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

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592 views

How would I calculate the convection coefficient in transient convection?

So I have faced a problem dealing with transient conduction and I need a little help with the problem solving concepts. I need to determine how long it would take to reach the final temperature but I ...
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1answer
336 views

What is lambda R in Richardson's Law?

I've got to calculate the thermionic emission through a diode, so I need to use Richardson's Law. However, one thing's got me confused - according to the Wikipedia page: $$J = A_GT^2e^\frac{-W}{kt}$$ ...
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397 views

Boltzmann Brain Immortality

This is a somewhat philosophical question. Given that even after person A's death, there's a non-zero chance that a brain fluctuates into existence with exactly the same structure and memories as A's ...
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1answer
3k views

How much disolved oxygen is removed by boiling water?

Apologies if this is a chemistry question I've read that drinking water contains dissolved oxygen to the tune of $10\:\rm{ppm}$. I've also read that raising the temperature of water will remove some ...
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263 views

2d or 1d conduction in this scenario?

There is a rectangular fin attached to a heat exchanger with a base temperature of 350K. The fin has uniform properties and experiencesa uniform heat generation. It also experiences heat transfer with ...
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2k views

In what situations do I use the characteristic length of a fin to find the surface area?

So I'm learning about fins in heat transfer and it seems that there are two separate formulas for the surface area of a rectangular fin of length L, width w and thickness t. The fin is attached to a ...
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94 views

Terminology question about energy

I'm looking for the appropriate term to use for what gets "used up" as potential energy is converted to heat and work. For example, some of the the energy in solar radiation is converted by ...
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2answers
208 views

How does cooling scale with volume?

What equation would give me the answer to the question, "If i have a cup of water at a tempature of say boiling, how long would that cup of water take to cool off compared to say half that size of a ...
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208 views

What is the molecular model of heat of mixing?

Heat can be evolved or absorbed when two dissimilar liquids are mixed together at constant temperature and pressure. This is called the heat of mixing of the mixture. I'm trying to understand the ...
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220 views

Depalma Free Energy fields

Few years ago I read some papers about Free Energy, written by Bruce de Palma, a physicist who is said to be the inventor of the N-Machine, which is an device that works with free energy latent in the ...
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2answers
396 views

simulations at constant pressure-volume and constant volume-temperature

I have read some papers where the authors perform simulations at constant pressure and constant volumen or sometimes constant volume constant temperature. My question is when is it better to use one ...
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1answer
149 views

SDE for particle, PDE for the density

Given a particle on the plane we can assume that it follows 2D Brownian motion. On the other hand if there is a lot of such a Brownian particles one can be interested in the evolution of the density ...
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158 views

thermal energy while calculating Langevin Forces

I have a quick question from thermodynamics. I remember that we take kT/2 as the kinetic energy per degree of freedom in kinetic theory of gases. But when we do langevin forces (for example in ...
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612 views

free adiabatic expansion

My question is mainly an engineering question. Assume I have a turbine in which I adiabatically expand compressed air. The air cools down and does work to its surroundings, which is captured by the ...
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3answers
7k views

Derivation of polytropic process equation

I know that a polytropic process equation is often the result of an empirical fit of a P-V curve that allows one to derive some nice analytical equations for the process. Is there actually a ...
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1answer
274 views

How to vizualize a heat pump microscopically?

Hey guys, while learning thermodynamics i wondered how a the principle of a heat pump would look at a microscopic level, not on a quantum mechanical level. I learned that when a hot and a less hot ...
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27 views

Heat transfer coefficients

I am trying to make a model of non-stationary heating on a plate and I have questions considering coefficients. I am using explicit method to solve it (have to use it). I am not sure if I am using the ...
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23 views

Internal reversibility

The concept of reversibility always gives me a hard time.In a reversible process the change of entropy is zero. On the other hand for irreversible process it is not.But there comes another topic which ...
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1answer
31 views

Double Pipe Heat Exchanger

I have a metal plate and I need to transfer heat out of the metal plate to the atmosphere. Hence, I can use a double pipe heat exchanger which contains 2 pipes. But what if I just use one pipe and let ...
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31 views

Why 'free energy' can contain different amount of information in different settings, and what's their connection to phase transition?

I have seen 'free energy' arising from several contexts in very different forms, and each contains different amount of information (as a number, 1D function, 2D surface, etc). For example free ...
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24 views

Internal Energy questions

The first law of thermodynamics states that $\Delta U=q+w$ where $ΔU$ is the increase in internal energy of the system, $q$ is the thermal energy supplied to the system and $w$ is the work done on the ...
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34 views

Sufficient conditions for Equipartition Theorem to hold

I was wondering what are the sufficient conditions for the Equipartition Theorem. I know there is another question (For which systems is the equipartition theorem valid?) that somewhats answers this ...
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17 views

Thermodynamics of a heated enclosure [closed]

i have to maintain an aluminum enclosure of size 200x300x400mm at around 80 degrees Celsius, there for i need to calculate the thermodynamics of the system, ie what power of heater i need to achieve ...
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42 views

Writing an equation for the INTERNAL energy for a BRAYTON cycle

I am trying to understand the equation for the change in internal energy in the Brayton Cycle. Both the websites below gave me this formula :Change in U=q1+q2−w=0 ...
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23 views

Is $\mathrm{d}H = C_P \mathrm{d}T$ and $\mathrm{d}U = C_V \mathrm{d}T$ always true?

Is $\mathrm{d}H = C_P \mathrm{d}T$ and $\mathrm{d}U = C_V \mathrm{d}T$ always true for ideal gases?
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33 views

Square of velocity = 2 times temperature times specific heat capacity ; seems too tidy [closed]

So I did some algebra and came up with the following $v^2 = 2Tc$ where $ v = $ average velocity of the particles $ T = $ tempreture $ c = $ specific heat capacity but this seems too tidy
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22 views

showing that polytropic exponent n = 5/3 corresponds to an isentropic process for a monoatomic ideal gas [closed]

Given an ideal monoatomic gas with $pV = NT$ and $E = \frac{3}{2}NT$ what are the types of processes described by $n = 0, 1, \frac{5}{3}$, where n is the polytropic exponent ($p V ^ n = const.$)? ...
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38 views

Why does transfer of $^3$He from the rich to sparse phases in a dilution refrigerator cause cooling?

Explanations of the dilution cooling process seem to state without explanation that moving a $^3$He atom across the boundary from the $^3$He rich to the $^3$He sparse phases takes up heat from the ...
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51 views

How do I model this Quantum Cascade Laser (QCL) using COMSOL? [closed]

I'm trying to model this: GaAs on-top of a heat-sink made of Copper. It's trying to simulate a type of Quantum Cascade Laser, I'm looking at the effect the temperature has by running a 10V voltage ...
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1answer
35 views

Equation involving; elastic modulus, poisson ratio, radius and pressure of a sphere [closed]

I am trying to determine the expansion of a spherical pressure vessel. I am certain that an equation exist that will define the change of radius of the sphere as the pressure increases. The vessel ...
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1answer
27 views

The potential profile of two batteries in series

I would like to know the potential profile in two batteries in series, like the battery in the attached image.
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51 views

Why temperature of liquid drops after spraying thru a nozzle?

We have tested in our lab as mentioned in the picture. We connected hot water at 130degree F at 40Psig to a nozzle(bottle sprayer).we measured the temperature differences inside tank and after ...
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2answers
106 views

How to calculate liquid pressure regardless of the gravity?

Assume we have a balloon in a tank filled with a liquid , without any gravitational forces: We just know the Temperature, V0 and V1 I want to calculate the pressure exerted from the liquid on the ...
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2answers
34 views

Which is a more effective cooling method?

Assume I have a refrigerator at 0C and I put a can of soda at the center of the refrigerator. The temperature inside the refrigerator will be increased to 5C after I put the soda in, but will go back ...
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8 views

How do I calculate temperature diffusion in a gas contained in a heatsink?

To simplify the problem, consider a sphere with a diameter of $25\mathrm{mm}$ inside an infinite heatsink at $300\mathrm{K}$. At $t=0$ I instantaneously raise the temperature of the gas in the ...
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27 views

Is the diffusion coefficient time-dependent?

It is known that in the partial differential equation: $$u_t=au_{xx} $$ within the limits $0<x<1$ and $a>0$, the diffusion coefficient, arises in the mathematical modelling of a process of ...
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2answers
39 views

Entropy change in an irreversible process between 2 equilibrium state

Calculating entropy change in an irreversible process between 2 states requires computing the change in entropy for any reversible process between the 2 same states, but why? If someone could provide ...
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46 views

Thermal de Broglie wave length

If we refer to this wiki page thermal de Broglie wave length, we can see there are two expressions. One is derived using equipartition theorem, which makes perfect sense. The other one used $\pi kT$ ...
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37 views

What is the gas entropy as a functional of a one-particle distribution function?

There are some discrepancies on how to introduce entropy in classical kinetic theory. In what follows $f(r,p,t)$ is the usual one-particle distribution function of a monatomic gas, normalised to the ...
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22 views

Given the Pair Correlation Functions for a Multi-component Fluid, is There a Simple Way to Obtain the Helmholtz Free Energy?

I think it may not be possible. Given all the pair correlation functions (e.g. calculated from Ornstein Zernike theory), it seems possible to obtain the internal energy (assuming different species ...
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27 views

How water anomalies affects the Mpemba effect?

I understand basic explanation of Mpemba effect like this: Distant molecules are easier to realign into new formations. But if we compared two bottles of water like in Mpemba effect and one of them ...
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49 views

What is the smallest engine theoretically possible?

Is there some law of physic forbiding small engines that use say petrol? I would like to know of some analogy between electric motors and heat engines. In the latter we can make a motor so small that ...
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1answer
63 views

Energy conservation of permanent magnets engine

How to explain the fact that magnet can attract an object (apply a work W) without losing a (significant) part of its internal energy? How to apply the energy conservation principle? Please think to ...
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2answers
113 views

An equation to describe a process of heating one room using hotter air from another room

There are two rooms completly isolated from the outer world (a closed system, no heat losses). They are connected with each other through a ventilation tube. Temperature in room A is 32 degrees ...
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32 views

When did the casual definition of the three laws of thermodynamics first appear? [closed]

Who is credited as the source, and when were the laws first stated in this form? (1st Law) You can't win. (2nd Law) You can't break even. (3rd Law) You can't get out of the game.
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40 views

Callen Thermodynamics 2.8-2 help? [closed]

A two component gaseous system has a fundamental equation of the form $S=AU^{1/3} V^{1/3} N^{1/3} + \frac{BN_1N_2}{N}$ where $N=N_1+N_2$ and $A$ and $B$ are positive constants. A closed cylinder of ...
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36 views

Is the system in equilibrium?

If the time derivative of the Liapunov functional related to free energy of my system is zero, can I say my system is in chemical equilibrium?
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28 views

Necessity of infinitesimal steps for reversible processes

Is there a mathematical proof for why a reversible process is one that has many infinitesimal equilibrium steps, rather than fewer large steps? Maybe something along the lines of this?: ...
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145 views

Confusing Chemical potential of mixtures

I feel that there are very few textbook that treat the chemical potential of mixtures in an understandable clear way, which is why I wanted to ask here about certain things? Although I do not have a ...
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22 views

Can a gas be modelled as a low density blackbody, if we want to consider how detectable it will be in space?

The answer to this question taught me about the sort of parameters I need to consider if I want to consider how "detectable" an object in space is. I want to consider the detectability of a ...