# 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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### Why potential energy is not considered in the internal energy of diatomic molecules?

In thermodynamics, I am taught that there are 5 degrees of freedom in diatomic molecules since there are 3 for translational and 2 for rotational. I interpret degrees of freedom as "ways you can ...
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### Gibbs Free Energy and Definitions of Entropy [duplicate]

When we speak about entropy, it's classically defined as $\delta S = \frac {\delta Q}{T}$ Where, I emphasize that it's the definition for only a change in entropy and not entropy itself (That's how I ...
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### Different Bekenstein bound equations – what’s the difference?

Can someone help me understand the difference between the Beckenstein bound equations that I’ve come across? They all appear to have different dimensions. I’ve been told that if you include the ...
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### Why does entropy obey the superposition principle?

I was deriving the Boltzmann's entropy formula $${\displaystyle S=k_{\mathrm {B} }\ln \Omega}$$ We start with two prepositions: Let's consider two systems and we know the entropy of the first is $S_1$...
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### Definition of entropy and microstates (Huang)

The definition of Boltzmann entropy given in Ref. 1 appears to be different from most sources I've seen up to this moment. Let me start from the latter: typically textbooks assume that in a given ...
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### Precise relation between theromdynamic beta and coupling constant in Euclidean QFT

In statistical mechanics, the thermodynamic is inverse of the temperature: $\beta \propto T^{-1}$. In Euclidean QFT, I have often run into the expression like $\beta \propto g^{-2}$ where $g$ is the ...
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### Bubble behaviour during water / vapour coexistence in the most idealised scenario without boundaries

Consider the following statement: during a 1st order phase transition, the temperature of the system stays constant and any extra heat goes into turning a larger portion of the system into the new ...
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### Feasibility of entropy at zero temperature

I remember a college lecturer of mine once gave me this equation of entropy during one of his lectures on thermodynamics: \begin{align*} \Delta{S} = \frac{ d {Q} }{T} \\ \end{align*} I found out ...
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### Can metal get hotter than heat source? [closed]

Trying to understand something I have actually witnessed.. My cousin is a welder, and claims that a 400 degree acetylene flame will in time melt steel. He showed me a 400 degree flame, then had a ...
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### Does Kirchhoff's law of thermal radiation fail for optically thin sources?

A box made of any material with a small hole in it will give blackbody radiation. According to my textbook by ARNAB RAI CHOUDHURI, If you place an optically thick source of same temperature as the box ...
1 vote
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### Why the work done by surrounding on the piston is same as average work done by surrounding on the gas inside the cylinder? [closed]

In thermodynamics when we derive the formula for work done by the surrounding on the gas, we actually derive the formula for work done by the surrounding on the piston and say that let the surrounding ...
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### In which situation the potential energy of a system is equal to its Gibbs free energy?

The other day I was giving a presentation about Transition State Theory, and I was showing both pictures of some potential energy surfaces (PES) and some Gibbs Free energy vs. reaction coordinate ...
1 vote
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### How is It possible to conceive heat in a reversible process? [duplicate]

Reversible processes are idealized processes. But even taking them as idealized processes i dont understand how can they be considered to go from one equilibrium state to another, specially if there ...
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### Why is a cavity with discrete modes necessary in the derivation of Planck's law?

In my readings, I have come across the concept that a finite cavity allows for the quantization of electromagnetic modes, leading to discrete energy levels. However, it's not clear to me why this step ...
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### What systems/subsystems does statistical mechanics actually apply to?

I've been trying to start learning statistical mechanics but I've run into a few roadblocks with the terminology being used and need clarification before I continue. Specifically, I want to clarify ...
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### Thermodynamics of Direct Air Capture of ${\rm CO_2}$: estimating the minimal energy requirements with the Gibbs Free Energy of Mixing

Recently I've watched a great video from a YouTube channel called "Cool Worlds" titled How Thermodynamics Hold Negative Carbon Tech. In that video, Professor David Kipping talks about how ...
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### Why don't atoms constantly emit electromagnetic radiation?

Consider a hydrogen atom: with one proton (and one neutron) and one electron. We can state that, on average, it is electrically neutral. We model the electron by a quantum wavefunction, and therefore ...
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### Piezoelectric effect in terms of classical thermodynamics

Non-volumetric work associated with the increase in polarization in a dielectric is equal to $dW = E dP$, where $E$ and $P$ are vectors of the electric field and the total polarization in the ...
1 vote
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### At what size of matter does the second law of thermodynamics breakdown?

I'm in high school and I was studying some topics related to heat transfer. While studying, I was curious about why heat strictly flowed from a higher temperature (more energy) to a lower temperature (...
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### Is the increase in the internal energy of an ideal gas equivalent to an increase in thermal energy?

ΔU = Q - W Q: Heat received by the gas from the surroundings (positive if received, negative if given to the surroundings) W: Work done by the gas on the surroundings (positive if done on the ...
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### Where do we get $\Delta U = nC_vT$ from? [duplicate]

The change in internal energy at constant volume is $Q=nC_v\Delta T$ Why does the same equation apply for constant pressure?
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### Thermodynamic work and potential functions

I was reading about the maximum thermodynamic work of a system (Z) that is going to equilibrium. $$dZ = dU + p_0 + T_0dS$$ I then came across the thermodynamic potential ...
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### Is the 2nd law a reason for the irreversibility of natural processes or a consequence of it?

I have been introduced to chemical engineering thermodynamics due to my academic background. I had learnt about internal energy, entropy etc and applied the equations to various scenarios of practical ...
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### Entropy change of an isolated system consisting of two thermally conductive blocks of equal isochoric heat capacity at different temperatures

A classic example considered in most introductory treatments of elementary thermodynamics consists of two thermally conductive (e.g., metal) blocks placed in thermal contact. In particular, I find ...
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### Relating specific heat capacity with the rate of cooling

By Newton’s law of cooling, the rate of heat loss (cooling) is directly proportional to the difference in the temperatures between the body and its environment. On the other hand, Specific heat ...
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### Measurability of the energy difference of two states with different mole numbers

In H.B. Callen's thermodynamics (2nd ed. p. 18), after expaining why "the methods of mechanics permit us to measure the energy difference of any two states with equal mole numbers", the ...
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### If I'm inside water, is the amount of heat energy I can receive capped because water can't go above 100°C?

I just learned about double-boiling, where instead of putting a pot directly on the flame stove (where it can get too hot), you put a big tub of water on the flame, and put the pot in the tub of water....
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### What do we mean when we say the CMB has a temperature and how do we measure it?

I have read this: An object without any internal degrees of freedom, like a single photon, can't really have a temperature. But an ensemble of photons can have a temperature. If you put an ensemble ...
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### Does the expansion of the universe cause the universe reach maximum entropy faster than a non expanding universe?

I've seen Does an expanding universe cool down? So I understand an expanding universe cools down, my question is, does an expanding universe reach maximum entropy faster than a non-expanding universe? ...
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### Thermodynamic diagrams in Hamiltonian mechanics

If we compare the fundamental thermodynamic relation $(1)$ and the Hamilton's principal function $(2)$, than we have two practically identical equations: $$dU=TdS-pdV \tag1$$ $$dS=pdq-Hdt\tag 2$$ In ...
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### Should I hold a baby formula bottle to cool it down faster?

This is an interesting and somewhat surprising physics problem - holding a hot object in your hand will cool it down faster, even if the air around is colder. I guess that 90% of people would be ...
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### What are good introductory books for contact geometry in non-equilibrium thermo?

As far as I can understand, Mrugala's paper on the geometry of thermodynamic processes covers introductory prerequisites for the contact formalism applied to thermodynamics, however, I know there's a ...
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### Landau and Lifshitz Principles of Statistical Mechanics

I've been reading Landau and Lifshitz book on Statistical Mechanicals and some aspects about how they lay out the principles has me a little confused. Let us now consider a macroscopic body or system ...
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### Unitary Evolution and the Second Law of Thermodynamics [duplicate]

Apologies for the long question. This problem has been mentioned multiple times on this platform, with no real resolution, in my opinion. So I'd like to gather all my thoughts below with hopes of ...
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### Estimating Heat transfering by measuring [closed]

Consider the setup with closed chamber with two flat metal objects with know dimensions. The first object can be heated or cooled by peltier module and the second one is placed on top of the first one....
1 vote
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### Contribution of rotational mode in ideal gas law

The ideal gas law is very accurate for an $N_2$ gas when the temperature is around 300K and the pressure is around 1atm. At these conditions, $N_2$'s compressibility factor is 0.997, which means that ...
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### Why can photon be treated like gas?

In Cosmology, especially when studying Cosmic Dynamic, sometime we will treat photons as gas to calculate its pressure, but according to my understanding, photon and gas are nothing alike. Why can ...
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### Net particle number density for relativistic particles at finite chemical potential (tricky integral)

Question: How does one show that the chemical potential of relativistic fermions is negligible at high energies? In particular, I would like to show that the difference between the particle density $n$...
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### Why is the First Law of Thermodynamic related to Fluid Equation?

In Cosmology, there is a equation called Fluid Equation: $$\dot{{\varepsilon}}+3\frac{\dot{a}}{a}(\varepsilon+P)=0.$$ It is derived by taking time derivative of the First Law of Thermodynamic: \$\dot{E}...
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### What book should I read to learn about the magnetocaloric effect

I am trying to study the magnetocaloric effect for a project but I can't find any good resources that deeply talk about it. Could you make any good recommandation to learn the basics about this effect ...