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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Is a world with constant/decreasing entropy theoretically impossible?

I'm not 110% sure exactly what I mean by this question. It was sparked by a friend who said he wished the law of entropy were reversed, so he wouldn't have to worry about cleaning the bathroom. ...
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Is there any proof for the 2nd law of thermodynamics?

Are there any analytical proofs for the 2nd law of thermodynamics? Or is it based entirely on empirical evidence?
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Could temperature have been defined as $-\partial S/\partial U$?

When coming up with a definition of temperature, it's typical to start with an empirical definition that a system with a hotter temperature tends to lose heat to a system with a colder temperature. ...
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How can it be that the beginning universe had a high temperature and a low entropy at the same time?

The Big Bang theory assumes that our universe started from a very/infinitely dense and extremely/infinitely hot state. But on the other side, it is often claimed that our universe must have been ...
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Can Lee-Yang zeros theorem account for triple point phase transition?

Now the prominent Lee-Yang theorem (or Physical Review 87, 410, 1952) has almost become a standard ingredient of any comprehensive statistical mechanics textbook. If the volume tends to infinity, ...
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Physical meaning of Legendre transformation

I would like to know the physical meaning of the Legendre transformation, if there is any? I've used it in thermodynamics and classical mechanics and it seemed only a change of coordinates?
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Is there some way to narrow down the Leidenfrost point for water?

Cooks sometimes use the Leidenfrost effect to estimate the temperature of a frying pan by flicking a few drops of water onto the heated pan. I had no idea, before looking into this, that this could be ...
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Mechanics + Thermodynamics: Bouncing Ball

In preparation for an exam, I'm revisiting old exam questions. This one seems neat, but also quite complicated: A soccer ball with Radius $R=11cm$ is inflated at a pressure of $P =9 \times 10^4 ...
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332 views

How is the logarithmic correction to the entropy of a non extremal black hole derived?

I`ve just read, that for non extremal black holes, there exists a logarithmic (and other) correction(s) to the well known term proportional to the area of the horizon such that $S = \frac{A}{4G} + K ...
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Difference between thermodynamics and statistical mechanics

I've just finished a class a few weeks ago which taught thermo and stat mech, and I still don't know the exact difference between the two. Can someone help clear this up for me? (Yeah it's sad, and ...
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Why isn't temperature measured in units of energy?

Temperature is the average of the kinetic energies of all molecules of a body. Then, why do we consider it a different fundamental physical quantity altogether [K], and not an alternate form of ...
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892 views

Is the total energy of earth changing with time?

Many years ego, Earth was hot. Over time, it has lost energy and has become colder. Is it now in equilibrium or is its total energy changing?
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834 views

What is the status of Mpemba effect investigations?

There is this puzzling thing that is called Mpemba effect: paradoxically, warm (35°C) water freezes faster than cold (5°C) water. As a physisist, I've been asked about it several times already. And I ...
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How to freeze the Niagara waterfalls?

Here is a picture of the usual vigorous Niagara Falls (in the winter). Here is the picture of Niagara Falls frozen in 1933 (in the very cold winter). Here is the picture of Niagara Falls frozen in ...
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What exactly is the difference between advection and convection?

After reading Wikipedia articles on advection and convection, I still cannot determine whether there is a consensus on a difference between these two terms. Sometimes, the term convection seems to ...
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How slow is a reversible adiabatic expansion of an ideal gas?

A truly reversible thermodynamic process needs to be infinitesimally displaced from equilibrium at all times and therefore takes infinite time to complete. However, if I execute the process slowly, I ...
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243 views

What makes now special?

I recently came across the paper Relation between the psychological and thermodynamic arrows of time (arXiv). Their argument makes sense to me, however their concept doesn't seem to address what ...
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353 views

Does this type of phase transition exist?

The short version of this question is: Is there, or could there be, a system with a phase transition where adding a small amount of heat causes a discontinuous jump in its temperature? Below are ...
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Entropy of radiation emitted into space

In several papers I see something equivalent to the following expression for the entropy of radiation given by an astronomical object such as the Sun (assuming the object can be approximated as a ...
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Does hot air really rise?

"Heat rises" or "warm air rises" is a widely used phrase (and widely accepted phenomenon). Does hot air really rise? Or is it simply displaced by colder (denser) air pulled down by gravity?
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Why do I feel cold air in the shower?

Our house has a glass sliding door to the shower. The shower has the dimensions of about 2 feet wide, 5 feet long, and 6 feet high. Above the door (and shower head) there is about 1 foot of open space ...
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967 views

Temperature of a neutron star

In our everyday experience termperature is due to the motion of atoms, molecules, etc. A neutron star, where protons and electrons are fused together to form neutrons, is nothing but a huge nucleus ...
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Can a single classical particle have any entropy?

recently I have had some exchanges with @Marek regarding entropy of a single classical particle. I always believed that to define entropy one must have some distribution. In Quantum theory, a single ...
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Would wearing clothing that is black on the inside and white on the outside keep you cooler?

The Straight Dope ran an explanation of why nomads often wear black clothing - it absorbs heat better from the body. On the other hand, white clothing reflects sunlight better. Is it possible to get ...
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Why is there more steam after a pot of water *stops* boiling?

I have a pot of vigorously boiling water on a gas stove. There's some steam, but not alot. When I turn off the gas, the boiling immediately subsides, and a huge waft of steam comes out. This is ...
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Why are materials that are better at conducting electricity also proportionately better at conducting heat?

It seems like among the electrical conductors there's a relationship between the ability to conduct heat as well as electricity. Eg: Copper is better than aluminum at conducting both electricity and ...
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The notion of an adiabatic process in thermodynamics -vs- quantum mechanics

I'm confused about the terminology in the two contexts since I can't figure out if they have a similar motivation. Afaik, the definitions state that quantum processes should be very slow to be called ...
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Why does the Boltzmann factor $e^{-E/kT}$ seem to imply that lower energies are more likely?

I'm looking for an intuitive understanding of the factor $$e^{-E/kT}$$ so often discussed. If we interpret this as a kind of probability distribution of phase space, so that $$\rho(E) = ...
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The entropic cost of tying knots in polymers

Imagine I take a polymer like polyethylene, of length $L$ with some number of Kuhn lengths $N$, and I tie into into a trefoil knot. What is the difference in entropy between this knotted polymer and ...
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Discontinuities and nondifferentiability in thermodynamics

In physics and engineering sources, calculus-based formalisms - whether differential forms on a manifold, or "differentials" of functions of several variables - are presented as a way of modeling and ...
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How efficient is an electric heater?

How efficient is an electric heater? My guess: greater than 95%. Possibly even 99%. I say this because most energy is converted into heat; some is converted into light and kinetic energy, and ...
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730 views

Light “diode” and 2nd law of thermodynamics

If I had a light "diode" - an object that only allowed light (at least for a range of frequencies) to travel through it in one direction, would this necessarily allow violations of the 2nd Law of ...
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References about rigorous thermodynamics

Can you suggest some references for rigorous treatment of thermodynamics? I want things like reversibility, equilibrium to be clearly defined in terms of the basic assumptions of the framework.
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452 views

Intuitive understanding of the definition of entropy

In Wikipedia, the definition of entropy goes like this: $ d S = \dfrac{\delta q_{\rm }}{T}$. The literal interpretation of this equation is that some amount of heat transferred into a system, if the ...
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On a hot day, when it's cooler outside than in; is it better to put a fan in an open window pointing inwards or outwards?

If it's really hot inside, but cooler outside; what is the best way to place a single fan to try and cool a room down? I always assumed it would be better pointing inwards (and this thread suggests ...
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Does the scientific community consider the Loschmidt paradox resolved? If so what is the resolution?

Does the scientific community consider the Loschmidt paradox resolved? If so what is the resolution? I have never seen dissipation explained, although what I have seen a lot is descriptions of ...
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A water drop in vacuum

Let's imagine the following situation: At an initial moment $t=0$, a large water drop with diameter for example $D=10\ \text{cm}$ is placed in deep space (Say an astronaut is experimenting). Let's ...
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Explanation of “thermite vs ice” explosion

There are several videos of the reaction, where some amount of burning thermite explodes on a contact with ice. An "original" video: http://www.youtube.com/watch?v=IuPjlYxUWc8 A Mythbusters ...
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Thermodynamics of supercooled water

Now that it's been freezing outside for the last few days, I experimented a bit with supercooling. I've left a bottle of clean water outside for a few hours, and behold, when I shook the bottle, the ...
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781 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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Quantum entaglement and the arrow of time

I have seen several claims to that quantum mechanics is required to explain the arrow of time which I take to mean the macroscopic irreversibility of physical systems. This is presumably to resolve ...
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Thermodynamically possible to hide a Dyson sphere?

You build a Dyson sphere around a star to capture all its energy. The outer surface of the Dyson sphere still radiates heat at much higher temperature than the cold space background, so you're easy to ...
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A formal procedure for thermodynamic relations

This is my third time taking a thermodynamics course (two in undergrad, one in grad), and I've finally become frustrated enough about something to post on here. A lot of thermodynamic questions want ...
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How badly could someone be injured by concentrated sunlight?

Recently-ish, I stumbled across an interesting short story (by way of Science Fiction & Fantasy Stack Exchange) where a soccer referee is apparently incinerated by concentrated sunlight. Where ...
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How efficient is a desktop computer?

As I understand it (and admittedly it's a weak grasp), a computer processes information irreversibly (AND gates, for example), and therefore has some minimum entropy increase associated with its ...
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807 views

Best way to chill a cup of coffee with cold water and 5 minutes [duplicate]

Initial data 1 x 3/4 full cup of hot coffee / tea / your favorite morning beverage cold water 5 minutes Considering that it's starting to get hot outside, and we all want to drink reasonably cold ...
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778 views

Is there a relativistic (quantum) thermodynamics?

Does a relativistic version of quantum thermodynamics exist? I.e. in a non-inertial frame of reference, can I, an external observer, calculate quantities like magnetisation within the non-inertial ...
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427 views

What happens to ice cream when you stir it?

I hope this is the appropriate forum for my question. I also considered posting it in the chemistry forum. When I eat ice cream I often stir it into a texture similar to that of soft serve. During ...
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Which direction does air flow?

I remember learning this in high school, but have forgotten it, and can't seem to find it anywhere online. Air travels from areas of high pressure to low pressure...correct? So if I have a cold room ...
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Why does adding solutes to pure water lower the the specific heat?

We found that water with salt, sugar, or baking soda dissolved in it cools faster than pure water. Water has a very high specific heat; how do these solutes lower it? We heated a beaker (300ml) of ...