GAIA -- THE EARTH SYSTEM
AN INTRODUCTION TO THERMODYNAMICS -- ENERGY AND CHANGE
Science is concerned with either the description of the natural world or the effort to explain the processes that occur in nature. The basic observation and description that went into organizing species of plants and animals or different kinds of rocks, or the positions and movements of planets and stars have given modern scientists a tremendous data base with which to work. And this very basic task continues. As a geologist, one of my research methods is to map the location and distribution of rock types within an area. I find the work enjoyable because it gives me an opportunity to spend time in the Adirondack Mountains and in Montana walking through areas that have been rarely traveled by others. It is important work because any explanation I make for the geologic history of these areas must be consistent with the geologic map. However, it is the other aspect of my work, trying to unravel the history of the rocks, the events that made them what they are today, that I find most fascinating. By using the map I draw and by studying the rocks I find, I am able to propose a model describing geologic events that occurred over 1 billion years ago and 20 to 30 km beneath the surface of the Earth. And, understanding these events in the distant past can help us understand the geologic processes that are reshaping our planet today.
A slightly different way of putting the scientist's interest in process is to say that we study change, and to think about change scientifically one needs a basic understanding of energy and the laws of thermodynamics. This is because change is work and work IS energy. In its simplest form work is moving something. You lift a book so that you can begin to study for an exam. The act of lifting the book is work and requires a transfer of chemical energy stored in your muscles to mechanical energy as your muscles contract so that you can bend your arms and lift them. But there are many other kinds of work as well. Water evaporating from the oceans is a form of work. Just the heating of the oceans that leads to evaporation can be considered work. A tree or a person growing is a kind of work. In fact our bodies are working even when we sleep.
Nowadays, much of the work we "do" is actually done by machines. We don't walk to the store, we drive our car and let the car do the work by moving itself and us. It wasn't always this way. Before the 1800's almost all work involved muscle -- either human or animal, but the invention of the steam engine and the resulting industrial revolution have gradually replaced more and more of human labor with mechanical devices. The economics of the situation were quite simple -- machines are cheaper than people. So a company would rather buy a machine to do the work than pay people to do the labor. If you think machines are more expensive, consider what it would cost to pay 4 people to carry you where you want to go instead of having a car. Even at minimum wage that would be $40,000 a year not counting payroll taxes and benefits. And, that wouldn't pay for weekend travel. So a car, even at today's prices, is a bargain.
Still machines aren't free even after the last payment has been made. It takes energy to make them work and energy costs money. So a second economic interest of the industrial revolution was to make the machines more energy efficient. This drive to understand energy and how it could be made to do more work became the branch of physics known as thermodynamics, literally heat-motion. The work of these physicists has produced a wealth of information concerning the interaction of matter and energy; we will be concerned with only a few.
The most important finding, known as the First Law of Thermodynamics, is that the amount of energy in the universe is constant. The First Law is commonly stated as "Energy can neither be created nor destroyed; it can only change form." This law can lead to some confusion because we talk about energy being used when we do work. What we really mean is that the form of energy is being changed as the work is done. For example when your car burns gasoline to release the chemical energy stored in the gas, heat, another form of energy, is released. The mechanical system of the car converts the heat to kinetic energy, the energy of the moving car. In theory one can measure the chemical energy of the gasoline and show that the amount of heat released is identical. One can also show that the sum of kinetic energy and heat lost by the car are equal to the original chemical energy of the gasoline.
The First Law is important because it allows us to understand the transformations of energy and how energy is used to accomplish work. The First Law reduces the problem of tracing the flow of energy to one of accounting. If some energy seems to disappear, for example when your arm becomes cold when washed with alcohol before getting a vaccination, the "loss" of energy or heat needs to be accounted for if you want to understand what has happened. Because energy cannot just disappear, the heat has to go somewhere. Your arm becomes cold because the heat is being stored in the alcohol molecules as they evaporate. If that's the case, what has to happen if the alcohol were to condense and become a liquid again?
The First Law is important, but two other relationships are even more important to understanding what controls change in the natural world. Think back to the example of the car. Perhaps you asked, if no energy disappears then what happens to the energy when the car stops? Why do we have to refill the gas tank; shouldn't one shot of energy be enough? The answer is that some of the energy the car had is lost to us as useful energy. During the 19th century the discovery of the First Law led to a fascination with perpetual motion machines. Inventors spent a great deal of time, labor and money trying to create a machine that would, once started, run forever without any additional input of energy. Machine after machine failed because no matter how beautifully designed or how carefully engineered, the machines all "lost" energy because even the best machines are subject to friction. Rub your hands together and you can easily demonstrate that friction makes heats. The First Law demands that the heat (energy) come from somewhere. In this case, it comes from the transformation of kinetic energy -- your moving hands. The same problem befalls all attempts at perpetual motion. Every moving object slows down, losing kinetic energy to frictional heating. Even the Earth is slowing down. The day today is nearly 3 times longer than it was three billion years ago.
Eventually it was realized that the loss of usable energy was as important to understanding machines and change as the fact that energy is neither created nor destroyed. This discovery became known as the Second Law of Thermodynamics which is stated in many ways. My favorite is "There is no such thing as a free lunch," meaning that anything we do uses energy. The textbook statement of the Law is, "Entropy is constantly increasing." Entropy is one of those terms and concepts that you will have to understand if you are to succeed in this course. It means disorder. So the Second Law also states that the Universe is becoming more disordered as time passes. Sometimes it is said that the universe is running down. Galaxies will stop spinning, stars will stop shining, and all life will disappear. Its a gloomy fate, but the good news is that it wont happen for a long, long, long time.
To a physicist, entropy or disorder means something different from a mess. Disorder is, of course, the opposite of order when everything is neatly arranged. So, a mess is an outcome of entropy increasing. But, DISORDER is also randomness in nature. So a cup of hot water is an ordering of nature because an amount of heat energy has been collected in the water and is not evenly distributed through the universe. Similarly the highly ordered molecular structure of water in ice is a low entropy state while the highly disordered structure of gaseous water (vapor) is a high entropy state, and so favored by the second law.
Water and its phase changes brings us to third discovery of thermodynamics that is crucial to this course. All change occurs within a system by either increasing entropy or by decreasing the potential energy in the system. System and potential energy are two more terms important to this course. By system I mean some set of things that I'm studying. For example, if I were interested in the movement of blood in our body I might study the circulatory system. If I wanted to study the interrelationships among living things in a pond, I would study the aquatic ecosystem. Systems can be as big as the universe and smaller than a drop of water.
Potential energy is stored energy. That is energy that can be released and possibly used to perform work. The most common form of potential energy is the energy stored in the position of an object. A brick balanced on a window ledge has the potential to fall. As it falls, it gains kinetic energy because the faster an object moves the more kinetic energy it has. In accordance with the First Law, that energy has to come from somewhere -- in this case from the work done to put the brick on the window ledge in the first place. Other common forms of potential energy are chemical energy such as the energy stored in gasoline or coal and nuclear energy, the energy stored in atoms that can be released in a nuclear reactor or in an atomic bomb. Potential energy is transformed and released through action that produces light, electricity, heat, or motion.
Back to the all important issue of thermodynamic tendencies that control change. The brick will fall because it loses potential energy. It really doesn't matter in terms of what will happen if it is falling off a pile of bricks or onto a pile. In the first case entropy would increase because the situation is becoming less ordered -- bricks are spreading out. In the second example entropy would decrease. It seems that the Second Law is not important to this example.
Water can be used as another example of the two tendencies. Solids are more ordered than liquids, and liquids more than gases. So water as a solid has less entropy than it has as liquid which has less entropy than it has as a gas or vapor. Therefore, if only entropy were important, all water would be a gas, and life as we know it would not exist. Similarly, when water vapor condenses it releases heat; so (according to the FIRST LAW) it must have potential energy as a gas that is released when it becomes a liquid. When water freezes, again heat is released to its surroundings, and so the liquid must have potential energy that is released as ice forms. If only potential energy were important in controlling change, then all water would be ice and life as we know it would not exist. But there are two tendencies controlling change and since they are competing with each other, which will win. You already know the answer -- if it's cold ice will form and if it's hot enough, water will become vapor. It would seem that in some way temperature is important. Higher temperature favors increasing entropy. Based on this and other evidence, it is possible to show that the tendency to lower potential energy is independent of temperature. No matter how hot or how cold it is the potential energy released or stored during a change is constant. In contrast, the importance or size of entropy change in a system increases as temperature rises. As a result at low temperature, potential energy changes are likely to control the changes that occur. At high temperature, entropy is likely to be the controlling factor.
One could consider four theoretically possible outcomes to an imagined change. Entropy and potential energy can both increase or both decrease or one can increase as the other decreases. For example if potential energy increases and entropy decreases will the change happen? No. Neither tendency favors the change. If PE and entropy increase, will the change occur? Here it depends on the temperature and the size of the PE and entropy changes. In other words, it may happen if entropy is the controlling tendency or it may not if PE is the controlling tendency. Think about the other possibilities and what affects the outcome of a potential change.
As you can see some changes are likely to occur and others can't. Think about burning gasoline in your car one more time. Potential energy is released (decreases) and entropy increases because a few large liquid molecules are turned into many small vapor molecules. So it is easy to burn gasoline. Can you turn CO2 and H2O the products of combustion back into gasoline? Certainly not by simply increasing or decreasing the temperature. Unlike the phase changes of water which are easily reversed by changing temperature, burning is considered an irreversible change. It is possible to make new gasoline, but only by a complex set of reactions in systems that release large amounts of potential energy and increase the total entropy of the universe -- those systems are living organisms or the industrial activity of humans.
Reading question: In terms of potential energy and entropy change, what is the difference between a reversible and an irreversible change?