The Second Law of Thermodynamics Or Energy is Forever, but Not Exactly

The Second Law of Thermodynamics
Or Energy is Forever, but Not Exactly

		Keeping It Simple (and Clear) Teachers and Learners: The Second Law of Thermodynamics is probably the most misunderstood principle of physics. Because of the confusion and pervasive misinformation regarding this principle, I've dragged my feet shamelessly when it came to dealing with it in this website. Well, I can't undo all the misinformation with one little web site. So I won't try. If you are a beginner start here; and if you are not going to be an engineer or scientist, stay here. It is probably all you will need to understand the basic energy change results the 2nd law predicts. Your head won't get filled with confusing non-thermodynamic and incorrect analogies. We don't need the silly, and wrong, examples of messy desks. Such things are not predicted by the 2nd Law of Thermo. Honest. Other than the next few paragraphs, we won't discuss entropy on this page (but someday its page will come). Thermodynamic entropy is a measurable property of matter, not a vague predictor of universal decay. We don't need to discuss measurements of disorder. We never need disorder. What are those units of disorder anyway? Disorderites? Messyisms? Gobbledygooks? Being a physical property, entropy has units. The units are energy divided by absolute temperature (We are talking about classical thermodynamics here - not getting into statistical thermo). I don't believe there are units of disorder in any field, though it sounds like it could be a good legal term for court room judges ("You are guilty of generating 3.5 units of disorder in my court room"). All of the various 2nd Law definitions listed in text books result from the basic energy change results I describe in these pages. But why start backwards? Start with the basic energy changes the 2nd Law describes. Very simple. Read the following at least 3 times: It is only about energy. It is only about energy changes. It is only about the condition of the energy before and after the change. To be sure, there are interesting concepts about organizational disorder, probability, complexity, and things getting messy (and the propagation of misinformation about thermodynamic entropy). But it is incorrect to create metaphors and anlogies from thermodynamic entropy to explain those concepts, and it only misleads beginners. Relax, you don't need them to explain this concept to students. The Second Law of Thermodynamics absolutely does NOT say everything tends toward disorder (or decay)! It is not a universal law of messiness. It is only about energy changes. Isn't that nice? We can all relax. My messy desk and your wrinkled shirt are not predicted or measured by entropy formulas and the 2nd Law of thermodynamics.
		Picture 1: The pressure in the volume on the left is higher than the pressure on the right. The pressure energy in the left side can be thought of as more "concentrated" than the pressure energy on the right side. Both sides take up the exact same amount of space (or volume), but there is more pressure energy in the left side. More pressure energy in the same space means it is more concentrated.
		Picture 2: The valve has just been opened. Immediately, air on the higher pressure left side starts to flow to the right side, because the pressure is lower there. Just like air escaping from a baloon. Energy is flowing from more concentrated to less concentrated.
		Picture 3: The air kept flowing through the opening until the pressure on both sides was equal. See the pressure guages? They show the same pressure on both sides. The pressures are now equal - no difference in concentration levels. There is no more air flow through the opening. We have reached equilibrium. The total energy hasn't changed (First Law), but it is more "spread out" or less concentrated now (2nd Law).
		Picture 1: Two tanks of water. The water on the left side is hotter than the water in the tank on the right side. There is only a thin piece of sheet metal, or maybe some glass, separating the water, so heat (thermal energy) can easily flow from one side to the other. Thermal Energy is more concentrated in the hotter water. A cubic inch of water on the left side, has more thermal energy in it than a cubic inch of water on the right side.
		Picture 2: The thermal energy continued to flow from the left side into the right side. The temperature on the right steadily increased, while the temperature on the left side got steadily cooler. Eventually the temperatures on both sides became the same, as shown by the cartoon thermometers above. Equilibrium has been reached. The concentration of thermal energy is the same on both sides, so there is no more energy flow.

How Everything Happens
Energy makes everything happen, and every time something happens, there is an energy change. There are two important natural "laws of energy" that describe what happens to the energy involved in every change. We call them "laws" because countless observations and thousands of experiments have shown them to always predict what will happen.

Ponder that for a moment - how everything happens. It means we don't understand much, if we don't understand both the first and second laws of Energy.


These next few pages will give you an overview of the famous, but often misunderstood, 2nd Law.

Beyond the First Law
The First Law of Thermodynamics tells us energy is conserved. The total amount never changes. But something does change. I will call it "re-usability", for now. It's not an official text book word, but pretty good for communicating the basic idea.

Remember that there has to be an energy transfer for something to happen; energy changes form or moves from place to place (heat flow, for example). As energy moves and changes, the total amount of energy stays the same, constant forever as far as we know.

That sounds good doesn't it?
Energy is forever.

But wait! If it's forever, why are all these do-gooders telling us we need to conserve energy by using less? Can't we just keep using it over and over? Why shouldn't everyone drive to work alone in a 300 horsepower car?

The Rest of the Story...
Alas, my friends, there is always a rub, and when it comes to energy, the rub is described by the Second Law of Thermodynamics. The first law would be quite happy to let us re-use energy over and over. The first law is happy as long as energy is conserved. It's the happy law.

The second law may seem a little less happy to some. It describes the aftermath of every energy change that makes something happen. The second law says that each time energy gets transferred or transformed, some of it, and eventually all of it, gets less useful. That's the truth. It gets less useful, until finally, it becomes mostly useless (at least as far as its ability to make things happen is concerned).

All of the energy we use ends up, sooner or later, as what we engineers like to call "low-grade" energy. This low-grade energy is only good for warming the air around us a little bit. We can't use it to do things we consider useful, like generate electricity or make a car go. Inevitably, most of it gets radiated out into the vast cold universe, lost to us forever.

To understand this, it is helpful to start with another aspect of the Second Law. Let's call it "the direction energy moves" aspect. 






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