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

« 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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