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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Chemical potentials for multicomponent solids

I have a problem understanding how to deal with the chemical potential when two thermodynamic systems are in contact. For simplicity, let us consider $T=0\ K$. We have a system that is composed of two ...
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1answer
204 views

Evaporating water in -30ºC

I don't really know anything about physics even though I pretended studying it for years. How is this explained?: ...
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1answer
19 views

Which function denotes the energy of thermal motion within a system?

In thermodynamics, the heat $Q$ is defined as a type of energy in transfer, and is not a state function, which function denotes the energy of thermal motion within a system?
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349 views

What is temperature?

Recently I read an interesting article about negative temperature. I was puzzled because I thought before that temperature has definite meaning in thermodynamics: it tells about how fast atoms jiggle. ...
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4answers
283 views

Mathematical proof of the Second Law of Thermodynamics [duplicate]

Is there some book or paper that formalizes statistical mechanics, like some people have done with relativity, and proves the second law of thermodynamics from more foundational axioms?
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21 views

heat supplied by two heat source in reversible heat engine [on hold]

A reversible engine is supplied with heat from two reservoirs at 900 K and 600 K and rejects heat to a sink at 300 K. The engine develops work equivalent to 90kJ/s and rejects heat at the rate of 56 ...
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3answers
78 views

Can we apply $pV^\gamma$=const only for quasistatic adiabatic process?

But If this is so, I have often seen people applying this formula for quick processes where no heat exchange is possible. Here is an example from where I am quoting the following: we will assume ...
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3answers
40 views

Intuitive explaination for why higher engine compression ratio is more efficient?

Intuitively to me it seems likes increasing compression ratio would require more work to compress the gasses before ignition, so you'd just end up getting back what you put in - like a spring. What am ...
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2answers
129 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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1answer
24 views

At what gap width between two plates does convection not occur?

Does the Grashof number lead to the answer? The Wikipedia article (https://en.wikipedia.org/wiki/Grashof_number) yields an equation for vertical plates $$Gr_L = \frac{g\beta(T_s-T_\infty ...
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1answer
31 views

Why is the outside run of high presure line on a ductless mini-split airconditioner insulated?

On every ductless mini-split air conditioner I've ever seen, both the high and low pressure lines are insulated between the compressor and the building. It seems like the liquid refrigerant coming ...
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2answers
32 views

Would incense burn at a different rate if oriented differently?

This is motivated by this question in the Puzzling.SE beta about measuring 90 minutes of time using two candles that burn for one hour. (Feel free to read up on that before I spoil the puzzle!) The ...
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2answers
316 views

Cooling down a container in outer space

If I have two containers filled with very hot water(~210F) with one in outer space and one on earth, which one has a higher rate of cooling initially? Imagine the containers are single wall metal ...
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3answers
3k views

How to calculate the evaporative cooling rate needed to protect a house from forest fire

Recently in our area there has been a large forest fire and I've been looking into home defense from such things. I am not a physicist - but can do some basic math. I was wondering how I could ...
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1answer
34 views

Extracting heat energy without a heat engine

Is it possible to extract the molecular kinetic energy from a system directly (without the use of a heat engine / temperature gradient) and convert that to another form of energy, such as electricity, ...
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3answers
489 views

How does a canvas water bag cool water?

I was reading about this water bottle by Botl that behaves like a canvas water bags to keep water cool. I found out that this idea is an old idea and cars would drive with water bags in front as shown ...
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1k views

What is the insulative value of water?

I am having a hard to finding what the R value of water is. I am trying to compare it to Styrofoam (which has an R-Value of 5). EDIT Based on the assumption that water has an insulating property, ...
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1answer
1k views

Mixing Water at Different Temperature

If I have cup of water at room temperature (say, $25^\circ$C). What would be the resultant temperature if I pour another cup of same amount of water at $100^\circ$C to it? Is it simply ...
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2answers
141 views

Why does the coefficient of performance need to be calculated with $W + Q$?

How much work must a heat pump with a COP of 2.50 do in order to extract 1.00 MJ of thermal energy from the outdoors (the cold reservoir)? The first formula that came into my mind after reading ...
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2answers
151 views

How does one calculate where the “surface” of a gas-giant would be?

Okay, so Jupiter, Saturn, et. al are gas giants. I understand that they have large gassy atmospheres, which, due to the pressure would eventually become more and more dense as one approaches the ...
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2answers
219 views

Calculating the change in entropy in a melting process

I have a homework question that I'm completely stumped on and need help solving it. I have a $50\, \mathrm{g}$ ice cube at $-15\, \mathrm{C}$ that is in a container of $200\, \mathrm{g}$ of water at ...
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3answers
49 views

Does the entropy of an adiabatically isolated system stays the same after we decrease/increase the pressure?

Even if the system is isolated and there is no heat exchange with surroundings, shouldn't the decrease/increase of pressure result in increase/decrease of entropy? Does this property of an ...
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3answers
204 views

Why is the Gibbs Free Energy $F-HM$?

With magnetism, the Gibbs Free Energy is $F-HM$, where $F$ is the Helmholtz Free Energy, $H$ is the auxiliary magnetic field, and $M$ is magnetization. Why is this? Normally, in thermodynamics, we ...
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1answer
46 views

Why does nuclear matter tend to maximize pressure?

I'm reading a text about equations of state of dense nuclear matter. It is often stated that the phase with maximum pressure is preferred. Why is that?
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6answers
692 views

How do you prove the second law of thermodynamics from statistical mechanics?

How do you prove the second law of thermodynamics from statistical mechanics? To prove entropy will only increase with time? How to prove? Please guide.
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1answer
72 views

Heat balance: how to obtain T(t) with energy in the form u(T(t))

I have the heat balance equation for a cooling case in the form: $$\frac{dU(T(t))}{dt}=-J$$ with $U$: energy (J) $T$: temperature (K) $t$: time (s) $J$: heat flux leaving the system (W) I have ...
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1answer
203 views

Heat transfer in a pipe

I have a gas (assuming air) at T = 500 K that enters a cylindrical pipe. The outlet target temperature is 330K. There will be heat transfer via: Forced convection from the gas to the inside of the ...
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7answers
4k views

Is it possible to “cook” pasta at room temperature with low enough pressure?

It is known fact, that boiling point of water decreases by decreasing of pressure. So there is a pressure at which water boils at room temperature. Would it be possible to cook e.g. pasta at room ...
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1answer
79 views

Can statistical mechanics explain the second law completely? [duplicate]

Statistical mechanics is restricted to the postulate of the equal a priori probability, but this postulate does not need to be considered for thermodynamics, so the valid ranges of statistical ...
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4answers
437 views

Why does a short circuit generate fire?

When a short circuit occurs it's obvious that there is fire. How come electric energy turns out to be heat energy? What causes the conductors to get hot when short circuit is present.
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4answers
527 views

Shock speed in air/vacuum shock tube

Some of you are probably aware of What If, xkcd's blog about interesting physics problems. One episode, Glass Half Empty, concerns itself with what would happen if a glass of water is half water, ...
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0answers
21 views

Derivation of Van der Waal's Equation from Helmholtz Free Energy

From the following equation $$ f(T,v) = f_{id}(T,v) - RT \ln\bigg(1-\frac{b}{v}\bigg)-\frac{a}{v}$$ how do I derive Van der Waal's equation. In the equation above, $f$ and $f_{id}$ are the Helmholtz ...
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1answer
32 views

Thermal Velocity

What is thermal velocity? What is it's physical significance? Wikipedia says: The thermal velocity or thermal speed is a typical velocity of the thermal motion of particles which make up a gas, ...
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0answers
20 views

How to calculate average temperature of Nichrome 80 resistance wire? [on hold]

I know the following: I need the resistance wire to hold a temperature of 400C with a steady current The resistance wire is 1cm in length I have a 1500 mAh battery with 3.2V available What gauge ...
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1answer
40 views

How fast do molecules move in objects?

I guess it depends on the heat or the type of the material but can you give some examples or formulas to calculate it ? The best example would be the average speed of the air molecules (all types in ...
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3answers
107 views

What would be non-ergodic physics processes?

As the title says, what would be non-ergodic processes that occur in statistical physics? Many textbooks do not really cover ergodicity really well so I ask this question. I can't suddenly remember ...
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1answer
36 views

What does adding a visible light blocking IR filter do with heat?

Lets say I have a strong halogen light source which has a lot of IR radiation (e.g. a construction lamp). If I change the transparent front glass from the enclosure with a IR filter that blocks ...
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2answers
155 views

What conditions do a bunch of atoms need to satisfy to have a temperature?

What conditions do a bunch of atoms need to satisfy to have a temperature? Suppose that we have a beam of helium atoms travelling in a common straight line, equally spaced with the same velocity. If ...
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1answer
25 views

Macro-Level Explanation of Fluid Heat Transfer

Here's something I've been struggling with for a while. Say you have a hot fluid and cold surface (or the reverse, so long as there is a temperature difference) where the fluid flows over the ...
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1answer
22 views

Do the characteristic vibrational modes of a material define how much will a material heat up upon radiation excitation?

Do, and if yes how, does the intensity and energy of the Raman modes of a material make it to heat more by radiation excitation. I see phonons and vibrations of the lattice as heat, but maybe I'm ...
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1answer
110 views

How do heated showers/faucets work? [closed]

I'm not sure if this question is for Physics SE, but I'd go ahead and ask. I was wondering how the heated showers work (the ones with the hot and cold knobs). I have always been curious and this may ...
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1answer
74 views

Which air will give more effective cooling ? Dry or Moist?

Yesterday, I had an innovative idea. I live in India, and in summer season, the temperature can reach up to 45 degree Celsius. We use Split 1.5 Ton AC in our small office. The idea is to put an ...
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1answer
20 views

Energy comparison, evaporate water vs compress air, same mass

I would like to compare the energy requirements of the 2 following tasks: Total energy to evaporate 1 litre of water at normal atmospheric pressure at normal temperature? Total energy to compress ...
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3answers
140 views

Why is Entropy's Definition Useful?

I have somewhat of an understanding for other physical quantities, but as far as entropy goes I only know it to be "disorder". Why is the change in entropy formula an appropriate/useful definition, ...
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1answer
52 views

Enthalpic Excess - Regular Solution Upon Mixing

My professor, in his handout, says the following about this diagram : For $\beta>+2$ there are two minima and phase separation occurs driven by unfavourable enthalpic interactions. I ...
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1answer
55 views

conducting heat to infinity

I am modeling a situation in which an object that is a small object that is spherical in shape is receiving heat at a constant rate and then conducting it away into a medium that can be model as ...
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3answers
395 views

Hot water freezing faster than cold water

This question has puzzled me for a long time. There is already a question like this on Physics.SE. John's answer to the question seems quite satisfying. But when I googled the cause I found this and ...
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1answer
64 views

Radiation emission and absorption

Any object can emit and absorb radiation and the power of emission can be represented by the Stefan-Boltzmann law: $$P=A\epsilon\sigma T^4$$ In many texts the net power radiated is the difference ...
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3answers
215 views

Why change in internal energy is zero in isothermal process

In isothermal process $\Delta U =0$. But I am having trouble understanding it. Say we have an ideal gas, and say my temperature is constant but I move the pressure, volume from $(P, V) \to (P-dP, ...
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2answers
223 views

Phase volume contraction in dissipative systems

I am confused about phase-volume contraction in dissipative systems. Please help me catch the flaw in my understanding. From a macroscopic point of view I understand that a dynamic system tends to go ...