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Thermodynamics I INTRODUCTION Thermodynamics, field of physics that describes and correlates the physical properties of macroscopic systems of matter and energy.

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Thermodynamics I INTRODUCTION Thermodynamics, field of physics that describes and correlates the physical properties of macroscopic systems of matter and energy. The principles of thermodynamics are of fundamental importance to all branches of science and engineering. A central concept of thermodynamics is that of the macroscopic system, defined as a geometrically isolable piece of matter in coexistence with an infinite, unperturbable environment. The state of a macroscopic system in equilibrium can be described in terms of such measurable properties as temperature, pressure, and volume, which are known as thermodynamic variables. Many other variables (such as density, specific heat, compressibility, and the coefficient of thermal expansion) can be identified and correlated, to produce a more complete description of an object and its relationship to its environment. When a macroscopic system moves from one state of equilibrium to another, a thermodynamic process is said to take place. Some processes are reversible and others are irreversible. The laws of thermodynamics, discovered in the 19th century through painstaking experimentation, govern the nature of all thermodynamic processes and place limits on them. II ZEROTH LAW OF THERMODYNAMICS The vocabulary of empirical sciences is often borrowed from daily language. Thus, although the term temperature appeals to common sense, its meaning suffers from the imprecision of nonmathematical language. A precise, though empirical, definition of temperature is provided by the so-called zeroth law of thermodynamics as explained below. When two systems are in equilibrium, they share a certain property. This property can be measured and a definite numerical value ascribed to it. A consequence of this fact is the zeroth law of thermodynamics, which states that when each of two systems is in equilibrium with a third, the first two systems must be in equilibrium with each other. This shared property of equilibrium is the temperature. If any such system is placed in contact with an infinite environment that exists at some certain temperature, the system will eventually come into equilibrium with the environment--that is, reach the same temperature. (The so-called infinite environment is a mathematical abstraction called a thermal reservoir; in reality the environment need only be large relative to the system being studied.) Temperatures are measured with devices called thermometers (see Thermometer). A thermometer contains a substance with conveniently identifiable and reproducible states, such as the normal boiling and freezing points of pure water. If a graduated scale is marked between two such states, the temperature of any system can be determined by having that system brought into thermal contact with the thermometer, provided that the system is large relative to the thermometer. III FIRST LAW OF THERMODYNAMICS The first law of thermodynamics gives a precise definition of heat, another commonly used concept. When an object is brought into contact with a relatively colder object, a process takes place that brings about an equalization of temperatures of the two objects. To explain this phenomenon, 18th-century scientists hypothesized that a substance more abundant at higher temperature flowed toward the region at a lower temperature. This hypothetical substance, called "caloric," was thought to be a fluid capable of moving through material media. The first law of thermodynamics instead identifies caloric, or heat, as a form of energy. It can be converted into mechanical work, and it can be stored, but is not a material substance. Heat, measured originally in terms of a unit called the calorie, and work and energy, measured in ergs, were shown by experiment to be totally equivalent. One calorie is equivalent to 4.186 × 107 ergs, or 4.186 joules. The first law, then, is a law of energy conservation. It states that, because energy cannot be created or destroyed--setting aside the later ramifications of the equivalence of mass and energy (see Nuclear Energy)--the amount of heat transferred into a system plus the amount of work done on the system must result in a corresponding increase of internal energy in the system. Heat and work are mechanisms by which systems exchange energy with one another. In any machine some amount of energy is converted into work; therefore, no machine can exist in which no energy is converted into work. Such a hypothetical machine (in which no energy is required for performing work) is termed a "perpetual-motion machine of the first kind." Since the input energy must now take heat into account (and in a broader sense chemical, electrical, nuclear, and other forms of energy as well), the law of energy conservation rules out the possibility of such a machine ever being invented. The first law is sometimes given in a contorted form as a statement that precludes the existence of perpetual-motion machines of the first kind. IV SECOND LAW OF THERMODYNAMICS The second law of thermodynamics gives a precise definition of a property called entropy. Entropy can be thought of as a measure of how close a system is to equilibrium; it can also be thought of as a measure of the disorder in the system. The law states that the entropy--that is, the disorder--of an isolated system can never decrease. Thus, when an isolated system achieves a configuration of maximum entropy, it can no longer undergo change: It has reached equilibrium. Nature, then, seems to "prefer" disorder or chaos. It can be shown that the second law stipulates that, in the absence of work, heat cannot be transferred from a region at a lower temperature to one at a higher temperature. The second law poses an additional condition on thermodynamic processes. It is not enough to conserve energy and thus obey the first law. A machine that would deliver work while violating the second law is called a "perpetual-motion machine of the second kind," since, for example, energy could then be continually drawn from a cold environment to do work in a hot environment at no cost. The second law of thermodynamics is sometimes given as a statement that precludes perpetual-motion machines of the second kind. V THERMODYNAMIC CYCLES Carnot Engine The idealized Carnot engine was envisioned by the French physicist Nicolas Léonard Sadi Carnot, who lived during the early 19th century. The Carnot engine is theoretically perfect, that is, it converts the maximum amount of energy into mechanical work. Carnot showed that the efficiency of any engine depends on the difference between the highest and lowest temperatures reached during one cycle. The greater the difference, the greater the efficiency. An automobile engine, for example, would be more efficient if the fuel burned hotter and the exhaust gas came out of the cylinder at a lower temperature. © Microsoft Corporation. All Rights Reserved. All important thermodynamic relations used in engineering are derived from the first and second laws of thermodynamics. One useful way of discussing thermodynamic processes is in terms of cycles--processes that return a system to its original state after a number of stages, thus restoring the original values for all the relevant thermodynamic variables. In a complete cycle the internal energy of a system depends solely on these variables and cannot change. Thus, the total net heat transferred to the system must equal the total net work delivered from the system. An ideal cycle would be performed by a perfectly efficient heat engine--that is, all the heat would be converted to mechanical work. The 19th-century French scientist Nicolas Léonard Sadi Carnot, who conceived a thermodynamic cycle that is the basic cycle of all heat engines, showed that such an ideal engine cannot exist. Any heat engine must expend some fraction of its heat input as exhaust. The second law of thermodynamics places an upper limit on the efficiency of engines; that upper limit is less than 100 percent. The limiting case is now known as a Carnot cycle. VI THIRD LAW OF THERMODYNAMICS The second law suggests the existence of an absolute temperature scale that includes an absolute zero of temperature. The third law of thermodynamics states that absolute zero cannot be attained by any procedure in a finite number of steps. Absolute zero can be approached arbitrarily closely, but it can never be reached. VII MICROSCOPIC BASIS OF THERMODYNAMICS The recognition that all matter is made up of molecules provided a microscopic foundation for thermodynamics. A thermodynamic system consisting of a pure substance can be described as a collection of like molecules, each with its individual motion describable in terms of such mechanical variables as velocity and momentum. At least in principle, it should therefore be possible to derive the collective properties of the system by solving equations of motion for the molecules. In this sense, thermodynamics could be regarded as a mere application of the laws of mechanics to the microscopic system. Objects of ordinary size--that is, ordinary on the human scale--contain immense numbers (on the order of 1024) of molecules. Assuming the molecules to be spherical, each would need three variables to describe its position and three more to describe its velocity. Describing a macroscopic system in this way would be a task that even the largest modern computer could not manage. A complete solution of these equations, furthermore, would tell us where each molecule is and what it is doing at every moment. Such a vast quantity of information would be too detailed to be useful and too transient to be important. Statistical methods were devised therefore to obtain averages of the mechanical variables of the molecules in a system and to provide the gross features of the system. These gross features turn out to be, precisely, the macroscopic thermodynamic variables. The statistical treatment of molecular mechanics is called statistical mechanics, and it anchors thermodynamics to mechanics. Viewed from the statistical perspective, temperature represents a measure of the average kinetic energy of the molecules of a system. Increases in temperature reflect increases in the vigor of molecular motion. When two systems are in contact, energy is transferred between molecules as a result of collisions. The transfer will continue until uniformity is achieved, in a statistical sense, which corresponds to thermal equilibrium. The kinetic energy of the molecules also corresponds to heat and--together with the potential energy arising from interaction between molecules--makes up the internal energy of a system. The conservation of energy, a well-known law of mechanics, translates readily to the first law of thermodynamics, and the concept of entropy translates into the extent of disorder on the molecular scale. By assuming that all combinations of molecular motion are equally likely, thermodynamics shows that the more disordered the state of an isolated system, the more combinations can be found that could give rise to that state, and hence the more frequently it will occur. The probability of the more disordered state occurring overwhelms the probability of the occurrence of all other states. This probability provides a statistical basis for definitions of both equilibrium state and entropy. Finally, temperature can be reduced by taking energy out of a system, that is, by reducing the vigor of molecular motion. Absolute zero corresponds to the state of a system in which all its constituents are at rest. This is, however, a notion from classical physics. In terms of quantum mechanics, residual molecular motion will exist even at absolute zero. An analysis of the statistical basis of the third law goes beyond the scope of the present discussion. See Gases; Quantum Theory; Uncertainty Principle. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.

« Carnot EngineThe idealized Carnot engine was envisioned by the French physicist Nicolas Léonard Sadi Carnot, who lived during theearly 19th century.

The Carnot engine is theoretically perfect, that is, it converts the maximum amount of energy intomechanical work.

Carnot showed that the efficiency of any engine depends on the difference between the highest andlowest temperatures reached during one cycle.

The greater the difference, the greater the efficiency.

An automobileengine, for example, would be more efficient if the fuel burned hotter and the exhaust gas came out of the cylinder at alower temperature.© Microsoft Corporation.

All Rights Reserved. All important thermodynamic relations used in engineering are derived from the first and second laws of thermodynamics.

One useful way of discussing thermodynamicprocesses is in terms of cycles—processes that return a system to its original state after a number of stages, thus restoring the original values for all the relevantthermodynamic variables.

In a complete cycle the internal energy of a system depends solely on these variables and cannot change.

Thus, the total net heat transferredto the system must equal the total net work delivered from the system. An ideal cycle would be performed by a perfectly efficient heat engine—that is, all the heat would be converted to mechanical work.

The 19th-century French scientistNicolas Léonard Sadi Carnot, who conceived a thermodynamic cycle that is the basic cycle of all heat engines, showed that such an ideal engine cannot exist.

Any heatengine must expend some fraction of its heat input as exhaust.

The second law of thermodynamics places an upper limit on the efficiency of engines; that upper limit isless than 100 percent.

The limiting case is now known as a Carnot cycle. VI THIRD LAW OF THERMODYNAMICS The second law suggests the existence of an absolute temperature scale that includes an absolute zero of temperature.

The third law of thermodynamics states thatabsolute zero cannot be attained by any procedure in a finite number of steps.

Absolute zero can be approached arbitrarily closely, but it can never be reached. VII MICROSCOPIC BASIS OF THERMODYNAMICS The recognition that all matter is made up of molecules provided a microscopic foundation for thermodynamics.

A thermodynamic system consisting of a pure substancecan be described as a collection of like molecules, each with its individual motion describable in terms of such mechanical variables as velocity and momentum.

At leastin principle, it should therefore be possible to derive the collective properties of the system by solving equations of motion for the molecules.

In this sense,thermodynamics could be regarded as a mere application of the laws of mechanics to the microscopic system. Objects of ordinary size—that is, ordinary on the human scale—contain immense numbers (on the order of 10 24) of molecules.

Assuming the molecules to be spherical, each would need three variables to describe its position and three more to describe its velocity.

Describing a macroscopic system in this way would be a task that eventhe largest modern computer could not manage.

A complete solution of these equations, furthermore, would tell us where each molecule is and what it is doing at everymoment.

Such a vast quantity of information would be too detailed to be useful and too transient to be important. Statistical methods were devised therefore to obtain averages of the mechanical variables of the molecules in a system and to provide the gross features of the system.These gross features turn out to be, precisely, the macroscopic thermodynamic variables.

The statistical treatment of molecular mechanics is called statistical mechanics,and it anchors thermodynamics to mechanics. Viewed from the statistical perspective, temperature represents a measure of the average kinetic energy of the molecules of a system.

Increases in temperature reflectincreases in the vigor of molecular motion.

When two systems are in contact, energy is transferred between molecules as a result of collisions.

The transfer will continueuntil uniformity is achieved, in a statistical sense, which corresponds to thermal equilibrium.

The kinetic energy of the molecules also corresponds to heat and—togetherwith the potential energy arising from interaction between molecules—makes up the internal energy of a system. The conservation of energy, a well-known law of mechanics, translates readily to the first law of thermodynamics, and the concept of entropy translates into the extentof disorder on the molecular scale.

By assuming that all combinations of molecular motion are equally likely, thermodynamics shows that the more disordered the stateof an isolated system, the more combinations can be found that could give rise to that state, and hence the more frequently it will occur.

The probability of the moredisordered state occurring overwhelms the probability of the occurrence of all other states.

This probability provides a statistical basis for definitions of both equilibriumstate and entropy. Finally, temperature can be reduced by taking energy out of a system, that is, by reducing the vigor of molecular motion.

Absolute zero corresponds to the state of asystem in which all its constituents are at rest.

This is, however, a notion from classical physics.

In terms of quantum mechanics, residual molecular motion will existeven at absolute zero.

An analysis of the statistical basis of the third law goes beyond the scope of the present discussion. See Gases; Quantum Theory; Uncertainty Principle. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation.

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