Elad’s blog

To my surprise, I found the best introduction to thermodynamics in a biology book.

I studied thermodynamics, a field of physics that started from a macroscopical explanation of heat and temperature and found a proper explanation in the quantum microscopic field, many years ago. However, I must admit that it was never entirely clear to me (and that it still ain’t, but that’s a different problem); anyhow, I didn’t need it too much in life - until recently.

Thermo is pretty popular, it seems, in explaining various phenomena - things like “why molecules move from this place to the other” tend to be explained by physicists in wierd words like “that would minimize the free energy of the system” or “the enthalpy of that state is this and that” and many other buzzwords. I spend a bit of time lately (when I’m not busy doing stuff like coating glass with sulfur, which also keeps me a bit busy these days) trying to unreveal the meaning of these terms.

And so I finally realized what all the thermo people mean by “intensive” properties and “extensive” properties of a system. Like I mentioned at the top, it took a biology book (something by a guy named Stanley Schultz, published many years ago) to explain it in such a simple words that even I could grab it. I will try to repeat the explanation, for the benefit of the novice physicist. Do remember that this is in the pre-introductory material, and I don’t aim at writing a lecture on thermodynaimcs here - just explain two basic, mostly overlooked,  terms.

The idea is, that if you take a system (which in thermodynamics usually means, ‘any part of the universe that interests you at the moment’) and divide it into two parts, let’s call them sub-systems, some of the properties of the entire system will be the sum of those properties of the sub-systems, and some - not. For example, the volume of the system is the sum of the volume of all its subsystems; but the pressure of the system is not the sum of the pressures - usually, it is the same of the pressure of each sub-system; or, another example, the total amount of charge (say, the number of electrons) in the system is the sum of the charges in all its sub-systems; but the electric potential is not additive.

Now, every property that is not additive is called an intensive property of the system; most common intensive properties are pressure, temperature, electric potential, and chemical potential. Conversely, a property that is additive is called an extensive property - obvious examples are volume, charge, amount of matter (”moles”), and entropy (which is a somewhat elusive monster for an uneducated physicit; it is a term that means “the number of possible states to a system”).

Here come two nice, pretty deep,  facts: the ratio of any two extensive properties is always an intensive property (that’s easy; for example, the ratio of amount of matter to volume gives the density, which is obviously an intensive property); and - more interesting - any intensive property can be expressed as a function of all other intensive properties of the system (without any extensive property around).

I will only go here one step further, before I lose all my readers. I just want to mention the idea of conjugate properties. The idea is, that to each intensive property we can pair one extensive property; a difference in the intensive property is a driving force of a change in the extensive property: a difference in pressure causes a displacement of volume; a difference in the electric potential causes the movement of charges; a difference in temperature drives enthropy changes; a difference in a chemical potential causes movement of matter, to name a few.

I read several thermodynamics books aimed at physicists, and must admit that non of them gave a good explanation of these particular terms. It is wierd that you sometimes have to look at books aimed for different disciplines in order to get easy explanations for professional terms; and I will not answer here what thermo introduction is doing in a biology book, we’ll address this in a different occasion.

This entry was posted on Wednesday, November 14th, 2007 at 16:05 and is filed under Life. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.