Which thermodynamics concepts confuse or confused you the most?

[pa5tabear]

I took a thermodynamics course last year and I know which topics confused me (fugacity for example) but I'm curious now in general about things people struggle with.

Please add the time at which it confused you (first time undergraduate class, graduate class, etc), or describe how it was difficult.

 

[Jorriss]

Entropy was the only thing that confused me. Unfortunately, effectively everything we did was just variants on the second law of thermodynamics ie the first law made sense, the second one was much, much less intuitive.

 

[Curl]

Applying thermodynamics to dynamic processes - the confusion is whether to think of the substance as a rarefied gas or as a continuum.

Also non-equilibrium thermodynamics

 

[pa5tabear]

Applying thermodynamics to dynamic processes - the confusion is whether to think of the substance as a rarefied gas or as a continuum.

Also non-equilibrium thermodynamics

Entropy was the only thing that confused me. Unfortunately, effectively everything we did was just variants on the second law of thermodynamics ie the first law made sense, the second one was much, much less intuitive.

What level in your education were/are you at when encountering these difficulties? And which degree programs are you in? Level and background content definitely influence these things.

 

[Jorriss]

What level in your education were/are you at when encountering these difficulties? And which degree programs are you in? Level and background content definitely influence these things.

I first encountered it in upper division thermodynamics, but without a microscopic basis, I was comfortable with not understanding it entirely. After graduate stat mech, when entropy was still fuzzy, is when I realized it was very complicated.

 

[Curl]

I encountered it when thinking about thermodynamics on my own. It is probably impossible to find a course on thermodynamics that poses such questions.

 

[Rap]

Entropy, without a doubt. The more I learn about it, the less I understand it. Anybody who thinks that if they learn the thermodynamic definition, the statistical mechanical description, the information theory description, and can solve any problem involving entropy, that they then understand entropy, is, I think, fooling themselves.

There's something really deep going on with entropy. There is no such thing as "before" and "after" in time in any physical law except the second law of thermodynamics, where entropy is defined. Yet we all understand "before" and "after" - our consciousness, our sense of time "flows forward". This is very unscientific, but I just have much the same weird feeling as when studying relativity, how our perception of the distinction between space and time is subjective, not "real" in some sense. In quantum mechanics, what you know about a system is what you choose to know, in some sense. The more you know about one aspect of a system, the less you know about another aspect. Regarding entropy as an information-theoretic concept, entropy is a measure of what you don't know. Classically, the more you know about a system, the less entropy it has. Only in classical physics can you completely "know" a system, and the concept of entropy evaporates, and there is no before or after. But, then, the real world is not classical.

I can calculate the hell out of most problems and get it right. I understand where many legs of the elephant are, but I still can't see the entropy elephant in the living room.

 

[Naty1]

I once saw a video of Feynman discussing his first thermodynamics class...He said something to the effect he did not understand entropy and he was the brightest guy in the class.

It reminded me of the thermodynamics class I had...I was baffled by entropy and I'm pretty sure so was the person teaching.

 

[anorlunda]

In college as an EE, I used to joke with ME friends: "I'll explain imaginary power to you, if you can make me understand entropy."

 

[0xDEADBEEF]

Why do Legendre transforms work, and why do thermodynamic potentials make sense? How does the mathematical structure of derivatives, partial derivatives, total derivatives and differential forms work?