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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Strange behaviour of water drops on a heated pan

I did an experiment in my home and I was surprised by the results. I heated a pan without anything in it. After some time it became very hot and I took some water (2 drops) and put it into the pan. It ...
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282 views

Chemical potential in Thermodynamics

In many scenarios, on computing the partial derivative of the internal energy (U) with respect to mole number (N) is negative. This implies that adding more moles of the substance decreases the U of ...
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7k views

Why does maximal entropy imply equilibrium?

From a purely thermodynamical point of view, why does that entropy have to be a maximum at equilibrium? Say there is equilibrium, i.e. no net heat flow, why can the entropy not be sitting at a ...
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4k views

Why was the universe in a extraordinarily low-entropy state right after the big bang?

Let me start by saying that I have no scientific background whatsoever. I am very interested in science though and I'm currently enjoying Brian Greene's The Fabric of the Cosmos. I'm at chapter 7 and ...
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198 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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492 views

Why isn't temperature frame dependent?

In (non-relativistic) classical physics, if the temperature of an object is proportional to the average kinetic energy ${1 \over 2} m\overline {v^{2}}$of its particles (or molecules), then shouldn't ...
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461 views

Modification of Newton's Law of Cooling

Yesterday I randomly started thinking about Newton's Law of Cooling. The problem I realized is that it assumes the ambient temperature stays constant over time, which is obviously not true. So what I ...
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594 views

How does a gas of particles with uniform speed reach the Maxwell-Boltzmann distribution?

Take an empty container and fill it with $N$ gas particles (ideally a monoatomic gas), each having the same kinetic energy $E$, then isolate the container. Since initially the speeds don't follow the ...
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133 views

How can point-like particles in an ideal gas reach thermodynamical equilibrium?

Having learned that the particles of an ideal gas must be point-like (for the gas to be ideal) I wonder how they can reach thermodynamical equilibrium (by "partially" exchanging momentum and energy). ...
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1k views

What are the six degrees of freedom of the atoms in a solid?

A monoatomic ideal gas has heat capacity $C_v=1.5$ which comes from the three translational degrees of freedom. For solids at high temperature, $C_v=3$, implying six degrees of freedom. What are ...
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528 views

What determines bubble locations in boiling water?

Something a little different to our usual fare. I was boiling a pan of water for cookery the other day, and got to wondering what caused the location of the bubble streams from the bottom of the pan. ...
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113 views

Are Carnot engine efficieny and Fourier heat trasmission law related?

It just occured to me that the efficiency of Carnot cycles is $\eta= \frac{T_1 - T_2}{T_1}$, that is, the efficiency decreases as the difference between reservoir temperatures decreases. On the other ...
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916 views

How much more energy does it take for a human body to heat 0C ice vs 0C water?

I'm trying to determine if going through the trouble of ingesting ice is worth the hassle versus ingesting ice-cold water, but my physics skills are rusty. If I drink a gram of ice water at ~0C, my ...
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275 views

Trying to understand a step in deriving Maxwell-Boltzman statistics

In the Wikipedia article on Maxwell–Boltzmann statistics, there is a point in the derivation that stumps me. When I get to where $\displaystyle W=N!\prod\frac{g^{N_i}}{N_i!}$ is quoted as a count ...
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107 views

Can the hot combustion products from a large flame be in “non-local thermal equilibrium”

Question: Does it take some time for the hot combustion products from a flame to reach local thermodynamical equilibrium (i.e. for the energy state populations to follow the Boltzmann distribution)? ...
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125 views

What precisely does the 2nd law of thermo state, considering that entropy depends on how we define macrostate?

Boltzmann's definition of entropy is $\sigma = \log \Omega$, where $\Omega$ is the number of microstates consistent with a given macrostate. If I understand correctly, this means that it only makes ...
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527 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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231 views

Will entropy continue to increase even if the universe begins to contract?

If the universe is heading for a big crunch, when the universe starts to collapse will entropy decrease and the arrow of time consequently reverse or not? I'm interested in the explanations, not just ...
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1k views

How can I understand a Vortex Tube and its efficiency?

A Vortex Tube takes a pressurized input stream, most typically of a gas, and creates two output streams with a temperature differential. Apparently, it has been described as a Maxwell's Demon. Both ...
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464 views

Thermodynamically reversed black holes, firewalls, Casimir effect, null energy condition violations

Scott Aaronson asked a very deep question at Hawking radiation and reversibility about what happens if black hole evolution is reversed thermodynamically. Most of the commenters missed his point ...
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11k views

How does a maple syrup evaporator work?

Some background info on what an evaporator is: It is a system of metal pans set over a heat source. Sap constantly enters the first pan controlled by a float valve to keep a constant depth. The pans ...
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204 views

Why aren't we surrounded by Black holes?

The Bekenstein bound is a limit to the amount of entropy a thermodynamical system can have. The bound is given by the following expression: \begin{equation} S \leq \frac{2 \pi k R E}{\hbar c} ...
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410 views

Entropy of the Sun

Is it possible to measure or calculate the total entropy of the Sun? Assuming it changes over time, what are its current first and second derivatives w.r.t. time? What is our prediction on its ...
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1k views

Are two thin blankets significantly warmer than a single thick blanket?

Almost every source I can find online maintains that two 0.5 cm blankets are significantly warmer than a single 1cm blanket due to air trapped between the thin blankets. However, the thermal ...
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222 views

radiation thermodynamics paradox

This question is concerned with a thermodynamic paradox for radiating bodies and radiation in a cavity of a specific shape. Consider two nested shells that are axisymmetric ellipsoids with the same ...
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335 views

Is there a relativity-compatible thermodynamics?

I am just wondering that laws in thermodynamics are not Lorentz invariant, it only involves the $T^{00}$ component. Tolman gave a formalism in his book. For example, the first law is replaced by the ...
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Why does thermal resistance go down as temperature goes up?

Here is the thermal resistance data for three speaker coils disengaged from the speaker cone. Any ideas? I would think it would be a horizontal line. http://en.wikipedia.org/wiki/Thermal_resistance ...
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973 views

What temperature can you attain with a solar furnace?

A solar furnace is a device that concentrates the sun's light on a small point to heat it up to high temperature. One can imagine that in the limit of being completely surrounded by mirrors, your ...
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143 views

Self Stirring Boiling Liquid

A couple of weeks ago, a friend and I were brewing a new batch of beer. We boil our wort in a turkey fryer. Pretty standard setup for beginners. Anyway, our wort was getting pretty close to boiling ...
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433 views

Is an entropic force an actual force that can be explained as a fundamental interaction?

Fundamental interactions, such as electromagnetism, the strong force, the weak force, and possibly gravitation, all have something in common: They can be described in terms of relativistic quantum ...
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447 views

Spontaneous conversion of heat into work at negative temperatures

Consider a heavy macroscopic object moving in a gas. Friction causes its kinetic energy to be converted into heat. Thermodynamically, there is (effectively) no entropy associated with the kinetic ...
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113 views

Finding two dimensional critical point

I'm reading an article about bi layered membranes which state that for the free energy function $f(\theta) = \theta \ln \theta + (1-\theta)\ln(1-\theta) + \chi \theta (1-\theta)$ Where $\phi_i$ is ...
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438 views

How is information defined from a thermodynamics point of view?

How is information defined from a thermodynamics point of view ? I came across some definitions using the concept of free energy of a system. If I have information stored in a finite volume of space ...
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849 views

Is it 11% hotter today than it was yesterday?

Yesterday the temperature outside was 0.5 °C. Today, the temperature is 30 °C. 30 is 5300% more than 0.5, but today is obviously not 5300% hotter than yesterday. In Fahrenheit, the temperatures are ...
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Why is there water coming out of a car’s tail pipe?

I notice yesterday that my neighbor’s car had water coming out of the exhaust pipe in the morning. My first response was since the hot exhaust is hotter than the cold tail pipe, heat is transferred ...
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4k views

What equation of state is needed for liquid states?

I'm familiar with the ideal gas law $$PV=nRT$$ but I don't think it applies to liquids like water. If I'm wrong, please correct me! If I'm right, then what equation of state applies to liquids such ...
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Sauna thermodynamics

Why does it get hotter (feel hotter) in a sauna when one pours water over the hot stones? Wikipedia says that the water condenses onto the skin, but the actual air humidity is so low that I doubt ...
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After what speed air friction starts to heat up an object?

I understand that air friction cools off an object at low speeds. For example, if you blow on a spoon of hot soup, it cools off. Or if you swing a hot frying pan in the air, it cools off faster. But ...
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Perpetual motion machine of the second kind possible in nano technology?

First of all sorry for my English - it is not my native language. During my engineering studies at the university the thermodynamics professor told us that the "second law of thermodynamics is not ...
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582 views

Definition of the entropy

In physics, the word entropy has important physical implications as the amount of "disorder" of a system. In mathematics, a more abstract definition is used. The (Shannon) entropy of a variable $X$ is ...
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550 views

Is speed an intensive property?

I remember being taught in elementary physics that while it makes sense to add volumes, masses, or heat, it makes no sense to add temperatures. As I wanted to use that to illustate some other issue, ...
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45k views

How can a wooden spoon be used to prevent water from over boiling?

This is an image I found via StumbleUpon Does this actually work? If so, why?
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1k views

Introduction to differential forms in thermodynamics

I've studied differential geometry just enough to be confident with differential forms. Now I want to see application of this formalism in thermodynamics. I'm looking for a small reference, to learn ...
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762 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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302 views

How to guess the content of a christmas present?

Let us assume that the present does not make any recognizable sounds when shaken (meow splat - the present now contains a dead kitten). Let us furthermore assume ...
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1k views

What would ACTUALLY happen to a person jettisoned into space?

[insert obligatory statement of my lack of knowledge in physics] Alright, so we have all seen the movies where someone gets blasted out of the airlock on their starship, or their suit decompresses ...
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450 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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958 views

Melting and freezing point

Why is the melting and freezing point of a substance are always the same ? This was quoted in my textbook but they didn't give a reason for this being so. Thank you.
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343 views

What is the deal with heat capacity?

For some time now I've been confused about heat capacity. The way I understand it, if I put in an amount of heat energy into the system, $dQ$, its temperature will change by $CdT$. But now, ...
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611 views

Entropy increase and end of the universe

While taking thermodynamics our chemistry teacher told us that entropy is increasing in day by day (as per second law of thermodynamics), and when it reaches its maximum the end of the world will ...