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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Are there materials with an adiabatic compressibility higher than that of common gases?

Are there materials with an adiabatic compressibility, measured at atmospheric pressure, higher than that of common gases? Is there any fundamental upper limit to the differential compressibility of ...
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17 views

Pressure components in a film

I am performing molecular-dynamics simulations of a polymer near a crystalline substrate (polymer film). I am comparing the mechanical properties in the film with the properties in the bulk polymer. ...
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1answer
80 views

Death by entropy

An idea struck me as I was walking to class today. According to Wikipedia, entropy is defined as the number of specific ways in which a thermodynamic system may be arranged, commonly understood as a ...
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1answer
28 views

Best airflow stategy for cooling a confined area?

I am working on a machine which has an upper side which processes a product. On the lower side of the machine there are a couple of motors with built in inverters and there is a laser unit. There are ...
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31 views

Calculating The Change in Thermodynamic Potentials for Reversible Adiabatic Expansion of Real Gas

The question asks to find which quantities have a change of zero for the reversible adiabatic expansion of a real gas: Entropy, Enthalpy, Internal Energy, Helmholtz Energy, and Gibbs Energy. Change ...
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50 views

Entropy of a system: microstates and macrostates

Currently we're deriving the Gibbs expression for the entropy during an introductory thermal physics course. We're dealing with a system with $N$ different equally likely microstates that are ...
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55 views

Why does the Nusselt Number for a Constant Surface Temperature, Thermally Fully Developed Circular Pipe Converge To a Constant of 3.66?

I have flow in a pipe with a constant surface temperature boundary condition. The velocity profile is fully developed and the temperature profile is also fully developed. In my textbook, it says that ...
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60 views

Definition of heat and work

What is the definition of heat and work in thermodynamics? In high school textbooks, heat is defined to be the energy transfer due to a temperature difference. I think this is not a good definition. ...
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1answer
56 views

Heat Equation with In-Depth Radiation Exact Solution

I am looking to solve the heat conduction equation in a semi-infinite solid with in-depth radiation on the domain $-\infty < x < 0$. The governing equation of this problem is: $$\rho c ...
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89 views

What would a normal thermometer read at the Sun's photosphere?

I was reading about the Earth's Thermosphere, and I quote this: "The highly diluted gas in this layer can reach 2,500 °C (4,530 °F) during the day. Even though the temperature is so high, one would ...
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1answer
39 views

Speedy Cooling - Ice based Cooler - Efficient Way

Ice is used in a cooler in order to cool its contents. Which of the following will speed up the process? A) Wrap the ice in a metal foil B) Drain the water from the cooler periodically C) Put the ...
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115 views

Ideal gas in ensemble

I want to calculate the phase space density for a single ideal gas particle in a microcanonical ensemble. I know that the partition function is given by the well-known expression that you find for ...
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1answer
49 views

Why do windows fog quickly?

I'm from Bogotá, Colombia. In my city the temperature is all the year between 12ºC and 25ºC (bus mostly under 19ºC) and when I drive and it's raining, there is accumulation of fog on the windows, but ...
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33 views

Heat Transfer, Water in a pot

I have a question I've been pondering for a while. How can I determine the temperature in a pot filled with water? I know these things: I have a known mass of water, of mass $m_w$ I have a pot of ...
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3answers
62 views

Comparison between entropy and internal energy

Why is entropy change a better way of determining a spontaneous process compared to the change in internal energy?
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1answer
79 views

How does heat travel?

How does heat travel? I have two competing thoughts here!! Firstly some form of atomic/molecular process liberates a photon in the infrared region of the spectrum which is detected as heat by a ...
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2answers
67 views

Why does a substance expands upon freezing? What conditions necessitate this?

Why does a substance expands upon freezing? What conditions necessitate this? For example, how does the slope of sublimation or fusion curve in a P-T diagram affect this?
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80 views

Is the atmospheric pressure the cause of a planet's surface temperature or is it the temperature the cause of a planet's atmospheric pressure?

I heard a climatologist on a talk show saying that one of the widely known arguments used by climate scientists to exemplify what a runaway greenhouse effect could cause to Earth's temperature and ...
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80 views

Is the physics of $j = \rho v$ questionable? What are the consequences? [closed]

In electrodynamics you have the quantity $\mathbf{j}$, which represents density of current per surface area. It is often said that $\mathbf{j} = \rho\mathbf{v}$, for geometric reasons. When I make ...
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80 views

Derivation of the Maxwell-Boltzmann speed distribution

We're currently deriving the Maxwell-Boltzmann speed distribution, but I'm struggling to squeeze out the right answer. For the first exercise we have to derive the fraction of molecules travelling ...
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98 views

What is the connection between the non-reversibility of the decay of unstable nuclei (as Uranium, Plutonium) and the 2nd principle of thermodynamics?

The 2nd principle of the thermodynamics says that if a system (e.g. an ideal gas) is left undisturbed, its number of microscopic states only increases. This is a statement of irreversibility of the ...
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2answers
75 views

Second Law from Statistics

Hi all I hope you can help me with the statistical origins of the Second Law. I cannot find anything that mathematically proves that order from disorder is impossible only improbable Leading me to ...
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1answer
44 views

Is state defined for non-equilibrium systems?

Equilibrium thermodynamic states follow the state principle and Gibb's phase rule when it comes to fixing a state when given a minimum of (typically) 2 state properties. For non-equilibrium states, I ...
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43 views

In statistical mechanics, what does integrating with respect to the position of a molecule mean?

So, this is probably a dumb question, but I cannot visualize or make sense of integrating over the position of a molecule in space. Okay, so an example in my thermodynamics textbook: we have N = 5 ...
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26 views

hydrodynamic entrance length [developing flow]. Non-native speaker needs some help [closed]

I am currently writing my thesis in English as an non-native speaker. From time to time, I struggle quite a bit with the correct expression of the sentences. Could you have a look on the highlighted ...
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1answer
65 views

Heat and the Temperature? [closed]

I have two chambers, one with oil and another with water. They are 1 kg each. When I supply an energy of 33600 J. Find the temperature that oil and water get heated to ? Specific heat capacity of oil ...
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2k views

Does a cooling mass lose mass as it radiates?

Assume we have a red hot cannonball in space. It starts off with mass M at 1000K. Later it has cooled by radiation to 100K. Has the mass decreased?
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70 views

Why is there a limited range of possible soap bubble size?

Soap bubbles are never "too small" or "too large". What defines the range of possible diameters of a soap bubble? Related questions: Why do steam bubbles increase in size as they rise, Why is the ...
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29 views

Where did the energy of the universe come from? [duplicate]

Last week my Physics teacher asked the class this question: If the internal energy of the universe is constant, then where did all the energy come from originally?
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44 views

Net work and the definitions of enthalpy and entropy and their implications in the Gibbs free energy

From the first law of thermodynamics: $\Delta U = q + w$ By definition: at constante pressure: $q_P = \Delta H = \Delta U + P \Delta V \quad Eq.1$ By definition, in the reversible process: $q_{rev} ...
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9 views

Calculating vapor quality in a two-phase region

I'm currently trying to find the vapour quality of a refrigerant in a two-phase region of cycle. I know $v_l$ and $v_g$, the specific volumes of the liquid and gas. I also know the temperature and ...
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1answer
25 views

What is the formula to find the temperature at a specific location of a material as a function of time that is gaining or losing heat?

Imagine a scenario where there is a homogenous material and at a any location in that object I know the temperature at time 0. Given some properties of that material, how can I find out at any other ...
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22 views

Solution to a kinetic temperature problem

I have a particular equation that needs to be solved for the kinetic temperature of a stream of gas particles. If we let the kinetic temperature be designated by the letter x, the equation becomes, ...
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15 views

Callen's Second Postulate - Composite or Closed+Composite systems?

In section 1.9 of Callen's Thermodynamics he says: The single, all-encompassing problem of thermodynamics is the determination of the equilibrium state that eventually results after the removal of ...
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33 views

Is collision in ideal gas because of physical collision or due to repulsion force

Is collision in ideal gas because of physical collision or due to repulsion force when they approach close by?
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50 views

A question about Memristors

A question about memristors: These semiconductor entities have been defined in terms of magnetic flux leakage, and a non-linear relationship to the electric charge that has flowed. In essence, the ...
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1answer
55 views

Boundary value problem

Consider the boundary value problem \begin{align} \frac{du}{dt}&= \frac{d^2u}{dx^2} , \\ u(0,t)&=0 \\ u(L,t)&=0 \\ u(x,0)&=f(x) \end{align} I know how to solve it using ...
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1answer
39 views

Change in Shannon entropy of a quantum circuit of Hadamard gate and a loop

The following Q&A about reversible computing is available here. It has listed a number of practical scenarios where a reversible circuit can still be dissipating heat. Let's assume that none of ...
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Entropy of this system

We have a system with two energy states $E_0$< $E_1=0$. We also know that state $E_0$ can only take at most $m$ particles. Curently, there are $n<m$ particles in $E_0$. Now, I am supposed to ...
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27 views

A model for surface adsorption

I have been trying to model the adsorption of an inert gas on the surface of a clean solid. So far my underlying goal is to express the Gibbs energy as a function of the number $N$ of adsorbed ...
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1answer
34 views

Entropy change at varying volumes

I have two questions both related to each other: 1.What is the effect of change in volume (whether increasing or decreasing), of a system, under isothermal conditions? How does this change in ...
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2answers
35 views

Clausius statement of 2nd Law

I'm slightly messed up with Clausius statement of 2nd Law. I've seen at least two versions, which seem to be conceptually different. a) It is impossible to transfer heat from a colder body to a ...
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31 views

Robin boundary conditions for the heat equation

Consider a 3D channel with fluid or gas with walls $\Gamma_1$, inflow part $\Gamma_2$ and outflow part $\Gamma_3$. The temperature is described by the heat equation: $$ \frac{\partial T}{\partial t} ...
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1answer
33 views

Problem Boltzmann distribution [closed]

I am trying to solve a problem about boltzmann distribution. If(A)=0.74 Uf(V)=0.037 Ia(A)=0.130 I have to find ...
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0answers
22 views

Boltzmann Distribution [duplicate]

I am trying to solve a problem about boltzmann distribution. If(A)=0.74 Uf(V)=0.037 Ia(A)=0.130 I have to find ...
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2answers
49 views

Only differences in chemical potential have physical meaning?

Is the statement in the title true? Does the chemical potential of a single pure substance that does not undergo any chemical reactions or phase changes have any physical meaning?
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23 views

maximun work in a reversible process

I often read maximum work done is by a reversible process. Shouldnt the more appropriate sensence should be maximum work done per unit of heat is maximum in a reversible process. ?
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1answer
33 views

Johnson-Nyquist noise for resistive element with temperature gradient

In Nyquist's 1928 paper, Thermal agitation of electric charge in conductors, the voltage noise for a resistor is derived assuming a circuit in thermal equilibrium and one temperature. How does the ...
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1answer
52 views

Energy without temperature

If you know the entropy $S$ of your system. Is there a general way to calculate the internal energy $U$ of your system? So the entropy $S$ is the only thing I know of my system. I have no information ...
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Why would the lack of air in a mine in an asteroid prevent you from flooding it?

In Isaac Asimov's I, Robot novel, two scientists stationed in an asteroid where mining activities are performed say "We could flood the mines, if this weren't an airless asteroid" when considering ...