Phenomenological theories: thermodynamics

In summary, Barut was talking about how different physical theories are based on different kinds of knowledge (atomistic vs. phenomenological), and how thermodynamics falls into the phenomenological category.
  • #1
"Phenomenological" theories: thermodynamics

Hello, I was reading A. O. Barut's Electrodynamics and Classical Theory of Fields and Particles and was interested by a phrase he used, "'atomistic' physical theories - in contrast to phenomenological theories like thermodynamics - ...". What could be meant here by "phenomenological theories like thermodynamics"? I have a vague sense of what Barut meant, but any more insight would be very helpful.
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  • #2
I'm not entirely sure what he would mean by that. I guess thermodynamics is a theory that does not really discuss the microscopics, and it requires measurements to make further predictions (i.e. you cannot calculate U(N,V,T) purely from thermodynamic considerations except in very special cases), but at the same time thermodynamics is quite robust. It holds regardless of the system in question, quantum or classical, so in that case it's less "atomistic" than the other branches of physics.

Although I have no idea why he would divide physics up like that.
  • #3
Statistical mechanics takes knowledge of the atoms of a system, and deduces from that some behaviour of the system as a whole.

But plain thermodynamics was phenomenological in the sense of describing just empirical results, without any other basis. In particular, the 2nd law of thermodynamics matches phenomena we measure, but almost contradicts our understanding of the underlying mechanisms by which atoms and so forth interact.

The phenomological approach is to measure some complex thing, plot the data, and try to guess an equation that will predict future measurements accurately. Fitting the equation involves choosing the best value for each parameter, with no prior reason or explanation to favour a particular value.

The atomistic approach is to study each element of the system in isolation, to learn how they work and interact. You can measure properties of each element, and then (by deduction) you write down the equation for the whole system. Hopefully you get the same equation as empirically, except now you also know exactly why each parameter has the value it does.
  • #4
That's a nice explanation, cesiumfrog. :approve:


1. What is the main focus of phenomenological theories in thermodynamics?

Phenomenological theories in thermodynamics aim to describe and understand the behavior and properties of macroscopic systems, such as gases and liquids, based on fundamental laws of thermodynamics. They focus on the observable phenomena and do not consider the underlying microscopic mechanisms.

2. How are phenomenological theories different from statistical mechanics?

Phenomenological theories and statistical mechanics both aim to describe macroscopic systems, but they approach this task in different ways. While phenomenological theories use empirical observations and general laws to explain macroscopic behavior, statistical mechanics uses the microscopic properties of individual particles to predict macroscopic behavior.

3. What are the limitations of phenomenological theories in thermodynamics?

Phenomenological theories have limitations, as they cannot explain the microscopic mechanisms behind macroscopic behavior. They also cannot predict behavior outside the range of observations used to develop the theory. Additionally, they may not accurately describe systems with complex interactions or non-equilibrium conditions.

4. Can phenomenological theories be applied to all types of thermodynamic systems?

Yes, phenomenological theories can be applied to all types of thermodynamic systems, including gases, liquids, solids, and mixtures. However, the accuracy of the predictions may vary depending on the complexity of the system and the range of observations used to develop the theory.

5. How are phenomenological theories used in practical applications?

Phenomenological theories have practical applications in various fields, such as engineering, chemistry, and materials science. They are used to model and predict the behavior of thermodynamic systems, which is crucial for designing and optimizing processes and products. They also provide a framework for understanding and interpreting experimental data.

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