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Questions tagged [thermodynamics]

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

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2
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2answers
48 views

Do liquids continuously boil in a pressure cooker?

First, I understand that the boiling point of water is increased as the pressure is increased. At 15 PSI, the boiling point is about 250F. My question, which seems like nobody can answer with ...
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1answer
22 views

Efficiency of sequential Carnot engines

A Carnot engine A operates between temperature $T_1$ and $T_2$ ($T_1>T_2$). A second Carnot engine B uses all the heat rejected by A as input and input and operates between temperature $T_2$ and $...
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13 views

On ways to solve a special case of 3D heat equation

How would I solve (even if numerically) the heat equation of a 3D box with initial uniform temperature & with a laser hitting the center of the top layer, heating it up at a constant rate? If it ...
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1answer
24 views

validity of Johnson noise model in current through vacuum

The Wikipedia page says Johnson noise is the electronic noise generated by the thermal agitation of the charge carriers (usually the electrons) inside an electrical conductor at equilibrium. I ...
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0answers
26 views

Hritik Nayaran said photons heats us [on hold]

and i saw this. "The photons of ionizing radiation that over time can possibly affect evolution, contribute to the more immediate problem of morbidity and mortality of cancer. ... This energy, when ...
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2answers
80 views

Heat Equation… with Newton Cooling?

I have the following differential equation that is purported to represent the equilibrium temperature at a point $x\in [0,L]$ on an uninsulated rod of length $L$, whose end points are kept constant $T(...
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1answer
38 views

Related to the Second Law

It is impossible to cross two adiabatic curves. Could someone better justify me why? Is it something related to the Second Law?
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2answers
38 views

Can Efficiency of a normal engine in other dimensions be 100%?

While I was imagining about carnot engine I got this question whether we can achieve a 100% efficient engine in other dimensions?
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2answers
29 views

How to calculate energy released by formation of frost on window during winter?

When the T ambient is low, we can see frost formation when the surface of window or surface is less than 0 degree C. But how to calculate the energy released during that frost formation at the window ...
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0answers
19 views

Volume of air required to keep electrical box cool [on hold]

I have a steel electrical box that is 15cm x 15cm x 10cm. The weight of the box is 1kg. Inside the box is a relay that loses 30 Watts of heat while in operation (it's attached to a heat sync that ...
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3answers
72 views

Measuring the entropy change in our body

What is an efficient way to measure the entropy change of our body in a day? Do we measure work done by the heart or such? Thanks for the help!
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1answer
51 views

Using Gibbs entropy to sum entropy of subsystems. Why does this not equal the Gibbs entropy of the total system?

I have found the Gibbs entropy of a simple system and the summed Gibbs entropy of its subsystems. I believed that these values should be equal. They are not. I'd be grateful for any ideas regarding ...
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3answers
165 views

Why don't people just put a water sprinkler on their roof for cooling? [on hold]

Some air conditioners work with evaporative processes. So I'm not sure why people wouldn't just use a sprinkler that turns on for a minute every hour or half hour or whatever in order to wet the roof. ...
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2answers
70 views

Clarifying Planck's law

I've edited this. The 2 answers i've received so far were confused as to my meaning. I'm in an endless argument with a guy who insists that Planck's law predicts that a black body where $T>0$ ...
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0answers
16 views

Entropy in Quantumstatistcs and Seckur-Tetrode-Equation

In my lecture we considerd a microcanonical ensemble with $E-\Delta \leq E_{\{n\}} \leq E$ with $E_{\{n\}}$ being the energy-eigenvalues of the Hamiltonian and $\{n\}$ being the quantum numbers. We ...
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0answers
8 views

Adapting this equation for vibrating string damping to a drumhead?

This article provides an equation for predicting decay rates (quality factors) of frequencies of an isolated vibrating string as the sum of: Air friction damping Viscoelastic/thermoelastic damping ...
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1answer
42 views

Are equilibrium states at fixed U and at fixed S the same state?

Consider a closed (which doesn't exchange matter with its surrounding) system initially in an equilibrium state $(S_a,U_a,V_a)$ with $U_a$, $V_a$ fixed by some external constraints. The constraint ...
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1answer
22 views

Mean value with respect to maxwell's velocity distribution

I don't understand why for an arbitrary $A$, the integral $$ \int \text{d}^3v \;f(\vec{v})A(\vec{v}) $$ is the mean value $\langle A\rangle_f$ of $A$ with respect to Maxwell's velocity distribution. ...
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1answer
17 views

Why isn't S concave in Maxwell's thermodynamic surface?

Maxwell's thermodynamic surface represents S(U,V) for a substance like water. We showed in class that S is concave, so why isn't it the case here? From Wikipedia:
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16 views

In a perfect fluid, is incompressibility incompatible with thermodynamic relations?

When we study fluid dynamics we assume that the hypothesis of local equilibrium holds. It means that every point of the fluid is in equilibrium with its immediate surroundings. If I understand it ...
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0answers
13 views

Critial temperature and phase transition temperature in Bose-Einstein Condensation, which is proper?

It is well known that at certain, usually extremely low, temperature, an ideal Bosonic gas starts to condense, and the Bose-Einstein condensation happens. It is a third-order phase transition. My ...
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0answers
30 views

Proof that entropy is a state function from the laws of thermodynamics

As far as I understand, the 4 laws of thermodynamics are like axioms from which all other theorems pertaining to macrostate thermodynamics can be derived. I was recently introduced to the concept of ...
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0answers
29 views

Evaporating water using water

I found a problem in the problem-sheet for 8-graders and I am curious how to solve it because I guess I have forgotten something important needed for this. The problem: how much water at 373 K can ...
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2answers
35 views

Fermi level in a solid

I am confused as to how fermi level is defined within a crystal/ solid It is normally situated midway between valence band and conduction band, is this how it is defined or is there other reasons this ...
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0answers
34 views

Is a Black Hole in Thermal Equilibrium with its Thermal Atmosphere?

A Schwarzchild black hole has a "thermal atmosphere" in the region $r_{s} < r < (3/2)r_{s}$ where $r_{s}$ is the Schwarzchild radius [1, 2]. The thermal atmosphere is comprised of radiation ...
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2answers
53 views

What does it mean to have Vin =/= Vout in a steady flow?

For a control volume and a steady flow inside a pipe $\dot{m_{in}} =\dot{m_{out}} $ -- >$\dot{V_{in}}\rho_1 =\dot{V_{out}}\rho_2 $ so for a compressable fluid where in general $\rho_1 \ne \rho_2$ the ...
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2answers
75 views

Some Questions about the Critical Point

I'm currently trying to understand the physics of phase transitions and I'm having a hard time doing that. First of all, the discussions on the topic seem to be confusing and there is no methodical ...
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1answer
67 views

Derivation of the Boltzmann distribution (in a specific form)

I am trying to derive a version of the Boltzmann distribution from statistical mechanics: $$n(\mathbf{x}) = n_{0}e^{-q\phi(\mathbf{x})/kT}.$$ This expression is commonly used in the derivation of ...
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4answers
147 views

What is wrong with this proof that an irreversible process does not always entail an increase in entropy?

In the paper, Clausius and Darwin Can Both Be Right, the author claims to show an apparent paradox in the status of the heat that is not converted into work in a modified version of the Carnot Cycle. ...
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0answers
22 views

Determine the modules of the following quantities:

Two cylinders (I and II) are provided with pistons whose masses are negligible and move without friction. One mole of an ideal gas is confined to each of cylinders I and II. Two types of expansion are ...
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0answers
12 views

What dictates the set of microcanonical ensembles of a system in the canonical ensemble?

So in the canonical ensemble we have a system inside of a heat reservoir, which can exchange energy with the system. This means the energy of the system is not necessarily a constant value but ...
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0answers
23 views

Entropy due to linearization of Freely-Jointed Chain

Say I model a polymer as a freely jointed chain in 3D with N bonds of length L each one. If I stretch the chain (initially coiled) along a 2D surface until it becomes linear (basically a 1D line of ...
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1answer
56 views

What is the obtainable work from cooling water? [closed]

In Callen's thermostatistics textbook, problem 4.5-14 says If the temperature of the atmosphere is 5°C on a winter day and if 1 kg of water at 90°C is available, how much work can be obtained as ...
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1answer
24 views

How does boiling-point elevation due to a solute affect the steam/gas that is created when boiling?

Consider the following situation: We have a pot of strong brine (for concreteness let's say 25g of NaCl per 100ml of water, at standard conditions) and heat it to a boil. How will the salt content ...
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0answers
27 views

Entrance question [closed]

A body of mass m is suspended by means of a string of length L.What is the minimum horizontal velocity to be given to the body so that (i)the body reaches the height of suspension and returns (ii)the ...
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1answer
47 views

Why is the temperature of a star not intensive?

The temperature of a star obviously depends on its mass. Does this mean that temperature is not always intensive?
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1answer
41 views

Final pressure of gases

Theory question here: If I have 4 sample of gases with same number of moles and same starting temperature, which one will have the highest pressure if I compress them 1/5 of its volume? Will it be :...
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0answers
16 views

What are the relative effects of convection and radiation in house curtains?

In the responses to a question about the effect of curtains in keeping a house warm, both convection and radiation were mentioned. In general I understand that convection is generally far more ...
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0answers
42 views

Heat transfer problem

I have a pipe with laminar flow and inlet temperature T1. Far away from the inlet (in the z direction) I have postulated the temperature (T) as a function of r and z (cylindrical coordinates). The ...
3
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1answer
70 views

Does a single atom have entropy?

In statistical thermodynamics, the entropy is given by $S = k_B \ln{\Omega}$ where $\Omega$ is the number of possible microscopic states for a given macroscopic system. But the atom as a system can ...
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0answers
87 views

Is this “Scientific Reports” article defendable as to the relativistic thermodynamics? [closed]

An article was published in Scientific Reports (one of the Nature Publishing Group journals) a couple of years ago about a correct transformation for thermodynamic quantities such as pressure and ...
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0answers
23 views

Irreversibility as an artificial concept and relationship with quantum mechanics

In my statistical physics classes we are following the book Statistical physics of particles - by Mehren Kardar and we were viewing the derivation of the boltzmann equation and irreversibility . ...
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2answers
41 views

Cooling of objects Thermodynamically unfavourable?

As I've learnt, systems in the Universe always tend to increase their entropy since it provides a larger number of availiable microstates for energy to be distributed and is hence statisticaly ...
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2answers
56 views

Degrees of freedom of a molecule

We all know that if we consider a mono-atomic molecule, it has $3$ translational degrees of freedom only, along the $3$ principal coordinates of the Cartesian coordinate system. In case of a ...
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4answers
3k views

How is the entropy at the time of the Big Bang calculated?

I've seen several reports that the entropy of the Universe at the time of the Big Bang was lower than it is now. This seems impossible. If we view entropy as a function of the number of micro ...
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2answers
59 views

Question about description of Gibbs free energy

When introduced to the gibbs free energy, it was derived as follows: First law: $dU=dq+dw$ Second law: $dS>dq/T$ for a spontaneous change. Note $dq$ and $dw$ are inexact differentials. ...
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1answer
43 views

Are there any example of reversible open system?

I went through different formulaes of work done under different system. the work done were for reversible system open and closed. Are there any reversible open system?
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0answers
14 views

Latent Heat and Ordinary Heat?

We have a name for the thermal energy associated with the change of potential energy during a phase transition. Unless I'm very mistaken, we call this latent heat. Here we have $Q=mL$. But what ...
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0answers
10 views

Question on thermodynamics and surface tension [on hold]

show that for a thin liquid film of area $A$ and surface tension $\sigma$ , $$2\left(\frac{\delta \sigma}{\delta p}\right )_{A,T}=\left(\frac{\delta V}{\delta A}\right)_{p,T}$$ I found this question ...
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1answer
43 views

Why the Earth's inner layers temperatures are layered and not distributed?

so I was having a debate with a flat earther, and he sent me this image which I couldn't explain well. at first, I thought that the different pressure levels inside the layers cause these levels of ...