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.

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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.