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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An isolated Earth

It is known the fact that there is no way to extract energy (in any form) from any system without introducing some energy. The Earth for example, gets energy from the Sun, from nuclear fusion of ...
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155 views

Derivation of the Thermal Noise Spectrum

The thermal noise spectrum is given by: $$\mathcal{S}(f) = \frac{\hbar f}{2(e^{\frac{\hbar f}{kT}} - 1)}$$ This equation seems really similar to the Dirac-Fermi distribution but where does it come ...
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183 views

Why does adding milk to my coffee produce a fractal?

Whenever I add milk to my morning coffee I often enjoy watching the patterns which are created. These patterns have a striking resemblance to certain fractals and my question is, "Why?" Oh dear, that ...
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73 views

Function describing an adiabatic process

I understand that an isothermic process, plotted on a PV diagram, can be described by a rectangular hyperbola. However, now I am wondering, what about an adiabatic process, what function best ...
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23 views

Work of an adiabatic/ compresion: sign?

As far as I am concerned the adiabatic work it is given by: $$ W_{adi}=\frac{P_{f}V_{f}-P_{i}V_{i}}{\gamma -1}$$ While I have seen in other examples that it is used with $1-\gamma$ in the ...
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53 views

Help wanted achieving cryogenic temperatures [closed]

My son's science project involves liquifying air. I found the Linde process which is the way it was done first. We are trying and not getting very far so I am looking for advice. I have an air ...
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84 views

Conduction– conservation of energy

So I know that conduction is transfer of thermal energy by direct contact via molecular collisions. For conduction, $Q/t$ is the rate of heat flow and is heat current ($I$). My textbook says that ...
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106 views

What happens to matter when in a thermodynamic equilibrium?

I am trying to gain a better understanding of thermodynamic equilibrium. Here's what (I think) I know: If a system is in Thermal, Radiative, Chemical, and Mechanical Equilibrium, then it is in ...
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137 views

General Thermodynamic equation of state

I heard my professor saying that the equation $$ PV = \frac{2}{3}U $$ is valid for any non-relativistic gas, be it Ideal or Real gas(includes quantum ideal gases). Is this true, If it is how can we ...
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404 views

What would give us more heat ? infrared or microwaves?

As we know that our body is made up mostly of water and the frequency of vibration of water molecules matches that of microwaves which is the working principle of microwave ovens. When we come in ...
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94 views

Trying to turn a nonlinear differential equation into a linear one

The problem I have today is to determine the differential equation for a square fin protruding from a wall that experiences surface to ambient radiation and has an internal heat generation of $\dot q$ ...
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62 views

Thermalising a sub-system of a larger, interacting system

I'm considering a joint system consisting of a spin-1/2 particle (qubit) and a spin-l particle (reference) coupled via a Hamiltonian $H_0$. At a certain point I want to couple the qubit to a bosonic ...
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60 views

What creates black streams in fountain-type fireworks?

For the New Year celebration of 2014, my nephews set off a series of fountain-type fireworks (as shown below). When one looks carefully into the shower of light, one notices that a fair number of ...
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250 views

Thermal equilibrium in general relativity

The Newtonian condition for thermal equilibrium for a static system is $T = \mathrm{const}$. In this homework I'm asked to show that it's curved space generalization is $T(-g_{00})^{\frac{1}{2}} = ...
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63 views

How does pressurized gas constantly push?

If a gas, such as hydrogen, is pressurized into an air tight container, a force in terms of pascals (or whatever unit you want to use) is exerted, correct? That is what pushes against every surface ...
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715 views

Where does the kinetic energy go?

A uniform cylinder was placed on a frictionless bearing and set to rotate about its vertical axis. After a cylinder has reached a specific state of rotation it is heated without any mechanical support ...
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173 views

How much heat needed to heat 400g of ice at 0°C to 20°C in a 200g aluminium pot? [closed]

An aluminium pot has a mass of 200g and contains 400g of ice at 0°C. How much heat would be needed to melt that ice and then raise the temperature of the resulting water to 20°C. The specific heat ...
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395 views

Does heat increase the volume of a gas, and in turn its pressure?

Lets say you have 1 liter of hydrogen in a sealed container, at 100 psi. If 50 cm^2 of the containers surface area is heated to a 1000 degree Celsius, will the psi increase over time? What would be ...
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125 views

“Single-shot” Heat engine efficiency limits

The sun is 5778K and Earth is ~290K. Using the sun as the hot reservoir and earth as a cold reservoir we get 95% Carnot efficiency. However, the solar power efficiency limit is only 86%, see: ...
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337 views

'Polar Vortex' Boiling Water to Snow is Mpemba Effect?

I am based far away from the icy storm currently blanketing the US - the 'polar vortex'. However, I have seen in the TV news footage of reporters throwing boiling water into the air, the water ...
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10k views

Why does a copper mug keep my Moscow Mule cold? (Or does it?)

The Moscow Mule is a delicious cocktail that is normally served ice cold in a copper mug. The general consensus among Moscow Mule drinkers is that the copper mug keeps the drink colder than a normal ...
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308 views

How does the internal energy and entropy depend on mass?

I've found this thermodynamics question: Given a fluid described by the following equations: $$PV^{1/3}=aT^3 ,\quad U=3aT^3V^{2/3}, \quad S=\frac{9}{2}aT^2V^{2/3}$$ The parameter $a(n)$ ...
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100 views

NPT simulations: what is the inertia of volume fluctuations (using periodic boundary conditions)?

NPT molecular dynamics (MD) are most often implemented by adding some kind of piston that regulates the pressure. The inertia of this piston determines the dynamics of volume fluctuations in the ...
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353 views

Explain internal energy and enthalpy

Internal energy and enthalpy. I am finding it hard to distinguish between the two. Confused. Can someone explain me the two terms and difference between them? I tried learning from wikipedia but it ...
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682 views

Setting up an equation for calculating how long it takes a body to change temperature in its sorroundings

The United States has just recently been hit by a massive vortex of Arctic air. These unusually bitter temperatures have sparked my interests to ask the following rheotical question: How much time ...
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91 views

How does this Stirling engine work?

I'm looking at this Stirling engine, which I believe is a variation of an alpha type. It was developed by Nasa back in the 1980's. The thing is, I can't figure out how the hydrogen cycles through the ...
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857 views

Why do high current conductors heat up a lot more than high voltage conductors?

120 volts x 20 amps = 2,400 Watts However, if I increased the voltage and lowered the current, you can also use a smaller wire size (more inexpensive), also have less heat and achieve the same watt ...
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199 views

Heat engines and “Angular momentum” engines?

We know that the theory of heat engines is that, if you accept the second law of thermodynamics, $\Delta S > 0$ then you can define temperature using $\frac{1}{T} = \frac{\partial S}{\partial E}$ ...
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162 views

Why 1 Month LHC Magnet Cooling Times

An often quoted figure is that the LHC magnets take a month to completely cool and a month to warm. There is never an explanation as to why that is. I can conjure any number of reasons (slow changes ...
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285 views

Does diamond dust conduct heat as well as diamond?

Diamond is one of the best thermal conductors you can get. If the diamond is crushed into dust and spread out over a flat surface, but still held fairly compact (for instance in a small petri dish), ...
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220 views

Is An Exploding Beer Bottle An Accurate Simulation Of A “Frost Quake”?

A Frost Quake or Cryoseism is similar to an earthquake except it is triggered by ice expansion instead of tectonic activity. A Cryoseism may also refer to the sudden movement of a glacier, but I am ...
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176 views

Questions about Statistical Mechanics

For grand partition ensemble, is it true that the introduction of chemical potential allows us to have the sum of number of the particles in each state to be the total number of particles ("On ...
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73 views

Would a cone shaped Stirling engine work?

A Stirling engine evidently functions by heating and cooling air, thus making the piston move up and down. What if the heated side of the cylinder were shaped as a sort of cone with a gentle slope, ...
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67 views

Spreading heat with water

If water is cycled through a thin pipe by a pump, and a certain spot on that pump is made of thin copper that is being heated by a 1000 C source, will the water, as a whole, attain a heat of 1000 C ...
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43 views

Why the pressure of water at 298 Kelvin is 25 mm Hg (torr)? [closed]

Here is the problem, is about the law of Dalton. NH4NO2 decomposes giving N2 (gaseous). NH4NO2 decomposes and collects on water at 198 Kelvin and 748 atmosphere (atm). Vokume 411 ml N2. How many grams ...
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86 views

Thermo-couple Eelectroomotive force

I am studying thermocouples. In a text book, the author said that, the electromotive force can be written as $$ E= \alpha \theta + \beta \theta^2 \tag{1}$$ where $\alpha$ and $\beta$ are constants ...
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50 views

Transfered thermal energy to a gas with varying mass

In my physics book (and wikipedia) it states that the thermal energy transferred to an object is: $Q = c \ m \ dT$, where $Q$ is the transferred thermal energy, $c$ is the specific heat capcity of the ...
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78 views

How to calculate the van der Waals force from the van der Walls equation?

Given the van der Waals equation $$\left(p+\frac{n^2a}{V^2}\right)\left(V-nb\right)=nRT$$ and the van der Waals constants $a$ and $b$, how can I find the van der Walls force between two atoms at ...
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829 views

Compression and expansion

Why is it that solids on compression [As in striking a hammer etc.] heat up, but liquids and gases on compression [Pressurizing liquids causes them to freeze or gases to liquify] cool down? Can ...
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959 views

A draft makes people feel cold. How can that be measured?

On some days our office feels very cold to the point at which we find it difficult to type because our hands have gone numb. But our facilities manager insists it is the same temperature as always. ...
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124 views

is this heat calculation equation correct?

If I have a line of copper wire (lets say $\textrm{1 meter}$ long, $\textrm{1 mm}$ thick) and one end is a flattened disk of copper about the size of a quarter, and I apply a lot of heat to it (I'm ...
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32 views

What groups of symmetry are most suited for filling uniformely a spherical 3D space, whilst possessing the lowest possible surface-to-volume ratio?

I am looking for the closest known approximate solution to Kelvin foams problem that would obey a spherical symmetry. One alternative way of formulating it: I am looking for an equivalent of ...
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710 views

How is NASA's mod II Stirling engine so powerful yet so small?

Is it because of the temperature difference? I just don't understand how it can propel a car. Here's the link to the engine: ...
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623 views

How effectively does heat flow through copper wire?

If I have a line of copper wire (lets say 1 meter long, 1mm thick) and one end is a flattened disk of copper about the size of a quarter, and I apply a lot of heat to it (I'm talking 800 Celsius) will ...
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1answer
31 views

Is using water in a charcoal smoker less efficient than not using water?

I have a charcoal smoker that uses water. My understanding is that the water serves as a buffer and as a way to add moisture to the cooking environment. Some say that using water wastes fuel because ...
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68 views

Second law of thermodynamics and motion

The second law of thermodynamics states that the entropy of an isolated system never decreases, because isolated systems spontaneously evolve toward thermodynamic equilibrium—the state of ...
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4k views

How do I predict volume loss due to evaporation when boiling water?

Suppose I have a pot with diameter $D$ containing a volume of water $V$, being heated by a flame under it. If the ambient air temperature is $T$ and relative humidity is $R$, how can I calculate the ...
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199 views

Debye temperature for the steel?

I'm looking for data on Debye temperature of steel (ideally with a known carbon concentration, structure, and set of phases), but find only data on elements. Do You happen to meet the data in papers, ...
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4k views

Realistic calculation of heat loss for pipe

Good day everyone, I am new on this site and I hope to find here help, since I am not going anywhere with the literature I have found. I try to calculate realistically the heat loss of a hot, ...
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137 views

How do Stirling engines work?

How do Stirling engines work? I understand the heating and cooling of air, but how much faster (or more force, I'm not sure which to use) does the piston move per degree Celsius that the temperatures ...