通常，热力学系统的周围环境也可以被看作是另一个热力学系统。在这种观点中，我们可以把系统及其周围环境看作是相互接触的两个系统，远程作用力也将它们联系在一起。系统的包围物是两个系统之间的接触面或边界。在热力学公式中，该表面被认为具有特定的渗透性质。例如，接触的表面可能被认为只能透热，使能量只能作为热传递。当远程力在时间上不发生变化，两个系统之间的热量传递减慢并最终永久停止时，这两个系统被称为热平衡; 这就是接触平衡的一个例子。其它类型的接触平衡可用其它类型的比渗透率来定义。当两个系统对于某一特定类型的渗透率处于接触平衡时，它们具有属于该特定类型渗透率的强度变量的共同值。这种强度变量的例子有温度、压力、化学势。

通常，热力学系统的周围环境也可以被看作是另一个热力学系统。在这种观点中，我们可以把系统及其周围环境看作是相互接触的两个系统，远程作用力也将它们联系在一起。系统的包围物是两个系统之间的接触面或边界。在热力学形式中，该表面被认为具有特定的渗透性质。例如，接触的表面可能被认为只能透热，使能量只能作为热传递。当远程力在时间上不发生变化，两个系统之间的热量传递减慢并最终永久停止时，这两个系统被称为热平衡; 这就是接触平衡的一个例子。其它类型的接触平衡可用其它类型的比渗透率来定义。当两个系统对于某一特定类型的渗透率处于接触平衡时，它们具有属于该特定类型渗透率的强变量的共同值。这种强度变量的例子有温度、压力、化学势。

A contact equilibrium may be regarded also as an exchange equilibrium. There is a zero balance of rate of transfer of some quantity between the two systems in contact equilibrium. For example, for a wall permeable only to heat, the rates of diffusion of internal energy as heat between the two systems are equal and opposite. An adiabatic wall between the two systems is 'permeable' only to energy transferred as work; at mechanical equilibrium the rates of transfer of energy as work between them are equal and opposite. If the wall is a simple wall, then the rates of transfer of volume across it are also equal and opposite; and the pressures on either side of it are equal. If the adiabatic wall is more complicated, with a sort of leverage, having an area-ratio, then the pressures of the two systems in exchange equilibrium are in the inverse ratio of the volume exchange ratio; this keeps the zero balance of rates of transfer as work.

A contact equilibrium may be regarded also as an exchange equilibrium. There is a zero balance of rate of transfer of some quantity between the two systems in contact equilibrium. For example, for a wall permeable only to heat, the rates of diffusion of internal energy as heat between the two systems are equal and opposite. An adiabatic wall between the two systems is 'permeable' only to energy transferred as work; at mechanical equilibrium the rates of transfer of energy as work between them are equal and opposite. If the wall is a simple wall, then the rates of transfer of volume across it are also equal and opposite; and the pressures on either side of it are equal. If the adiabatic wall is more complicated, with a sort of leverage, having an area-ratio, then the pressures of the two systems in exchange equilibrium are in the inverse ratio of the volume exchange ratio; this keeps the zero balance of rates of transfer as work.

接触平衡也可视为交换平衡。在接触平衡状态下，两系统之间某些量的传递速率存在零平衡。例如，对于只能透热的壁，内能作为热在两个系统之间的扩散速率是相等并相反的。两个系统之间的绝热壁只对作为功传递的能量有渗透作用; 在机械平衡，两个系统之间作为功的能量传递速率相等且相反。如果是一个简单的壁，那么通过它的体积转移率也是相等且相反的; 即它两边的压力是相等的。如果绝热壁比较复杂，有一种杠杆，有一个面积比，那么两个体系在交换平衡中的压力与体积交换比成反比，这使得转移率的零平衡作功。

接触平衡也可视为交换平衡。在接触平衡状态下，两系统之间某些量的传递速率存在零平衡。例如，对于只能透热的壁，内能作为热在两个系统之间的扩散速率是相等并反向的。两个系统之间的绝热壁只对作为功传递的能量有渗透作用; 在力学平衡，两个系统之间作为功的能量传递速率相等且相反。如果是一个简单的壁，那么通过它的体积转移率也是相等且相反的; 即它两边的压力是相等的。如果绝热壁比较复杂，有一种杠杆，有一个面积比，那么两个体系在交换平衡中的压力与体积交换比成反比，这使得转移率的零平衡作功。