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Name: Linda G.
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Question:
I need a simple definition of entropy and the three laws of thermodynamics. I do teach middle school but the simpler the better.



Replies:
Thermodynamics is not an easy subject at any level, but possibly easier to teach at the middle school level -- surprise! The reasons are historical and pedagogical. Historical because thermodynamics is an abstract subject that predates the general belief by scientists of the existence of atoms and molecules. Pedagogical, because most of us have been sold a "bill of goods" regarding the laws of thermodynamics.

Here is what I mean. The first law concerns "energy". What we were fed, at least in my experience, was a sort of shell game. Typically, "energy" is defined for us unsuspecting students as "the ability to do work". OK but what is work? Work is an applied force acting against an object in the direction of the objects motion. And immediately before the student has a chance to absorb this as being so obvious, which it is not, we are off talking about blocks sliding down planes, pulleys, and all the other garbage (sorry for the tough term) that is flung out, and of course there is heat, with is another "form" of energy, that can do work too. In fact, "energy" is an abstract concept, idealized from various observations. It is as abstract as lines and points, which in geometry are also presented as so "obvious" how dare the student challenge the definitions. It is only much later, if ever, that a student finds out that there are geometries, that are equally valid that are based on very different assumptions and definitions. Now back to thermodynamics: First Law: There is this abstract concept "energy" which be experience gives a number that if we do something to get from A to B and back to A ends up having a value of zero -- more or less. It is as abstract as electric fields which were 'invented' to explain electricity and magnetism, even though Maxwell 'invented' the field without having ever actually observed one. He only said if you let me 'invent' this abstract quantity, the field, I can with four equations explain all of classical electromagnetism! The First Law says, if I divide up a wide variety of processes into abstract quantities called work and heat and if I carry out this wide variety of processes starting at A, going to B, and returning to A and compute these abstract quantities work and heat add up to zero. That is not an easy concept, and certainly not self-evident. Second Law: Most simply stated -- Some things happen. Some things do not. What do the things that happen have in common? What do the things that do not happen, have in common? The answer is this abstract quantity we call entropy. Forget the mathematical scaffolding we with which the abstract quantity is shrouded. Stripped to the core, the Second Law proposes an abstract quantity, with formulas on how to calculate it, called entropy that says that things that happen show a positive increase in this quantity we call entropy, and the things that do not happen (under some specified constraints) show a decrease in this quantity we call entropy. But it is an abstraction, not something that is at all self-evident.

For historical reasons, scientists were not really sure atoms and molecules existed, the history of the Second Law is distorted and convoluted to make it valid even if atoms and molecules did not exist. But today there is no valid reason to make what is reasonable, very "scriptural" and "pure" void of any discussion of the existence of atoms and molecules. The Third Law cannot even be discussed properly without invoking atoms and molecules, because it states, again an abstraction, that there is a lowest possible energy (again an abstraction) for collections of atoms and molecules that are perfectly ordered. Its implications are sweeping, but the concept in terms of atoms and molecules, is simple.

I hope this helps, but it is not an easy task you face. Thermodynamics has been mangled so badly for the last century and a half that it requires very careful analysis to avoid being caught in a circular presentation.

Vince Calder



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