The Concept of Entropy

FROM THE LECTURE SERIES: The Joy of Science

By Robert Hazen, Ph.D., George Mason University

It is found that no system can operate with 100 percent efficiency. In this context, the second law of thermodynamics holds tremendous power and sway over people’s lives. At the core of the concept of entropy are such human experiences as aging, death, and decay. The second law defines the very direction of time.

Image showing the second law of thermodynamics.
According to the second law of thermodynamics, heat flows spontaneously from hot to cold. (Image: Fouad A. Saad/Shutterstock)

Energy as a Unifying Theme in Science

In simple terms, the entropy of an object is a measurement of the amount of energy not available for the work. The idea of entropy follows logically from other statements of the second law of thermodynamics—the statements, for example, that heat spreads out, or that no system can operate with 100 percent efficiency. Every natural system uses energy. Energy is one of the handful of unifying themes in all science.

Indeed, every natural system has to obey the two laws of thermodynamics. The first law says that energy can change form from one type to another as many times as required, and the total amount of energy is constant. However, the second law places restrictions on how energy can transfer. Heat must flow from hot to cold, spreading out and evening out. There is a built-in direction to the way energy is used.

Learn more about the nature of science.

Behavior of Heat Energy

One statement of the second law of thermodynamics is that heat flows spontaneously from hot to cold. Another equivalent statement is that one cannot build an engine of 100 percent efficiency that converts heat to work.

There are far-reaching consequences of the second law that go beyond just the way heat energy behaves. In its most general form, the second law of thermodynamics comments on the state of order of the entire universe.

Learn more about universal gravitation.

The Tendency of Systems to Become Disordered with Time

As scientists of the 19th century thought about the implications of energy, they eventually came to a startling realization—the great principle that every isolated system becomes more disordered with time. The concept of entropy displays the tendency of systems to become disordered quite spontaneously, in spite of everything done to maintain control. A new car, for example, looks spotless and shiny when it is purchased. A year later, one looks carefully, and the car has dents and dings and is going to be dirty. Similarly, people’s bodies also experience aging—the decay.

Image showing the tendency of systems to become disordered.
Systems in the universe have the tendency to become disordered with time. (Image: Fouad A. Saad/Shutterstock)

All systems in the universe have this tendency to become more disordered with time. For example, a person can take a jar and put two different types of colored candies in them. He or she can layer those, but if the person shakes the jar around just a little bit, the candies are no longer ordered. Now, the person could shake this jar as many times as he or she wanted to, and the candies would keep coming up with new ordered arrangements, but they would never be as perfectly ordered as they were when the person started this.

This is a transcript from the video series The Joy of Science. Watch it now on The Great Courses Plus.

Energy to Reorder a System That Has Become Disordered

If a person wanted, he or she could reverse the process that rendered the mess in the system, such as the disordered state of the candy jar. He or she could fix the candies by putting them back in layered order. And in theory, if one knew more about human aging, one could reverse that process too.

There is no intrinsic reason why humans could not extend their lifespan if they knew how to fix the gradual damage—the gradual aging process. All it takes in theory in all of these cases is the use of energy. One can take energy, and can reorder a system that has become disordered.

Learn more about magnetism and static electricity.

Rudolf Julius Emanuel Clausius

The concept of entropy, the disorder of a system, was introduced in 1865 by a German physicist, Rudolf Julius Emanuel Clausius. Clausius was born in Pomerania, which is now a part of Poland, in 1822. He studied at several German universities and graduated with distinction from the University at Halle. He accepted a professorship at the Imperial School of Artillery and Engineering in Berlin, and subsequently, taught in universities in Zurich and in Würzburg and Bonn, where he died in 1888.

The Concept of Entropy by Rudolf Julius Emanuel Clausius

Clausius devised the concept of entropy in order to quantify this natural tendency of systems to become more disordered with time.

He synthesized the ideas of Nicolas Léonard Sadi Carnot, James Prescott Joule, and others, and published the first clear statement of the two laws of thermodynamics in 1850. It was the first time in a publication there was a statement about these two laws.

It gradually came to being understood over a period of many decades in the 19th century.

Common Questions about the Concept of Entropy

Q: What is the second law of thermodynamics?

According to the second law of thermodynamics, heat flows spontaneously from hot to cold, and one cannot build an engine of 100 percent efficiency that converts heat to work.

Q: What did the scientists of the 19th century think about the implications of energy?

As scientists of the 19th century thought about the implications of energy, they eventually came to a startling realization—the great principle that every isolated system becomes more disordered with time.

Q: Who introduced the concept of entropy as the disorder of a system?

The concept of entropy, the disorder of a system, was introduced in 1865 by a German physicist named Rudolf Julius Emmanuel Clausius.

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