I was rummaging through old files looking for something, and came across this rant I wrote back in 1986 when I was teaching thermodynamics at KU. I decided not to give it to the students for obvious reasons. I am posting this with minimal cleanup.
Thermodynamics is a subject that starts with very simple physical observations of the real world. The whole field is based on two laws (actually four: Two biggies and a couple of backups) and there isn’t a hell of a lot of theoretical background to any of them. The laws just are. If that’s not good enough for you, the Nobel prize is waiting.
The zero’th law says that if A is at the same temperature as B, and B is at the same temperature as C, then A is at the same temperature as C. Pretty profound stuff huh! And this is college? Well, somebody had to start somewhere, and that’s why they call it the zero’th law anyway.
Actually, when you really start looking at it, it gets pretty philosophical. Nobody’s actually proved the zero’th law mathematically, so it has to be a postulate. But it is necessary implicitly for the other laws to have any meaning. Luckily, it is borne out by observation, and nobody’s contradicted it yet. So mankind builds its first thermodynamic foundation on pretty sturdy ground. The stuff after that? Just you wait.
Next is the first law of thermodynamics which is simply the conservation of energy. What goes in must come out. But what makes it a bit more complicated is that nobody really knows for sure what energy is. Oh sure, we can all talk about it: OPEC screws us for it; it flies out the attic if we aren’t in the pink; we run out of it if we weren’t good little boys (sorry, persons) and finished all our porridge for breakfast like mummy told us to, etc. But we can’t hold it in our hands (1) and it’s invisible. But in some way we all know that it exists (I think). It cannot be created or destroyed.(2) The net result of all this crap is that we usually get a pretty circular definition for the first law. The first law defines energy, but the first law is itself defined by energy. Or something like that.
Everybody sneaks around this little problem by arbitrarily defining two other things first: Heat and Work (and let’s not get into that headache just yet). This takes up to Chapter 4 in Van Wylen and Sonntag. Then in Chapter 5 we get this beaut: “The first law of thermodynamics states that during any cycle a system undergoes, the cyclic integral of the heat is proportional to the cyclic integral of the work”(3). Are you kidding me? A much more sensible definition is as follows, but notice that we still really don’t say what the heck energy is:
energy input – energy output = change in stored energy
Anyway, throwing up our hands, we define energy well enough so we can use the concept and go on, keeping quiet along the way so we still get paid by those bean counters.
The second law is a deceptively easy concept but really tough in application. Or maybe it’s simple in application, but a damn difficult concept. I can never remember which. Basically, it says you can’t get something for nothing. Energy isn’t free and it’s a one way street. For instance, cups of coffee don’t sit around getting hotter. Energy in the form of heat flows from the hot cup to the cool surroundings, not vice-versa. Air flows into a vacuum, it doesn’t spontaneously turn around and head back out. The accursed friction is real. Even perfect engines cannot be 100% efficient. All quacks who build perpetual motion machines are full of doggy poo. All this from the immortal words of Clausius:
“Die Entropie der Welt strebt einem Maximum zu”.
To get around this little game, everybody defines entropy. And this little guy can be a monster, until you simply stop questioning it and go with the flow. The most universal application of the second law (or perhaps the starting point) is that the entropy of the universe always is tending toward a maximum. Think of entropy as a state of disorder. The greater the entropy, the more the mess. Yes folks, at last scientific proof that dirty dishes are nature’s way and lost single socks have an afterlife.
The third law of thermodynamics harps back to the fact that we don’t really know what energy is. Nor entropy, dorm food, and Vanna White’s phone number. Actually, the third law is something of an anticlimax when you get to it. It states that at absolute zero degrees, the entropy is zero. This is reasonably logical, and it gives us a point of reference. Once the energy or entropy of something is defined somewhere (zero at zero degrees, for instance), its state is fixed/determined at all other conditions. If all this makes sense, then I hate to spoil the party, but to be perfectly honest, the third law is no use to anybody except for a few mad scientists, and a few even madder philosopher-engineers. For one thing, all engineers are ever really interested in are the differences in energy and entropy, so we can define a reference point as zero anywhere we want. Not to mention that one of the teansy weansy problems is that absolute zero is unattainable, by definition of the actual law which defines it. Now that’s a real circular definition.
All this is enough to pick up a cold sixpack, drive out to Lone Star Lake, blow up the extra large inflatable raft, and set yourself adrift. Or perhaps you might say a few words to the big guy who made all this wonderful stuff possible and ask him why on earth he made it so weird. Actually, civilization hasn’t fallen yet so the whole field of thermodynamics is probably doing ok. The above bullshit won’t help you solve the homework problems or do well on the tests. But I hope it will help you take everything a little less seriously when you don’t understand something, and skeptically when you think you do. Above all, never trust the teacher. He just started studying this stuff a few years before you did, and look how screwed up he is.
(1) Actually we can a la the potential energy of the column of air we are supporting, the potential energy of the hands being held off the ground, the chemical metabolic potential of our bodies, etc.
(2) Except for E=mc2, of course, and even then this only states that mass is another form of energy.
(3) To those of you that this is all sounding gibberish, congratulations, you are normal.