Thermodynamics system/boundary/surroundings

A thermodynamic system (closed system) is one that interacts with the surroundings by exchanging heat and work thru its boundary an isolated system is one that does not interact with the surroundings. The state of a system is determined by the values of its various properties, eg, pressure, volume, internal energy, etc. A system can be composed of a finite number of homogeneous parts, called phases, or there can be a single phase. For some applications, it may... 

The laws of thermodynamics are statistical laws. This means that they describe large assemblies of particles called systems. The system is defined as some arbitrary part of the universe with defined boundaries. If neither heat nor matter is exchanged between the system and its surroundings, it is called an isolated system. If matter cannot cross the system boundaries, it is said to be a closed system if it can cross, then it is an open system. If it is thermally insulated, it is an adiabatic system. 

We define a heat reservoir as any body, used as part of the surroundings of a particular system, whose only interaction with the system is across a diathermic boundary. A heat reservoir is then used to transfer heat to or from a thermodynamic system and to measure these quantities of heat. It may consist of one or more substances in one or more states of aggregation. In most cases a heat reservoir must be of such a nature that the addition of any finite amount of heat to the system or the removal of any finite amount of heat from the system causes only an infinitesimal change in the temperature of the reservoir. 

A work reservoir is similarly defined as any body or combination of bodies, used as part of the surroundings, whose only interaction with the system is one that may be described in terms of work. We may have a different type of reservoir for each mode of interaction other than thermal interaction. A work reservoir then is used to perform work across the boundary separating the reservoir and the thermodynamic system and to measure these quantities of work. In the following we are, in order to simplify the discussion, primarily concerned with mechanical work, but this limitation does not alter or limit the basic concepts. A reservoir for mechanical work may be a set of weights and pulleys in a gravitational field, an idealized spring, or a compressible fluid in a piston-and-cylinder arrangement. In any case the reservoir must... 

In physics and chemistry we call an ensemble of substances a thermodynamic system consisting of atomic and molecular particles. The system is separated from the surroundings by a boundary interface. The system is called isolated when no transfer is allowed to occur of substances, heat, and work across the boundary interface of the system as shown in Fig. 1.1. The system is called closed when it allows both heat and work to transfer across the interface but is impermeable to substances. The system is called open if it is completely permeable to substances, heat, and work. The open system is the most general and it can be regarded as a part of a closed or isolated system. For instance, the universe is an isolated system, the earth is regarded as a closed system, and a creature such as a human being corresponds to an open system. 

A thermodynamic system is a part of the physical universe with a specified boundary for observation. A system contains a substance with a large amount of molecules or atoms, and is formed by a geometrical volume of macroscopic dimensions subjected to controlled experimental conditions. An ideal thermodynamic system is a model system with simplifications to represent a real system that can be described by the theoretical thermodynamics approach. A simple system is a single state system with no internal boundaries, and is not subject to external force fields or inertial forces. A composite system, however, has at least two simple systems separated by a barrier restrictive to one form of energy or matter. The boundary of the volume separates the system from its surroundings. A system may be taken through a complete cycle of states, in which its final state is the same as its original state. 

If the boundary of a system does not permit the transfer of matter between the system and its surroundings, the system is said to be closed, and its mass is necessarily constant. The development of basic concepts in thermodynamics is facilitated by a careful examination of closed systems, and for this reason they are treated in detail in the following sections. Far more important for industrial practice are processes in which matter crosses the system boundary as streams tliat enter and leave process equipment. Such systems are said to be open, and tliey are treated later in tliis cliapter, once the necessary foundation material lias been presented. 

So far it has been assumed that although heat and work may pass through the boundaries of thermodynamic systems, no transport of matter takes place. Thermodynamics can, however be extended to take account of exchanges of chemical substances as well as energy between a system and its surroundings. For instance, equation (15), Chapter 5, can be modified by the addition of terms as follows... 

We begin with the application of the first law of thermodynamics first to a dosed system and then to an open system. A system is any bounded portion of the universe, moving or stationary, which is chosen for the application of the various thermodynamic equations. For a closed system, in which no mass crosses the system boundaries, the change in total energy of the system, dE, is equal to the heat flow to the system. 8Q. minus the work done by the system on the surroundings. W. For a closed sy.sreni. the energy balance is... 

Thermodynamic systems make mechanical contact through the pressure that is exerted on the boundaries that separate them. At equilibrium, this pressure is equal on both sides of the boundary. If a pressure imbalance arises between the system and its surroundings, the boundaries of the system must move in response to the mechanical force. Such imbalance may arise from the application of a mechanical force that acts to compress or expand the system, or through the application of heat, which causes the volume to expand or contract. The movement of boundaries involves the exchange of work, which we call PFwork. 

Thermodynamic Systems Any quantity of matter that is separated from its surroundings by rigid or imaginary boundaries and whose properties may be unequivocally and completely described by thermodynamic, macroscopic state variables. 

Open Systems Thermodynamic systems in which matter and/or energy may transfer with the surroundings, through the boundaries. 

The definition of a thermodynamic system in nanoscale is the same as the macroscopic systems. In thermod5mamics, a system is any region completely enclosed within a well-defined boundary. Everything outside the system is then defined as the surroundings. The boundary may be either... 

A thermodynamic system is separated from the remainder of the universe by a boundary and everything outside this boundary is known as the surrounding. Exchanges of work, heat, or matter between the system and the surroundings take place across this boundary. The systems are categorized as open, closed and isolated depending on the types of interactions involved. 

Straining a body involves forces and displacements. Therefore, work is done in this process. To calculate this work we consider the body as a thermodynamic system that is closed but not isolated. All matter and points not belonging to the body under consideration constitute the surroundings of this system. By designating the system to be closed we infer that the boundary between the system and its surroundings does not allow any mass transfer. For an isolated system the boundary would in addition exclude the transfer of all energy. 

The set of molecules of a fluid phase, i. e. a gas or a liquid, which is within the field of forces of the atoms or molecules of the external or internal surface of a solid sorbent is called the adsorbate of the fluid on this sorbent material. This set may be considered as a thermodynamic system or phase in the sense of W. Schottky [1.63]. Such a system is defined as set ofbodies or molecules divided from its surroundings by clearly defined boundaries and exchanging with its surroundings only mechanical work, heat and mass. Obviously, any adsorbate is an inhomogeneous phase as - by definition - its molecules are subject to the surface forces of the sorbent atoms. Hence the conditions for local (x = (xi, X2, X3)) thermodynamic equilibrium conditions, derived from the Second Law of thermodynamics, are [1.64]... 

A body or substance placed in a calorimeter constitutes a thermodynamic system. Such a system can be characterized by indicating the boundary conditions relative to the surroundings and the values of all relevant physical quantities, namely, temperature, pressure, and volume (for solids, the stress and deformation tensors). 

The quantity Hk ik represents potential energy shared by both the system and surroundings on account of forces acting across the system boundary, other than gravitational forces or forces from other external fields. The forces responsible for the quantity J k ik generally significant only between particles in the immediate vicinity of the system boundary, and will presently turn out to be the crucial forces for evaluating thermodynamic work. 

In thermodynamics we are interested in the quantity of work done on macroscopic parts of the system during a process, rather than the work done on individual particles. Macroscopic work is the energy transferred across the system boundary due to concerted motion of many particles on whieh the surroundings exert a force. Macroscopic mechanical work occurs when there is displacement of a macroscopic portion of the system on which a short-range contact force acts across the system boundary. This force could be, for instance, the pressure of an external fluid at a surface element of the boundary multiplied by the area of the surface element, or it could be the tension in a cord at the point where the cord passes through the boundary. 

The equation shows how we can evaluate the thermodynamic work w done on the system. For each moving surface element of the system boundary at segment r of the interaction layer, we need to know the contact force exerted by the surroundings and the displacement d/ in the local frame. 

Figure 2.1. Thermodynamic considerations refer to a system which is separated from its surroundings by an imaginary or real boundary. System and surroundings constitute the thermodynamic universe.

All thermodynamic considerations refer to a predeSned system. A thermodynamic system can be a quantity of substance, a specimen, or a working machine which in a well-defined way is set apart from its surroundings. The boundary between the system and its surroundings can be real, as for example a container wall or the surface of a specimen, or it can be an imaginary mathematical envelope that separates the system from its environments. Collectively a system and its surroundings are denoted as the thermodynamic universe. 

Solution. The thermodynamic system consists of the steel rod the boundary between the system and its surroundings is the surface of the rod. Everything beyond the surfcice of the rod is considered to be surroundings. This is a closed system, because the system is able to exchange energy, but not matter with its surroundings. 

You are making plans to stay warm in the winter. Due to your busy schedule, you are typically away from your house all day. You know it costs a lot to operate the electric heaters to keep your house warm. However, you have been told that it is more efficient to leave your house warm all day rather than turn off the heat during the day and reheat the house when you get home at night. You think that thermodynamics may be able to resolve this issue. Draw a schematic of the system, the surroundings, and the boundary. Illustrate the alternative processes. What is your choice to save power Justify your answer. 

A system is the region in space that is the subject of the thermodynamic study. It can be as large or small, or as simple or complex, as we want it to be, but it must be carefully and consistently defined. Sometimes the system has definite and precise physical boundaries, such as a gas enclosed in a cylinder so that it can be compressed or expanded by a piston. However, it may be also something as diffuse as the gaseous atmosphere surrounding the earth. 

Figure 4.3 We call the sum of the system and its surroundings the thermodynamic universe . Energy is exchanged between the system and its surroundings no energy is exchanged beyond the surrounds, i.e. outside the boundaries of the thermodynamic universe. Hence, the definition a universe is that volume large enough to enclose all the thermodynamic changes ...

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