thermodynamics training chpt15

ThermodynamicsThermodynamicsThermodynamics is a branch of physics which deals with the energy and work of a system Thermodynamics is the study of the effect of work, heat, and energy on the system.Power (W) = Rate at which energy is being transferred Work (J)  ‐ amount of energy transferred from one system to another2ThermodynamicsThere are three principal laws of thermodynamics. Each law leads to the definition of thermodynamic properties which help us to understand and predict the operation of a physical system - Zeroth Law - First Law; Work, Heat, and Energy - Second Law; Entropy3The The Zero(th) Zero(th) Law LawThis law states that if object A is in thermal equilibrium with object B, and object B is in thermal equilibrium with object C, then object C is also in thermal equilibrium with object A. This law allows us to build thermometers.“C” for methane at  +15C = 2.2 kJ/kg KThe concept of thermodynamic equilibrium, in which two objects have the same temperature. If we bring two objects that are initially at different temperatures into physical contact, they eventually achieve thermal equilibrium. During the process of reaching thermal equilibrium, heat is transferred between the objects. The amount of heat transferred delta Q is proportional to the temperature difference delta T 5 between the objects and the heat capacity c of the object.The amount of heat transferred delta Q is proportional to the temperature difference delta T between the objects and the heat capacity c of the object.6First First Law Law of of Thermodynamics ThermodynamicsThe first law introduces the concept that heat and work are equivalent. It states that: The heat lost from a source is equal to the total heat gained and work done on bodies that receive that heat.Second Second Law Law of of Thermodynamics ThermodynamicsThe second law introduce the concept of directional heat flow. It states that: Heat always flows from the hot body to a cooler one.Heat Heat Engine EngineForward Heat EngineA forward heat engine has a positive work output such as Rankine or Brayton cycle. Applying the first law of thermodynamics to the cycle gives: Q1 - Q2 - W = 0 The second law of thermodynamics states that the thermal efficiency of the cycle η has an upper limit (the thermal efficiency of the Carnot cycle), i.e. It can be shown that: Q1 > W, which means that it is impossible to convert the whole heat input to work and Q2 > 0, which means that a minimum of heat supply to the cold reservoir is necessary.Heat Heat Engine EngineForward Heat EngineHeat engine is defined as a device that converts heat energy into mechanical energy or more exactly a system which operates continuously and only heat and work may pass across its boundaries. The operation of a heat engine can best be represented by a thermodynamic cycle.Heat Heat Engine EngineReverse Heat EngineA reverse heat engine has a positive work input such as heat pump and refrigerator. Applying the first law of thermodynamics to the cycle gives: - Q1 + Q2 + W = 0 In case of a reverse heat engine the second law of thermodynamics is as follows: It is impossible to transfer heat from a cooler body to a hotter body without any work input i.e. W>0 which means that the co-efficiency of performance for a heat pump is greater than unity.Heat Heat Engine EngineReverse Heat EngineThe effectiveness of a reversed heat engine is defined in terms of a coefficient of performance (COP). The COP for a refrigerator is defined as: COP2 = Q2 / W and for a heat pump as: COP1 = Q1 / WEnthalpy Enthalpyƒ Enthalpy is a thermodynamic measure of the total heat content of a liquid or vapor at a given temperature and is expressed in energy per unit mass (k Joules per 1 kg) from absolute zero. ƒ Therefore, for a liquid/vapor mixture, it will be seen that it is the sum of the enthalpy of the liquid plus the latent heat of vaporizationhx = h′+ χ（h″- h″) χ: Dryness factorEnthalpy Enthalpy -- Pressure Pressureƒ In the previous examples the transition temperatures of water were 0° C and 100° C at atmospheric pressure. ƒ By altering pressure, we will affect the transitional temperature. ƒ When the relationship of enthalpy to pressure is plotted on a chart the resulting diagram is known as Mollier diagram.Enthalpy EnthalpyFrom a study of the FIRST LAW of thermodynamics, we find that the internal energy of a gas is also a state variable. - For a gas, a useful additional state variable is the enthalpy which is defined to be the sum of the internal energy E plus the product of the pressure p and volume V. Using the symbol H for the enthalpy: H=E+p*V - Propulsion engineers use the specific enthalpy in engine analysis more than the enthalpy itself. For a system with heat transfer Q and work W, the change in internal energy E from state 1 to state 2 is equal to the difference in the heat transfer into the system and the work done by the system: E2 - E1 = Q - W15Enthalpy Enthalpy- For the special case of a constant pressure process, the work done by the gas is given as the constant pressure p times the change in volume V: W = p * [V2 - V1] - Substituting into the first equation, we have: E2 - E1 = Q - p * [V2 - V1] - Let's group the conditions at state 2 and the conditions at state 1 together: (E2 + p * V2) - (E1 + p * V1) = Q - The (E + p * V) can be replaced by the enthalpy H; H2 - H1 = Q16Enthalpy EnthalpyFrom our definition of the heat transfer, we can represent Q by some heat capacity coefficient Cp times the temperature T. (H2 - H1) = Cp * (T2 - T1) -The SPECIFIC HEAT CAPACITY cp is called the specific heat at constant pressure and is related to the universal gas constant of the equation of state. This final equation is used to determine values of specific enthalpy for a given temperature. - Enthalpy is used in the energy equation for a fluid. Across shock waves, the total enthalpy of the gas remains a constant.17Entropy Entropy- Entropy, like temperature and pressure, can be explained on both a macro scale and a micro scale. Since thermodynamics deals only with the macro scale, the change in entropy delta S is defined here to be the heat transfer delta Q into the system divided by the temperature T: delta S = delta Q / T, dS = dQ / T - For gases, there are two possible ways to evaluate the change in entropy. We begin by using the first law of thermodynamics: dE = dQ - dW - If we use the definition of the enthalpy H of a gas: H = E + p * V, Then, dH = dE + p dV + V dp18Entropy Entropy- Substitute into the first law equation: dQ = dH - V dp - p dV + p dV - dQ = dH - V dp is an alternate way to present the first law of thermodynamics. For an ideal gas, the equation of state is written: p * V = R * T ; R is the gas constant - The heat transfer of a gas is equal to the heat capacity times the change in temperature; in differential form: dQ = C * dT - If we have a constant volume process, the formulation of the first law gives: dE = dQ = C (constant volume) * dT19Entropy Entropy- If we assume that the heat capacity is constant with temperature, we can use these two equations to define the change in enthalpy and internal energy. If we substitute the value for p from the equation of state, and the definition of dE in the first energy equation, we obtain: dQ = C (constant volume) * dT + R * T dV / V - Similarly substituting the value of V from the equation of state, and the definition of dH we obtain the alternate form: dQ = C (constant pressure) * dT - R * T dp / p - Substituting these forms for dQ into the differential form of the entropy equation gives: dS = C (constant volume) * dT / T + R * dV / V, and dS = C (constant pressure) * dT / T - R * dp / p20Entropy Entropy- These equations can be integrated from condition "1" to condition "2" to give: S2 - S1 = Cv * ln ( T2 / T1) + R * ln ( V2 / V1), and S2 - S1 = Cp * ln ( T2 / T1) - R * ln ( p2 / p1) - If we have a constant volume process, the second term in the equation is equal to zero, since v2/v1 = 1. We can then determine the value of the specific heat for the constant volume process. But if we have a process that changes volume, the second term in the equation is not zero. We can think of the first term of the equation as the contribution for a constant volume process, and the second term as the additional change produced by the change in volume. A similar type of argument can be made for the equation used for a change in pressure.21Operation Operation of of a a Simple Simple Cargo Cargo Reliq Reliq System System• There are two main types of liquefaction plants, namely: • Direct Reliquefaction Cycle; and Indirect Reliquefaction Cycle.In a Direct Reliquefaction Cycle, the evaporated or displaced cargo vapour is compressed, condensed and returned to the tank. This is the most commonly used system.••In an Indirect Reliquefaction Cycle, an external refrigeration system is employed to condense the cargo vapour without it being compressed. This cycle requires a very cold refrigerant and large surfaces. Enthalpy is a thermodynamic measure of the total heat content of a liquid or vapour at a given temperature and is expressed in energy per unit mass (k Joules per 1 kg) from absolute zero. Therefore, for a liquid/vapour mixture, it will be seen that it is the sum of the enthalpy of the liquid plus the latent heat of vaporisation.22Operation Operation of of a a Simple Simple Cargo Cargo System SystemVapour from cargo tankbCondensera dCompressorSeawaterTankcExpansion ValveReceiverHeatWith an understanding of the gas laws and the law of thermodynamics, the operation of the simple cargo system on board a liquefied gas ship can be followed. The cargo tank contains cold liquid, although it is insulated, some heat will still come through from outside. This is a consequence of second law of thermodynamics. This heat will overcome the latent heat of vaporisation and so some of the cargo will boil off. The vapour formed will raise the pressure in the tank. The vapour is drawn off and passed to a compressor.Operation Operation of of a a Simple Simple Cargo Cargo System SystemVapour from cargo tankbCondensera dCompressorSeawaterTankcExpansion ValveReceiverHeatThe work done by the compressor raises the temperature of the vapour according to the first law of thermodynamics. This temperature rise necessary so that it becomes hotter than the cooling medium that is subsequently used to cool it again.Operation Operation of of a a Simple Simple Cargo Cargo System SystemVapour from cargo tankbCondensera dCompressorSeawaterTankcExpansion ValveReceiverHeatThe hot gas or vapour now passes to a condenser where it is cooled by seawater. The removal of heat means that the molecules slow down and the vapour condenses to form a warm liquid. This liquid is also under pressure. The liquid is then passed through an expansion valve that reduces the pressure. This results in a partial evaporation, which causes the temperature of the bulk liquid to fall back down to tank temperature.Operation Operation of of a a Simple Simple Cargo Cargo System SystemVapour from cargo tankbCondensera dCompressorSeawaterTankcExpansion ValveReceiverHeatSo cold condensate is returned to the tank. These valves are also referred as JouleThompson valves, the physicist who discovered the cooling-expansion phenomenon. It is important to note that the joule-Thompson expansion results in cooling because the small amount of gas generated absorbs its latent heat of vaporisation from some of liquid.Operation Operation of of a a Simple Simple Cargo Cargo System SystemVapour from cargo tankbCondensera dCompressorSeawaterTankcExpansion ValveReceiverHeatJoule-Thompson expansion takes place at constant Enthalpy. The real cooling effect of this cycle is the removal of sensible heat and the latent heat of vaporisation from the full flow of gas as it condensed and cooled.28Operation Operation of of a a Simple Simple Cargo Cargo Reliq Reliq System SystemPressurecb aEnthalpyd29Ammonia Reliq Proces – direct single stage+33C 11.7 bar+140C 11.7 barCondenserExpansion valve Cargo Tank-33C 1.031 barCompressor-33C 1.031 bar3031Mollier Mollier Chart Chart of of Methane MethaneThe thermodynamic calculation of various gases or the liquids is done by using the thermodynamic diagram in general. Five (5) values of:1.pressure p, 2.specific volume v, 3.temperature T, 4.heat content h or i (enthalpy) 5.entropy sare used for the thermodynamic calculation. The p - i diagram is a so-called Mollier diagram, and “pressure p” is shown on VERICAL Axis and the “heat content h” (enthalpy i) on horizontal axisEnthalpyMollier Mollier Chart Chart of of Methane MethaneIsoEnthalpyMollier Mollier Chart Chart of of Methane MethaneThe following characteristics are included in this diagram:connected.）① Iso-bar（It is the horizontal in the line where the point of the same pressure was② Iso-enthalpy line （The normal in the line where the point of same was connected. ）liquid and the right side are moist steam at the left of this line. ）③ Saturated liquid line （In the line where the saturated liquid is shown, the under cooling ④ Saturated vapor line （In the line where the saturated vapor is shown, moist steamthe right side are superheated steam at the left of this line. ） andnormal in the liquid, and the line of a right descending in moist steam in the horizon and superheated steam as well as the isobar. ）⑤ Iso-thermal （In the line where the same point of the temperature is shown, near the⑥ Iso-entropy curve （Line where point of the same entropy was connected） ⑦ Iso-specific volume line （Line where point of the same bulkiness was connected） ⑧ Iso-Dryness line （Line where point of constant dryness was connected. Dryness X = 0.1,it means that 10% dry saturated steam in moist steam.） 34Mollier Mollier Chart Chart of of Methane MethaneK Kj/K j/KgK gKCR I TI CA LC Con onst stan ant tT Tem empe pera ratu ture re oC oCTEConstant Constant Enthalpy Enthalpy kcal/kg kcal/kg or or Kj/kg Kj/kgC Con onst stan ant tE Ent ntro ropy pyGAS AREAM PE RA TURE=-82 .5C)CRITICAL PRESSURE (ie. CH4 = 43 Bar)IsoSUB COOLED LIQUIDS SA AT TU UR RA AT TE ED DV VA AP PO OU UR RL LI IN NE EL LII Q QU UII D DS SA AT TU UR RA AT TE ED Di spec o s IS CON e olum fi c VL LII N NE Ekg m3/ E M OLU V T TANSUPERHEATED VAPOURArea of PV=mRTSATURATED REGION – LIQUID / VAPOR MIXTURELATENT HEAT OF VAPORISATIONEnthalpyPRACTICAL USE OF MOLLIER DIAGRAM361ST EXAMPLE DIFFERENCE IN VOLUME BETWEEN LNG LIQUID AND LNG VAPOR AT SAME TEMPERATURE37Specific Volume 0.00235 m3/kgFor Density 425 kg/m3 (1/425 = 0.00235)s e  ime m lu 55 t o V 2 ic  ce   f i ec ren p S iffe D.66 ity 1 s n e For Dfic V i c e Sp3/kg m   0 .600 0   e olum 001/ 1 m3 (  0.6 .66 =  kg/LNG Liquid  >>> LNG Vapour at Boiling temp – 160  C1.4 51. LNG liquid2. LNG Vapour38Specific Volume 0.00235 m3/kgFor Density 425 kg/m3 (1/425 = 0.00235)Specific Volume change from  LNG Liquid – 160 C >>> LNG  Vapour at 20 C = Difference  619  timesSpecific Volume 1.45000 m3/kg @ 20CFor Density 0.69  kg/m3 (1/0.696 = 1.45 m3/kg1.4 51. LNG Liquid 3. LNG Vapour @ 20C 39LATENT HEAT REQUIRED FROM LIQUID to VAPOR KJ / KGLNG liquidLNG VapourLNG Latent Heat = 675 – 164 kj/kg = 511 kj/kgEnthalpy 164 kj/kg Enthalpy 675 kj/kg 40SENSITIVE HEAT KJ / kg C (K)Sensitive Heat  from – 160 C  to – 100   = 800 kj/kg   ‐ 675  kj/kg = 125 kj/kg For 1 kg temperature to increase 60 C  required 125 kj  i.e. 125 kj / 60 C   =  Cp  =   2.083 kJ/kg/CLNG VapourLNG VapourEnthalpy 675 kj/kgEnthalpy 800   kj/kg 41Mollier Mollier Chart ChartIn summary, it can be seen that the heat flowing into the tank has been returned to the environment via the seawater, together with the additional work done by Mollier chart. Point ‘a’ represents the vapour in the dome of the tank. As it passes to the compressor it picks up some heat. The compressor raises the temperature and pressure of the gas to point ‘b’ it is then cooled and condensed and becomes a saturated liquid at point ‘c’ in the condenser. The expansion valve then produces cold liquid with a proportion of vapour and this is returned to the tank at point ‘d’.Pressurecb aEnthalpyd。

Thermodynamics (from the Greek θερμη, therme, meaning "heat" and δυναμις, dynamis, meaning "power") is a branch of physics that studies the effects of changes in temperature, pressure, and volume on physical systems at the macroscopic scale by analyzing the collective motion of their particles using statistics.[1]In this context, heat means "energy in transit" and dynamics relates to "movement;" thus, thermodynamics is the study of the movement of energy and how energy instills movement. Historically, thermodynamics developed out of need to increase the efficiency of early steam engines.[2]Typical thermodynamic system—heat moves from hot (boiler) to cold (condenser), (both not shown) and work is extracted, in this case by a series of pistons.The starting point for most thermodynamic considerations are the laws of thermodynamics, which postulate that energy can be exchanged between physical systems as heat or work.[3]The first law of thermodynamics states a universal principle that processes or changes in the real world involve energy, and within a closed system the total amount of that energy does not change, only its form (such as from heat of combustion to mechanical work in an engine) may change. The second law gives a direction to that change by specifying that in any change in any closed system in the real world the degree of order of the system's matter and energy becomes less, or conversely stated, the amount of disorder (entropy) of the system increases.[4]In thermodynamics, interactions between large ensembles of objects are studied and categorized. Central to this are the concepts of system and surroundings. A system comprises particles whose average motions define the system's properties, which are related to one another through equations of state defining the relations between state variables such as temperature, pressure, volume, and entropy. State variables can becombined to express internal energy and thermodynamic potentials, which are useful for determining conditions for equilibrium and spontaneous processes.[5]With these tools, thermodynamics describes how systems respond to changes in their surroundings. This can be applied to a wide variety of topics in science and engineering, such as engines, phase transitions, chemical reactions, transport phenomena, and even black holes. The results of thermodynamics are essential for other fields of physics and for chemistry, chemical engineering, aerospace engineering, mechanical engineering, cell biology, biomedical engineering, and materials science to name a few.[6]Thermodynamics, with its insights into the relations between heat, energy, and work as exemplified in mechanical systems, provides a foundation for trying to understand the behavior and properties of biological, social, and economic systems, which generally maintain an ordered pattern only by consuming a sustained flow of energy.Thermodynamics is the branch of physical science concerned with heat and its relation to other forms of energy and work. It defines macroscopic variables (such as temperature, entropy, and pressure) that describe average properties of material bodies and radiation, and explains how they are related and by what laws they change with time. Thermodynamics does not describe the microscopic constituents of matter, and its laws can be derived from statistical mechanics.Thermodynamics can be applied to a wide variety of topics in science and engineering, such as engines, phase transitions, chemical reactions, transport phenomena, and even black holes. The results of thermodynamics are essential for other fields of physics and for chemistry, chemical engineering, aerospace engineering, mechanical engineering, cell biology, biomedical engineering, materials science, and are useful for other fields such as economics.[1][2]Thermodynamics is one of the best logically structured branches of physics and has become one of the classical branches of theoretical physics.[citation needed]Much of the empirical content of thermodynamics is contained in its four laws. The first law specifies that energy can be exchanged between physical systems as heat and thermodynamic work.[3]The second law concerns a quantity called entropy, that expresses limitations, arising from what is known as irreversibility, on the amount of thermodynamic work that can be delivered to an external system by a thermodynamic process.[4]Historically, thermodynamics developed out of a desire to increase the efficiency of early steam engines, particularly through the work of French physicist Nicolas Léonard Sadi Carnot (1824) who believed that the efficiency of heat engines was the key that could help France win the Napoleonic Wars.[5] Scottish physicist Lord Kelvin was the first to formulate a concise definition of thermodynamics in 1854:[6]Thermo-dynamics is the subject of the relation of heat to forces acting between contiguous parts of bodies, and the relation of heat to electrical agency.Initially, the thermodynamics of heat engines concerned mainly the thermal properties of their 'working materials', such as steam. This concern was then linked to the study of energy transfers in chemical processes, for example to the investigation, published in 1840, of the heats of chemical reactions[7] by Germain Hess, which was not originally explicitly concerned with the relation between energy exchanges by heat and work. Chemical thermodynamics studies the role of entropy in chemical reactions.[8][9][10][11][12][13][14][15][16] Also, statistical thermodynamics, or statistical mechanics, gave explanations of macroscopic thermodynamics by statistical predictions of the collective motion of particles based on the mechanics of their microscopic behavior.The plain term 'thermodynamics' refers to macroscopic description of bodies and processes.[17]"Any reference to atomic constitution is foreign to ... thermodynamics".[18] The qualified term 'statistical thermodynamics' refers to descriptions of bodies and processes in terms of the atomic constitution of matter.Thermodynamics is built on the study of energy transfers that can be strictly resolved into two distinct components, heat and work, specified by macroscopic variables.[19][20]Thermodynamic equilibrium is one of the most important concepts for thermodynamics. As the systems and processes of interest are taken further from thermodynamic equilibrium, their thermodynamical study becomes a little more involved but also of much more practical value. In many important cases, such as heat engines or refrigerators, the systems consist of many subsystems at different temperatures and pressures. Thermodynamics is a practical science and also deals with these inhomogeneous dynamic systems provided the thermodynamical parameters are well-defined. The present article takes a gradual approach to the subject, starting with a focus on cyclic processes and thermodynamic equilibrium, and then gradually beginning to further considernon-equilibrium systems.For thermodynamics and statistical thermodynamics to apply to a process in a body, it is necessary that the atomic mechanisms of the process fall into just two classes: those so rapid that, in the time frame of the process of interest, the atomic states effectively visit all of their accessible range, and those so slow that their effects can be neglected in the time frame of the process of interest.[21] The rapid atomic mechanisms mediate the macroscopic changes that are of interest for thermodynamics and statistical thermodynamics, because they quickly bring the system near enough to thermodynamic equilibrium. "When intermediate rates are present, thermodynamics and statistical mechanics cannot be applied."[21] The intermediate rate atomic processes do not bring the system near enough to thermodynamic equilibrium in the time frame of the macroscopic process of interest. This separation of time scales of atomic processes is a theme that recurs throughout the subject.Basic for thermodynamics are the concepts of system and surroundings.[12][22]There are two fundamental kinds of entity in thermodynamics, states of a system, and processes of a system. This allows two fundamental approaches to thermodynamic reasoning, that in terms of states of a system, and that in terms of cyclic processes of a system.For thermodynamics and statistical thermodynamics to apply to a process in a body, it is necessary that the atomic mechanisms of the process fall into just two classes: those so rapid that, in the time frame of the process of interest, the atomic states effectively visit all of their accessible range, and those so slow that their effects can be neglected in the time frame of the process of interest.[21] The rapid atomic mechanisms mediate the macroscopic changes that are of interest for thermodynamics and statistical thermodynamics, because they quickly bring the system near enough to thermodynamic equilibrium. "When intermediate rates are present, thermodynamics and statistical mechanics cannot be applied."[21] The intermediate rate atomic processes do not bring the system near enough to thermodynamic equilibrium in the time frame of the macroscopic process of interest. This separation of time scales of atomic processes is a theme that recurs throughout the subject.Basic for thermodynamics are the concepts of system and surroundings.[12][22]There are two fundamental kinds of entity in thermodynamics, states of a system, and processes of a system. This allows two fundamental approaches to thermodynamic reasoning, that in terms of states of a system, and that in terms of cyclic processes of a system.A thermodynamic system can be defined in terms of its states. In this way, a thermodynamic system is a macroscopic physical object, explicitly specified in terms of macroscopic physical and chemical variables which describe its macroscopic properties. The macroscopic state variables of thermodynamics have been recognized in the course of empirical work in physics and chemistry.[13]A thermodynamic system can also be defined in terms of the processes which it can undergo. Of particular interest are cyclic processes. This was the way of the founders of thermodynamics in the first three quarters of the nineteenth century.The surroundings of a thermodynamic system are other thermodynamic systems that can interact with it. An example of a thermodynamic surrounding is a heat bath, which is considered to be held at a prescribed temperature, regardless of the interactions it might have with the system.The macroscopic variables of a thermodynamic system in thermodynamic equilibrium, in which temperature is well defined, can be related to one another through equations of state or characteristic equations.[23][24][25][26] They express the constitutive peculiarities of the material of the system.Classical thermodynamics is characterized by its study of materials that have equations of state that express relations between mechanical variables and temperature that are reached much more rapidly than any changes in the surroundings. A classical material can usually be described by a function that makes pressure dependent on volume and temperature, the resulting pressure being established much more rapidly than any imposed change of volume or temperature.[27]This is another expression of the concept of separation of time scales of atomic processes mentioned above.Thermodynamic facts can often be explained by viewing macroscopic objects as assemblies of very many microscopic or atomic objects that obey Hamiltonian dynamics.[12][28][29] The microscopic or atomic objects exist in species, the objects of each species being all alike. Because of this likeness, statistical methods can be used to account for the macroscopic properties of the thermodynamic system in terms of the properties of the microscopic species. Such explanation is called statistical thermodynamics; also often it is also referred to by the term 'statistical mechanics', though this term can have a wider meaning, referring to 'microscopic objects', such as economic quantities, that do not obey Hamiltonian dynamics.[28]The study of thermodynamical systems has developed into several related branches, each using a different fundamental model as a theoretical or experimental basis, or applying the principles to varying types of systems.[edit] Classical thermodynamicsClassical thermodynamics is the description of the states (especially equilibrium states) and processes of thermodynamical systems, using macroscopic, empirical properties directly measurable in the laboratory. It is used to model exchanges of energy, work, heat, and matter, based on the laws of thermodynamics. The qualifier classical reflects the fact that it represents the descriptive level in terms of macroscopic empirical parameters that can be measured in the laboratory, that was the first level of understanding in the 19th century. A microscopic interpretation of these concepts was provided by the development of statistical thermodynamics.[edit] Statistical thermodynamicsStatistical thermodynamics, also called statistical mechanics, emerged with the development of atomic and molecular theories in the second half of the 19th century and early 20th century, supplementing thermodynamics with an interpretation of the microscopic interactions between individual particles or quantum-mechanical states. This field relates the microscopic properties of individual atoms and molecules to the macroscopic, bulk properties of materials that can be observed on the human scale, thereby explaining thermodynamics as a natural result of statistics, classical mechanics, and quantum theory at the microscopic level.[edit] Chemical thermodynamicsChemical thermodynamics is the study of the interrelation of energy with chemical reactions and chemical transport and with physical changes of state within the confines of the laws of thermodynamics.[edit] Thermodynamic equilibriumEquilibrium thermodynamics studies transformations of matter and energy in systems at or near thermodynamic equilibrium. In thermodynamicequilibrium, a system's properties are, by definition, unchanging in time. In thermodynamic equilibrium no macroscopic change is occurring or can be triggered; within the system, every microscopic process is balanced by its opposite; this is called the principle of detailed balance. A central aim in equilibrium thermodynamics is: given a system in awell-defined initial state, subject to specified constraints, to calculate what the equilibrium state of the system will be.[40]Within a simple isolated thermodynamic system in thermodynamic equilibrium, in the absence of externally imposed force fields, all properties of the material of the system are spatially homogeneous.[41]Much of the basic theory of thermodynamics is concerned with homogeneous systems in thermodynamic equilibrium.[8][42]Most systems found in nature or considered in engineering are not in thermodynamic equilibrium, exactly considered. They are changing or can be triggered to change over time, and are continuously and discontinuously subject to flux of matter and energy to and from other systems.[43] For example, according to Callen, "in absolute thermodynamic equilibrium all radioactive materials would have decayed completely and nuclear reactions would have transmuted all nuclei to the most stable isotopes. Such processes, which would take cosmic times to complete, generally can be ignored.".[43] Such processes being ignored, many systems in nature are close enough to thermodynamic equilibrium that for many purposes their behaviour can be well approximated by equilibrium calculations.[edit] Quasi-static transfers between simple systems are nearly in thermodynamic equilibrium and are reversibleIt very much eases and simplifies theoretical thermodynamical studies to imagine transfers of energy and matter between two simple systems that proceed so slowly that at all times each simple system considered separately is near enough to thermodynamic equilibrium. Such processes are sometimes called quasi-static and are near enough to being reversible.[44][45][edit] Natural processes are partly explained by tendency towards thermodynamic equilibrium and are irreversibleSimple isolated thermodynamic systems, as time passes, tend naturally towards thermodynamic equilibrium. In the absence of externally imposed force fields, they become homogeneous in all their local properties.Many thermodynamic processes can be can be modeled by compound or composite systems, consisting of several or many contiguous component simple systems, initially not in thermodynamic equilibrium, but allowed to transfer mass and energy between them. Natural thermodynamic processes can be explained in terms of a tendency towards thermodynamic equilibrium within simple systems and in transfers between contiguous simple systems. Such natural processes are irreversible.[46][edit] Non-equilibrium thermodynamicsNon-equilibrium thermodynamics[47]is a branch of thermodynamics that deals with systems that are not in thermodynamic equilibrium; it is also called thermodynamics of irreversible processes. Non-equilibrium thermodynamics is concerned with transport processes and with the rates of chemical reactions.[48] Non-equilibrium systems can be in stationary states that are not homogeneous even when there is no externally imposed field of force; in this case, the description of the internal state of the system requires a field theory.[49][50][51] One of the methods of dealing with non-equilibrium systems is to introduce so-called 'internal variables'. These are quantities that express the local state of the system, besides the usual local thermodynamic variables; in a sense such variables might be seen as expressing the 'memory' of the materials. Hysteresis may sometimes be described in this way. In contrast to the usual thermodynamic variables, 'internal variables' cannot be controlled by external manipulations.[52]This approach is usually unnecessary for gases and liquids, but may be useful for solids.[53] Many natural systems still today remain beyond the scope of currently known macroscopic thermodynamic methods.[edit] System modelsA diagram of a generic thermodynamic systemAn important concept in thermodynamics is the thermodynamic system, a precisely defined region of the universe under study. Everything in the universe except the system is known as the surroundings. A system is separated from the remainder of the universe by a boundary which may benotional or not, but which by convention delimits a finite volume. Exchanges of work, heat, or matter between the system and the surroundings take place across this boundary.The boundary is simply a surface around the volume of interest. Anything that passes across the boundary that effects a change in the internal energy needs to be accounted for in the energy balance equation. The volume can be the region surrounding a single atom resonating energy, as Max Planck defined in 1900; it can be a body of steam or air in a steam engine, such as Sadi Carnot defined in 1824; it can be the body of a tropical cyclone, such as Kerry Emanuel theorized in 1986 in the field of atmospheric thermodynamics; it could also be just one nuclide (i.e. a system of quarks) as hypothesized in quantum thermodynamics.Boundaries are of four types: fixed, moveable, real, and imaginary. For example, in an engine, a fixed boundary means the piston is locked at its position; as such, a constant volume process occurs. In that same engine, a moveable boundary allows the piston to move in and out. For closed systems, boundaries are real while for open system boundaries are often imaginary.Generally, thermodynamics distinguishes three classes of systems, defined in terms of what is allowed to cross their boundaries.Interactions of thermodynamic systemsType of system Mass flow Work HeatOpenClosedIsolatedIn theoretical studies, it is often convenient to consider the simplest kind of thermodynamic system. This is defined variously by different authors.[77][78][56][79][80][81] For the present article, the following definition will be convenient, as abstracted from the definitions of various authors.A region of material with all intensive properties continuous in space and time is called a phase. A simple system is for the present article defined as one that consists of a single phase of material with no interior partitions.Engineering and natural processes are often described as compounds of many different component simple systems, sometimes with unchanging or changing partitions between them.[edit] States and processesThere are two fundamental kinds of entity in thermodynamics, states of a system, and processes of a system. This allows two fundamental approaches to thermodynamic reasoning, that in terms of states of a system, and that in terms of cyclic processes of a system.The approach through states of a system requires a full account of the state of the system as well as a notion of process from one state to another of a system, but may require only a partial account of the state of the surroundings of the system or of other systems.The notion of a cyclic process does not require a full account of the state of the system, but does require a full account of how the process occasions transfers of matter and energy between the system and its surroundings, which must include at least two heat reservoirs at different temperatures, one hotter than the other. In this approach, the notion of a properly numerical scale of temperature is a presupposition of thermodynamics, not a notion constructed by or derived from it.The method of description in terms of states has limitations. For example, processes in a region of turbulent flow, or in a burning gas mixture, or in a Knudsen gas may be beyond "the province of thermodynamics".[82][83][84] This problem can sometimes be circumvented through the method of description in terms of cyclic processes. This is part of the reason why the founders of thermodynamics often preferred the cyclic process description.[edit] Thermodynamic state variablesWhen a system is at thermodynamic equilibrium under a given set of conditions of its surroundings, it is said to be in a definite thermodynamic state, which is fully described by its state variables.If a system is simple as defined above, and is in thermodynamic equilibrium, and is not subject to an externally imposed force field, such as gravity, electricity, or magnetism, then it is homogeneous, that is say, spatially uniform in all respects.[85]In a sense, a homogeneous system can be regarded as spatiallyzero-dimensional, because it has no spatial variation.If a system in thermodynamic equilibrium is homogeneous, then its state can be described by a few physical variables, which are mostly classifiable as intensive variables and extensive variables.[51][86][87][12][28]Examples of extensive thermodynamic variables are total mass and total volume. Examples of intensive thermodynamic variables are temperature, pressure, and chemical concentration; 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.[28] Temperature is a thermal variable; according to Guggenheim, "the most important conception in thermodynamics is temperature."[12]Intensive variables are defined by the property that if any number of systems, each in its own separate homogeneous thermodynamic equilibrium state, all with the same respective values of all of their intensive variables, regardless of the values of their extensive variables, are laid contiguously with no partition between them, so as to form a new system, then the values of the intensive variables of the new system are the same as those of the separate constituent systems. Such a composite system is in a homogeneous thermodynamic equilibrium. Examples of intensive variables are temperature, chemical concentration, pressure, density of mass, density of internal energy, and, when it can be properly defined, density of entropy.[88]Extensive variables are defined by the property that if any number of systems, regardless of their possible separate thermodynamic equilibrium or non-equilibrium states or intensive variables, are laid side by side with no partition between them so as to form a new system, then the values of the extensive variables of the new system are the sums of the values of the respective extensive variables of the individual separate constituent systems. Obviously, there is no reason to expect such a composite system to be in in a homogeneous thermodynamic equilibrium. Examples of extensive variables are mass, volume, and internal energy. They depend on the total quantity of mass in the system.[89]Though, when it can be properly defined, density of entropy is an intensive variable, for inhomogeneous systems, entropy itself does not fit into this classification of state variables.[90][91] The reason is that entropy is a property of a system as a whole, and not necessarily related simply to its constituents separately. It is true that for any number of systems each in its own separate homogeneous thermodynamic equilibrium, all with the same values of intensive variables, removal of the partitions between the separate systems results in a composite homogeneous system in thermodynamic equilibrium, with all the values of its intensive variables the same as those of the constituent systems, and it is reservedly orconditionally true that the entropy of such a restrictively defined composite system is the sum of the entropies of the constituent systems. But if the constituent systems do not satisfy these restrictive conditions, the entropy of a composite system cannot be expected to be the sum of the entropies of the constituent systems, because the entropy is a property of the composite system as a whole. Therefore, though under these restrictive reservations, entropy satisfies some requirements for extensivity defined just above, entropy in general does not fit the above definition of an extensive variable.Being neither an intensive variable nor an extensive variable according to the above definition, entropy is thus a stand-out variable, because it is a state variable of a system as a whole.[90]A non-equilibrium system can have a very inhomogeneous dynamical structure. This is one reason for distinguishing the study of equilibrium thermodynamics from the study of non-equilibrium thermodynamics.The physical reason for the existence of extensive variables is the time-invariance of volume in a given inertial reference frame, and the strictly local conservation of mass, momentum, angular momentum, and energy. As noted by Gibbs, entropy is unlike energy and mass, because it is not locally conserved.[90] The stand-out quantity entropy is never conserved in real physical processes; all real physical processes are irreversible.[92] The motion of planets seems reversible on a short time scale (millions of years), but their motion, according to Newton's laws, is mathematically an example of deterministic chaos. Eventually a planet will suffer an unpredictable collision with an object from its surroundings, outer space in this case, and consequently its future course will be radically unpredictable. Theoretically this can be expressed by saying that every natural process dissipates some information from the predictable part of its activity into the unpredictable part. The predictable part is expressed in the generalized mechanical variables, and the unpredictable part in heat.There are other state variables which can be regarded as conditionally 'extensive' subject to reservation as above, but not extensive as defined above. Examples are the Gibbs free energy, the Helmholtz free energy, and the enthalpy. Consequently, just because for some systems under particular conditions of their surroundings such state variables are conditionally conjugate to intensive variables, such conjugacy does not make such state variables extensive as defined above. This is another reason for distinguishing the study of equilibrium thermodynamics from the study of non-equilibrium thermodynamics. In another way of thinking, this explains why heat is to be regarded as a quantity that refers to a process and not to a state of a system.。