Covers the study of (mostly homogeneous) macroscopic systems from a heat/energy/entropy point of view. Maybe combine with [tag:statistical-mechanics].

learn more… | top users | synonyms (3)

8
votes
5answers
2k views

How is thermodynamic entropy defined? What is its relationship to information entropy?

I read that thermodynamic entropy is a measure of the number of microenergy states. What is the derivation for $S=k\log N$, where $k$ is Boltzmann constant, $N$ number of microenergy states. How is ...
8
votes
3answers
3k views

How long does it take an iceberg to melt in the ocean?

This is a quantitative question. The problem is inspired by this event: On August 5, 2010, an enormous chunk of ice, roughly 97 square miles (251 square kilometers) in size, broke off the Petermann ...
8
votes
6answers
3k views

Does placing a lid on a pot affect the rate of boiling?

On the subject of cooking, someone once told me "If you have too much water in your pot after cooking a dish, just turn up the heat and let it boil without a lid for a few minutes". Now I've thought ...
8
votes
1answer
538 views

Camp Fire Crackle

I was sitting around the camp fire and there was a periodic sound of cracking and pieces of the burning wood flew off. My question is what is the mechanism for the cracking of the wood? I was thinking ...
8
votes
2answers
59k 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?
8
votes
5answers
953 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, ...
8
votes
2answers
261 views

Internal energy of an ideal gas as a function of volume

Okay so I've been reading a bit on Thermodynamics and I found something that I couldn't wrap around my head. For an ideal gas, the change in internal energy is equal to $$\Delta U = Q + W$$ And ...
8
votes
4answers
2k 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.
8
votes
4answers
553 views

Doesn't the use of a thermometer alter the temperature of the system?

If I place a mercury thermometer in hot water, heat energy will transfer from the water to the mercury inside the thermometer. Will this continue until thermal equilibrium is reached and thus the ...
8
votes
1answer
2k views
8
votes
3answers
576 views

What made the keys in my pocket so hot?

Not sure where to ask this question - thought you guys would probably have the best idea! Today a single key on my keychain in my pocket heated up so that it was too hot to handle and scalded my leg. ...
8
votes
3answers
541 views

How to work out the relation between the “mean relative speed” and the “mean speed”?

I'm a freshman and am taking the general physics course. I just learned intro thermodynamics. One problem that really puzzles me is the calculation of "collision mean-free path", where calculating the ...
8
votes
4answers
817 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 ...
8
votes
1answer
427 views

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 ...
8
votes
2answers
492 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 ...
8
votes
4answers
11k views

Efficiency of Stirling engine and Carnot's theorem

I want to calculate the efficiency of this Stirling cycle for an ideal gas $pV = nRT$ The mechanical work is $$ \Delta W_{12} = - \int_{V_1}^{V_2} p(V) \mathrm{d}V = -nRT_2 \ln \frac{V_2}{V_1}\\ ...
8
votes
3answers
2k views

Maxwell's Demon Constant (Information-Energy equivalence)

New Scientist article: Summon a 'demon' to turn information into energy The speed of light c converts between space and time and also appears in e=mc^2. Maxwell's Demon can turn information supplied ...
8
votes
5answers
5k 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 ...
8
votes
2answers
250 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 ...
8
votes
1answer
617 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 ...
8
votes
2answers
417 views

How much energy from extreme coldness?

Let's say I have: 1: one mole of extremely cold ideal gas 2: unlimited amount of ideal gas at temperature 300 K 3: one ideal heat engine Can I generate for example 1 MWh of mechanical energy using ...
8
votes
1answer
376 views

Why does the majority of smoke leave when flames appear?

When I burn wood, paper, or other plant matter, where there is a flame, there generally isn't too much smoke, but it I blow the fire out, smoke starts billowing up. It billows thicker and thicker as ...
8
votes
5answers
156 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). ...
8
votes
2answers
632 views

Why does Platinum evaporate if left long enough?

I have been reading into research relating to the redefining the 1 kg weight as the current Platinum-Iridium is becoming smaller. In this article, here, it mentions that the original metal weight ...
8
votes
2answers
6k views

Is there a simple way to derive a T-S diagram from a p-V diagram for arbitrary processes?

Often, for thermodynamic processes only a p-V diagram is shown. Even without hard figures, the shape of the curve can be helpful to evaluate the process. However, it is hard to figure out for real ...
8
votes
4answers
14k views

Are specific heat and thermal conductivity related?

Are there any logical relationship between specific heat capacity and thermal conductivity ? I was wondering about this when I was reading an article on whether to choose cast iron or aluminium ...
8
votes
1answer
226 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?: ...
8
votes
3answers
3k 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 ...
8
votes
2answers
291 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 ...
8
votes
1answer
839 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. ...
8
votes
1answer
121 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 ...
8
votes
3answers
528 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 ...
8
votes
2answers
1k 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 ...
8
votes
1answer
289 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 ...
8
votes
2answers
121 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)? ...
8
votes
1answer
172 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 ...
8
votes
3answers
745 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 ...
8
votes
1answer
307 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 ...
8
votes
4answers
2k 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 ...
8
votes
5answers
16k 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 ...
8
votes
2answers
222 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} ...
8
votes
1answer
2k 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 ...
8
votes
3answers
2k views

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 ...
8
votes
2answers
5k views

What is the relationship between Energy, Entropy, and Information?

What is the relationship between Energy, Entropy, and Information? I read this - What Is Energy? Where did it come from? - and the top answer says that 'energy' is an abstract number that is a ...
8
votes
1answer
1k 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 ...
8
votes
1answer
738 views

What do we mean when we talk about Gibbs Free Energy?

Before I start, I'm aware that this question may be better suited on the Chemistry or Biology site, but it's my belief that physicists are more likely to have a clear understanding on what certain ...
8
votes
4answers
614 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 ...
8
votes
2answers
642 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 ...
8
votes
1answer
125 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 ...
8
votes
1answer
518 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 ...