# Entropy - “Wiggle”?

The title is not a reference to a Jason Derulo song.

In any case:

1) How is change in entropy measured, experimentally? I've Googled this for a bit and I've found all sorts of mathematical equations such as delta S = q/T as a way to get a grasp on entropy. However, I want to know how entropy is actually determined in the lab (as opposed to on paper). I understand that the units on entropy are in terms of energy/temperature. So if I had to guess I would suppose that some sort of calorimetry is used.

2) What is the best way to teach entropy (at an introductory level)? I know that textbooks like to say it's a "measure of disorder." On the other hand I have lots of websites telling me that entropy is best NOT described at a measure of disorder. So what is it really, and what do you think about the term "disorder" as a synonym? My professor refers to it as the degrees of freedom in a system; he also notes that longer carbon chains tend to have higher entropy values due to a greater ability to "wiggle" along their carbon chains. What do you think of this?

Discarding the archaic idea of "disorder" in regard to entropy is essential. It just doesn't make scientific sense in the 21st century and its apparent convenience often is flat-out misleading. As of November 2005, fifteen first-year college texts have deleted “entropy is disorder” although a few still retain references to energy “becoming disorderly”. (This latter description is meaningless

-
I really like Boltzmann's statistical view of entropy $$S=\mathcal{k}_\mathrm{B}\cdot\ln W$$ in which $W$ is the number of microstates, that contribute to a macrostate defined through $S$. In this way you can relate it to the degree of freedom of a system, as well as the overall uncertainty of finding a particle in a particular state. It is derived for the ideal gas, but I think the philosophical touch is quite general. But I cannot answer your question. (Esp. not 1) –  Martin - マーチン Jul 27 '14 at 7:14
I found classical thermodynamics utterly and completely incomprehensible back when learned it. Luckily, a year later I took course in statistical physics, that provided me with really working insight into meaning of thermodynamical functions, entropy included. It was much more intuitive for me, so if you have troubles with classical thermodynamics, statistical physics may be a way to go. –  permeakra Jul 27 '14 at 17:08

You can't measure entropy directly, any more than you can measure interatomic distances. You measure other quantities -- for instance often you can measure energy gain/loss and temperature, and then you integrate $dS=dE/T$.

How to explain it? One of the best expositions I know is The Second Law by Henry A. Bent. It is full of insightful examples, lays the ground carefully and avoids woolly talk, unlike many other thermodynamics books.

Edit: Greg reminds us that energy and enthalpy are not directly measurable either. That's why I was careful to write energy gain/loss, which are somewhat more accessible to measurement; but of course even when doing a calorimetric experiment you're not measuring heat per se but other quantities: how much gas you burned or how intense the current was and how long it was left on. And even some of these quantities again are indirectly measured.

I agree 100% that "entropy is not more abstract than energy" -- they are all abstractions. Ultimately, however, I do think the measurement of thermodynamic entropy is one step more indirect than that of energy, if only because it involves the notion of temperature, which (if you think about it carefully) is subtle indeed.

-
Just for the record: you measure heat and such, and not energy or enthalpy or whatnot. Yes, if you do the experimental design well than heat will be equal to energy, or at least a good estimation of it. Entropy is not more abstract than energy or free enthalpy, and its measurement is not indirect than those. –  Greg Jul 27 '14 at 13:33
Wait are you sure you can't measure absolute entropy? Don't perfect crystals have 0 entropy? So can't we actually measure absolute entropy? –  Dissenter Jul 30 '14 at 20:46
Perfect crystals have 0 entropy only at 0 K, and this value is not measured (you can't get to absolute zero). –  Silvio Levy Jul 30 '14 at 21:15