Is Enthalpy a Valid Concept Despite Dimensional Mismatch?

[PeterDonis]

I think you will find that the temperature rise from the compression of air on airplane wings and fuselages is not a chemical process and so is not exothermic.

You're quibbling over words. There is heat exchange between the air and the airplane's fuselage and wings. That is precisely the kind of case for which using the enthalpy of the air in the analysis is useful.

 

[Sailor Al]

You're quibbling over words.

I do not think that distinguishing between a physical change and a chemical change is quibbling. In fact, I think it is an important distinction to make, as it can help us to understand the nature of matter and how it can be transformed.

 

[PeterDonis]

I do not think that distinguishing between a physical change and a chemical change is quibbling.

Yes, it is. The atoms don't know that they're supposed to be doing "chemistry" instead of "physics" and adjust their heat exchange behavior accordingly. The heat exchange between the air and the airplane's fuselage and wings is the same whether you label it as "chemistry" or "physics".

I think it is an important distinction to make, as it can help us to understand the nature of matter and how it can be transformed.

Which, even if it is true (and you would need to give some very good references to back up your claim here--but that would be a separate discussion), has nothing to do with what we're discussing in this thread. You asked about enthalpy, and you have been given multiple examples now of when and how it is useful. That's what this thread is about.

 

[russ_watters]

The problem was, as I indicated then in response to his explanation:

and to niels nielsen on the same thread:

As there were no responses to my inquiries on Stack Exchange, I am hoping that Physics Forums may be able to provide an answer.

But the pressure changes are big enough to provide the lift, so while they may be small compared to the atmospheric pressure, they are still very significant and thus will generate temperature changes (PV = nRT). It's not the use of thermodynamics that I am questioning. It's the use of enthalpy that I'm questioning.

The question is a repeat and the answer hasn't changed so there was no need to repeat it: you can use enthalpy or not - it's up to you.

What is the point of all this though? Why does this question matter so much to you? What's wrong?

 

[Sailor Al]

Yes, it is. The atoms don't know that they're supposed to be doing "chemistry" instead of "physics" and adjust their heat exchange behavior accordingly. The heat exchange between the air and the airplane's fuselage and wings is the same whether you label it as "chemistry" or "physics".

Yes, indeed, the atoms don't care if it chemistry of physics. The only way to influence the atoms is by plasma physics or nuclear physics. But the molecules do care, and that's the vital difference between chemistry and classical physics. Chemical processes alter the composition of the molecules in the system. Physical processes, such as those occurring in aerodynamics (sub-sonic, atmospheric conditions), don't alter the molecular composition of the air. The molecules of the air (O₂,N₂, etc.) have the same composition before and after any aerodynamic process. When discussing enthalpy, it is important to distinguish between chemical and physical processes. This is not a matter of quibbling, as the nature of the process can have a significant impact on the enthalpy change.

 

[PeterDonis]

Thread closed for moderation.