Thermodynamics is the science that deals with work and heat—and the transformation of one into the other. It is a macroscopic theory, dealing with matter in bulk, disregarding the molecular nature of materials. The corresponding microscopic theory, based on the fact that materials are made up of a vast number of particles, is called statistical mechanics.

Historical background

The origins of thermodynamics can be traced to the late eighteenth century. English-American physicist Benjamin Thomson, Count Rumford (1753–1814), became intrigued by the physical changes accompanying the boring of cannons. (Boring is the process of making a hole—in this case the barrel of the cannon—with a twisting movement.) He found that the work (or mechanical energy) involved in the boring process was converted to heat as a result of friction, causing the temperature of the cannon to rise.

Some of the fundamental relationships involved in thermodynamics were later developed by English physicist James Joule (1818–1889), who showed that work can be converted to heat without limit. Other researchers found, however, that the opposite is not true—that is, that there are limiting factors that operate in the conversion of heat to work. The research of French physicist Sadi Carnot (1796–1832), British physicist William Thomson, Lord Kelvin (1824–1907), and German physicist Rudolf Clausius (1822–1888), among others, has led to an understanding of these limitations.

The laws of thermodynamics

The most basic facts about thermodynamics can be summarized in two general laws. The first law of thermodynamics is actually nothing other than the law of conservation of energy: energy can neither be created nor destroyed. It can be converted from one form to another, but the total amount of energy in a system always remains constant.

For example, consider the simple example of heating a beaker of water with a gas flame. One can measure the amount of heat energy given off by the flame. One also can measure the increase in the heat energy of the water in the beaker, the beaker itself, and any air surrounding the beaker. Under ideal circumstances, the total amount of energy produced by the flame is equal to the total amount of energy gained by the water, the beaker, and the air.

The first law of thermodynamics is sometimes stated in a somewhat different form because of the kinds of systems to which it is applied. Another statement is that the internal energy of a system is equal to the amount of work done on the system plus any heat added to the system. In this definition, the term work is used to describe all forms of energy other than heat.

The first law can be thought of as a quantitative law (involving measurement of some quantity or amount): the amount of energy lost by one system is equal to the amount of energy gained by a second system. The second law, in contrast, can be thought of as a qualitative law (involving quality or kind): the second law says that all natural processes occur in such a way as to result in an increase in entropy.

To understand this law, it is first necessary to explain the concept of entropy. Entropy means disorder. Consider the dissolving of a sugar cube in water. The sugar cube itself represents a highly ordered state in which every sugar particle is arranged in an exact position within the sugar crystal. The entropy of a sugar cube is low because there is little disorder.

Words to Know

Energy: The capacity for doing work.

Entropy: The amount of disorder in a system.

First law of thermodynamics: The internal energy of a system is increased by the amount of work done on the system and the heat flow to the system (Conservation of Energy).

Heat: A form of energy produced by the motion of molecules that make up a substance.

Second law of thermodynamics: All natural processes proceed in a direction that leads to an increase in entropy.

Submicroscopic level of phenomena: Phenomena that cannot be observed directly by any of the five human senses, aided or unaided.

Work: Transfer of energy by a force acting to move matter.

But consider what happens when the sugar cube is dissolved in water. The cube breaks apart, and sugar molecules are dispersed completely throughout the water. There is no longer any order among the sugar molecules at all. The entropy of the system has increased because the sugar molecules have become completely disorganized.

The second law of thermodynamics simply says that any time some change takes place in nature, there will be more entropy—more disorganization—than there was to begin with. As a practical example, consider the process by which electricity is generated in most instances in the United States today. Coal or oil is burned in a large furnace, heating water and changing it to steam. The steam then is used to run turbines and generators that manufacture electricity. The first law of thermodynamics says that all of the energy stored in coal and oil must ultimately be converted to some other form: electricity or heat, for example. But the second law says that some of the energy from coal and oil will end up as "waste" heat, heat that performs no useful function. It is energy that simply escapes into the surrounding environment and is distributed throughout the universe.

The second law is sometimes described as the "death of the universe" law because it means that over very long periods of time, all forms of energy will be evenly distributed throughout the universe. The waste energy produced by countless numbers of natural processes will add up over the millennia until that is the only form in which energy will remain in our universe.

[ See also Gases, properties of ; Heat ; Temperature ]

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