Technology & Engineering
Thermodynamic Variables
Thermodynamic variables are properties that describe the state of a system, such as temperature, pressure, and volume. These variables are used to quantify and analyze the behavior of energy and matter within a system. Understanding and manipulating thermodynamic variables is crucial in the design and operation of various engineering systems, including power plants, refrigeration systems, and chemical processes.
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8 Key excerpts on "Thermodynamic Variables"
No longer available |Learn more- (Author)
- 2014(Publication Date)
- Learning Press(Publisher)
________________________ WORLD TECHNOLOGIES ________________________ tration; intensive Thermodynamic Variables are defined at each spatial point and each instant of time in a system. Physical macroscopic variables can be mechanical or thermal. Temperature is a thermal variable; according to Guggenheim, the most important conception in thermodynamics is temperature. If a system is in thermodynamic equilibrium and is not subject to an externally imposed force field, such as gravity, electricity, or magnetism, then (subject to a proviso stated in the following sentence) it is homogeneous, that is say, spatially uniform in all respects. There is a proviso here; a system in thermodynamic equilibrium can be inhomogeneous in the following respect: it can consist of several so-called 'phases', each homogeneous in itself, in immediate contiguity with other phases of the system, but distinguishable by their having various respectively different physical characters; a mixture of different chemical species is considered homogeneous for this purpose if it is physically homogeneous. For example, a vessel can contain a system consisting of water vapour overlying liquid water; then there is a vapour phase and a liquid phase, each homo-geneous in itself, but still in thermodynamic equilibrium with the other phase. For the immediately present account, systems with multiple phases are not considered, though for many thermodynamic questions, multiphase systems are important. In a sense, a homogeneous system can be regarded as spatially zero-dimensional, because it has no spatial variation. If a system in thermodynamic equilibrium is homogeneous, then its state can be described by a number of intensive variables and extensive variables.
eBook - PDFPhase Equilibria, Phase Diagrams and Phase Transformations
Their Thermodynamic Basis
- Mats Hillert(Author)
- 2007(Publication Date)
- Cambridge University Press(Publisher)
1 Basic concepts of thermodynamics 1.1 External state variables Thermodynamics is concerned with the state of a system when left alone, and when inter-acting with the surroundings. By ‘system’ we shall mean any portion of the world that can be defined for consideration of the changes that may occur under varying conditions. The system may be separated from the surroundings by a real or imaginary wall. The proper-ties of the wall determine how the system may interact with the surroundings. The wall itself will not usually be regarded as part of the system but rather as part of the sur-roundings. We shall first consider two kinds of interactions, thermal and mechanical, and we may regard the name ‘thermodynamics’ as an indication that these interactions are of main interest. Secondly, we shall introduce interactions by exchange of matter in the form of chemical species. The name ‘thermochemistry’ is sometimes used as an indication of such applications. The term ‘thermophysical properties’ is sometimes used for thermodynamic properties which do not primarily involve changes in the content of various chemical species, e.g. heat capacity, thermal expansivity and compressibility. One might imagine that the content of matter in the system could be varied in a number of ways equal to the number of species. However, species may react with each other inside the system. It is thus convenient instead to define a set of independent components , the change of which can accomplish all possible variations of the content. By denoting the number of independent components as c and also considering thermal and mechanical interactions with the surroundings, we find by definition that the state of the system may vary in c + 2 independent ways. For metallic systems it is usually most convenient to regard the elements as the independent components. For systems with covalent bonds it may sometimes be convenient to regard a very stable molecular species as a component.
- Pierre Gaspard(Author)
- 2022(Publication Date)
- Cambridge University Press(Publisher)
1 Thermodynamics 1.1 Generalities Thermodynamics aims to describe many-particle systems on the macroscale, i.e., on spatial scales larger than the distances between the particles and temporal scales longer than the corresponding time intervals. Thermodynamics enunciates general principles governing the balance of physical quantities characterizing such macroscopic systems. These physical quantities are the state variables, also called macrovariables, that are defined by observing the system on the macroscale. The state variables include mechanical variables such as the energy E and the particle numbers N k , which are defined in the framework of the underly- ing microscopic mechanics, as well as the nonmechanical variable called entropy S . This latter was introduced by Clausius (1865), who established its existence at the macroscale in addition to the mechanical properties, in particular, using the study of Carnot (1824) on the behavior of gases in idealized steam engines. Basically, the system is delimited by a boundary and has a volume V . The system can be an engine, a device, a machine, a motor, or part of a larger system, such as a volume element in a continuous medium like a fluid or a solid. The time evolution of the system may result from internal transformations and also from exchanges with its environment, as schematically represented in Figure 1.1. During the evo- lution of any kind (i.e., spontaneous time evolution or evolution under some external drive), some state variable X changes by some infinitesimal amount dX at every infinitesimal step of the evolution. Mathematically speaking, dX is the differential of X. This differential may have two contributions dX = d e X + d i X. (1.1) The contribution d e X is due to the exchanges of X with the exterior of the system (i.e., its environment) and the contribution d i X is caused by the transformations inside the system (Prigogine, 1967).
eBook - PDF- Milo D. Koretsky(Author)
- 2012(Publication Date)
- Wiley(Publisher)
► Given two properties, identify the phases present on a PT or a Pv phase diagram, including solid, subcooled liquid, saturated liquid, saturated vapor, and superheated vapor and two-phase regions. Identify the critical point and 2 ► Chapter 1. Measured Thermodynamic Properties and Other Basic Concepts triple point. Describe the difference between saturation pressure and vapor pressure. ► Use the steam tables to identify the phase of a substance and find the value of desired thermodynamic properties with two independent properties specified, using linear interpolation if necessary. ► Use the ideal gas model to solve for an unknown measured property given measured property values. Science changes our perception of the world and contributes to an understanding of our place in it. Engineering can be thought of as a profession that creatively applies science to the development of processes and products to benefit humankind. Thermodynamics, perhaps more than any other subject, interweaves both these elements, and thus its pur- suit is rich with practical as well as aesthetic rewards. It embodies engineering science in its purest form. As its name suggests, thermodynamics originally treated the conversion of heat to motion. It was first developed in the nineteenth century to increase the efficiency of engines—specifically, where the heat generated from the combustion of coal was con- verted to useful work. Toward this end, the two primary laws of thermodynamics were postulated. However, in extending these laws through logic and mathematics, thermo- dynamics has evolved into an engineering science that comprises much greater breadth. In addition to the calculation of heat effects and power requirements, thermodynamics can be used in many other ways. For example, we will learn that thermodynamics forms the framework whereby a relatively limited set of collected data can be efficiently used in a wide range of calculations.
eBook - PDFThermo and Fluid Dynamics
Recent Advances
- Dritan Hoxha(Author)
- 2019(Publication Date)
- Arcler Press(Publisher)
Here I will make a parenthesis. People often confuse heat and thermal energy or worse, they consider them as the same thing. Heat and thermal energy, earlier, used to be considered as synonyms. But they differ as follows: Heat is the thermal energy transferred across a boundary of one region of matter to another. As a process variable, heat is a characteristic of a process, not a property of the system. Thermal energy is the internal energy present in a system by virtue of its temperature. The average kinetic energy (translational) possessed by free particles in a system of free particles under thermodynamic equilibrium. One of the most important objectives when dealing with thermodynamics is to measure the amount of thermal energy involved in a certain situation. However, this is a very complicated process because the systems we consider when considering a thermodynamic related situation have a very large number of particles which interact with each other in many ways. Thus, we must have any facility when dealing with these situations in order to make them easier to study. Hence, if these systems are more or less balanced a situation known as equilibrium then it is possible to describe it through a small number of measurements or as otherwise known, “system parameters” [1]. Such a situation exists only in theory and it may occur only when the system is idealized as well as all quantities involved in this system such as the mass, pressure, and volume, or any other equivalent set of numbers. These numbers describe a very large number of variables ranging from 10 26 to 10 30 nominal independent variables. A General View on Thermodynamics 3 Below a map showing several branches of Physics is shown. It will help the reader to understand the place and importance of Thermodynamics as one of key Physics branches. 1.2. ETYMOLOGY OF THERMODYNAMICS It has not been so easy to determine the name of this branch of Physics, although it is relatively a new one.
- Allan D. Kraus, James R. Welty, Abdul Aziz(Authors)
- 2011(Publication Date)
- CRC Press(Publisher)
2 Thermodynamics: Preliminary Concepts and Definitions Chapter Objectives • To briefly introduce the subject of thermodynamics. • To provide precise definitions of some of the working terms used in a study of thermodynamics. • To consider the dimensions and units that pertain to thermodynamics. • To examine density and its related properties. • To define pressure and consider how it is measured. • To define temperature and to present the zeroth law of thermodynamics. • To outline a problem-solving methodology. 2.1 The Study of Thermodynamics When most people think about thermodynamics, they think about the transfer of energy and the utilization of such energy transfer for the useful production of work. This often leads many engineering students in fields such as computer science and electrical or civil engineering to wonder why this particular subject is relevant to them. In reality, thermody-namics deals with much more than the study of heat or energy transfer and the development of work. Indeed, it deals with virtually all aspects of our lives, from the combustion pro-cesses that run our automobiles and produce our electric power in power plants to the refrigeration cycles that cool our beer, from the cryogenic pumping of liquids and gases in space to the distillation processes used to produce the gasoline that runs our automobiles. Thermodynamics is important to electrical engineers so that they can better understand that the limiting factor in the microminiaturization of electronic components is the rejec-tion of heat. It is important to civil engineers because a knowledge of thermal expansion and thermal stresses is requisite to the design of structures and to the computer scien-tists who need to thoroughly understand the systems that they are trying to model and develop.
eBook - PDF- Marc J. Assael, William A. Wakeham, Anthony R. H. Goodwin, Stefan Will, Michael Stamatoudis(Authors)
- 2011(Publication Date)
- CRC Press(Publisher)
A system is characterized both by its contents and the system boundary; the latter in the end is always virtual. For example, if one considers a container with a rigid enclosure, the boundary of the system is set in a way to include all the material inside but to exclude the walls. Especially in engineering applications, a careful and advantageous choice of the system boundary is of enormous importance; defining the right system boundary may considerably ease setting up energy and mass balances, for example. 1.3.2 What Is a State? The state of a system is defined by specifying a number of thermodynamic vari-ables for the system under study. In principle, these could be any or all of the measurable physical properties of a system. Fortunately, not all of the variables or properties need to be specified to define the state of the system because only a few can be varied independently; the exact number of independent variables depends on the system but rarely exceeds five. The exact choice of the inde-pendent variables for a system is a matter of convenience, but pressure and Definitions and the 1 st Law of Thermodynamics 4 temperature are often included within them. As an illustration of this point, if the temperature and pressure of a pure gas are specified then the density of the gas takes a value (dependent variable) that is determined. The general rule for calculating the number of independent variables for a system at equi-librium is given by the phase rule that will be introduced and discussed in Question 4.1.1. 1.3.3 What Are the Types of Property: Extensive and Intensive? For a system that can be divided into parts any property of the system that is the sum of the property of the parts is extensive. For example, the mass of the system is the sum of the mass of all parts into which it is divided.
eBook - PDFThermomechanics Of Nonlinear Irreversible Behaviours, The
An Introduction
- Gerard A Maugin(Author)
- 1999(Publication Date)
- World Scientific(Publisher)
77 78 Chapter 4- Thermodynamics with Internal Variables present in the statement (2.3.53) of the first law of thermodynamics. This is what gives them this special status of being internal or hidden. In spite of this, however, the tensorial nature of the internal variable a (scalar, vector, tensor, n-vector) as well as its physical nature must in general be specified. Does it represent the average of some microscopic effect or is it the measure of some local structural rearrangement? This identification is the most difficult part of phenomenological analysis. One must also point out at that stage that the notion of being internal for a variable of state depends on the level of observation. We can very easily think of a variable that might be considered as internal from the macroscopic observation point of view, say, at the usual macro-scale of continuum mechanics — in which a strong nonlocality is not taken into account (see below for this) — or as observable from a point of view of mesoscopic observation which, while already outside the usual scope of phenomenological physics, might still be understood in an enlarged (in length and time scales) phenomenological framework. Therefore, along with J. Man-del (1980) we can always say that a clever physicist will always manage to detect the 'internal' variables and measure them. Controlling them may be outside his power, so that they are indeed internal variables. Thus in practice, these variables are measurable but not controllable, i.e. they cannot a priori be adjusted to a prescribed value through a direct action via a surface or body (volume) stimulus. We shall return to this critical point later (Sec. 4.7). Is the idea of internal variable proper to thermomechanics or is it an old and recurring idea in physics? This question deserves a short digression if we are to believe M. Jammer (1974) in his discussion of a related idea in quantum mechanics.
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