# Questions tagged [thermodynamics]

Covers the study of (primarily homogeneous) macroscopic systems from a heat/energy/entropy point of view. Consider also using the tag: [statistical-mechanics].

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### Modified Joule-Thomson expansion

I was trying to solve a thermodynamics problem about the Joule-Thomson (Joule-Kelvin) expansion and I can't reach the answer. The book has the solution, but there's a term which I don't get where it ...
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### Why different references express mass densities as $\mathrm{MeV}/\mathrm{fm}^3$ or $1/\mathrm{fm}^3$?

I was reading some papers about equations of state and have found that some of them express the mass density in terms of $\mathrm{MeV}/\mathrm{fm}^3$ and others in terms of $1/\mathrm{fm}^3$. Why is ...
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### Is there an analogous Unruh effect for observers on a rigidly rotating ring?

I read up on the Unruh effect recently and what I got from it is that its basically a result of transforming to Rindler coordinates and using a Bogoliubov transformation to change the creation and ...
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### Can we derive friction from collisions with environment?

I would like to know if friction, that is, the term $-\alpha \dot{x}$ in the equation of a conservative system $\ddot{x} = -\nabla U(x) -\alpha \dot{x}$ can be derived in some way, by assuming elastic ...
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### Gibbs phase rule argument about battery voltage dependency on the state of charge

I'm studying battery physics, particularly the dynamics of full discharge, and I am having trouble understanding an argument that is based on the Gibbs phase rule $$F=C-P+2.$$ I understand the phase ...
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1 vote
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### Meixner and the Impossibility of Non-equilibirum Entropy [closed]

Meixner in a series of publications, see a short list below, starting from 1959 criticized and ultimately denied the possibility of defining the entropy concept in an irreversible process. His ...
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### Entropy in irreversible adiabtic process

We know that, $$dS=\dfrac{\delta Q_{rev}}{T}$$ If you have an irreversible adiabatic process between two thermodynamic equilibrium end states of a system, there exists no possible reversible adiabatic ...
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### What causes the latent heat of solidification?

Every explanation that I read says: "When a crystal (solid state) forms from a liquid, heat is being produced." This seems rather weird, why would the production of something as ordered as ...
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### Is it possible to arrange two or more lenses and mirrors so that I can focus both sun’s rays at a single point

I know similar questions have been asked but I’m still not convinced. I understand it would violate 2nd law of thermodynamics to generate a temperature hotter than the sun using one lens. I’m asking ...
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### How does doppler cooling work?

I have trouble understanding how doppler cooling works. I understand that an atom moving towards laser sees the laser light blue-shifted and if the laser frequency is slightly below the atom's ...
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### Why does fat get stuck to ice after solidification?

Suppose we put ice cubes into soup, which consists of saturated fat in liquid form and water in liquid form. When we put ice cube inside of it, fat gets solidified and when we remove ice cube, fat is ...
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### Are calories an appropriate measurement of energy for biological systems? [closed]

According to this source, a calorie is an inappropriate unit to quantify the energy contained in food because a "calorie is heat energy" and "humans don't absorb photons" but ...
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### Friedmann fluid equation in the non-relativistic case

The Friedmann fluid equation I am referring to is: $$a\frac{d\rho}{da} = -3(\rho+P) .$$ In the non-relativistic (low temperature) case for an ideal gas universe (representing matter), I know that the ...
1 vote
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### Do neutrons have a higher entropy than protons?

Since free neutrons want to undergo beta decay into a proton while protons are relatively stable does that mean that neutrons have higher entropy than protons?
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### $q$-potential in grand canonical ensemble

I was recently going through R.K. Pathria's Grand canonical ensemble chapter and found the following section confusing under physical significance of statistical quantities. The author mentions; To ...
1 vote
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### What is the relationship between Clausius Inequality and 2nd Law?

I am confused about the application of the 2nd Law for reversible and irreversible processes and cycles. I want to know how the Clausius principle, the Kelvin-Planck statement, and the Clausius ...
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