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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Why does a thermometer in wind not show a lower temperature than one shielded from it?

I'm a little familiar with the physics and thermodynamics of the wind chill effect, but this question seems to come up from time to time: Why, given two temperature sensors or thermometers in the ...
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Crash course in classical thermodynamics

I've been working on some projects lately where it would be very handy to know more about thermodynamics than I do, but sadly I never had a chance to take a proper thermodynamics course in college. ...
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Do photons lose energy while travelling through space? Or why are planets closer to the sun warmer?

My train of thought was the following: The Earth orbiting the Sun is at times 5 million kilometers closer to it than others, but this is almost irrelevant to the seasons. Instead, the temperature ...
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Why do power lines sag when they are heated up?

I was reading some information about the 2003 power blackout in the Northeastern US. Beginning early in the afternoon of August 14, 2003 big transmission lines began to fail in First Energy's ...
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889 views

How does heat actually stay kept in the carbon molecules in the atmosphere? [duplicate]

We have all learned that the earth is getting heat up because of the CO2 and CO molecules absorbing heat. However, how is heat actually kept in those molecules. When photons heat them up, their ...
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667 views

Why does large curvature of spacetime imply high temperature?

I`ve just stumbled about a sentence which says that high curvature of spacetime implies that any matter present is at high temperature. This somehow confuses me, so my probably dumb question(s) are: ...
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Why does the gas get cold when I spray it?

When you spray gas from a compressed spray, the gas gets very cold, even though, the compressed spray is in the room temperature. I think, when it goes from high pressure to lower one, it gets cold, ...
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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 ...
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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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Would a perfectly clean and perfectly smooth champagne glass have no bubbles?

My understanding is that nucleation sites for bubbles in a champagne glass are either due to defects in the glass or due to fibers in the glass. (See this article for details on that statement.) Does ...
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468 views

If temperature is average KE per particle, and heat is total KE of all the particles, how can molar heat capacity vary?

If temperature is defined as the average kinetic energy per particle, and heat energy is defined as the total kinetic energy of all the particles (or more strictly, heat transferred is the total ...
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Does tea stay hotter with the milk in it?

A little thought experiment, similar to this one: Imagine you are making a cup of tea when the door bell rings. You've poured the boiling water into a cup with a teabag in it. As you're just about to ...
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Why is black the best emitter?

Why are emitters colored black better emitters than other colors? Why is white a worse emitter?
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What happens when you heat vodka in a microwave?

Since ethanol has a lower dielectric constant than water would the water heat up and boil before the ethanol? Would the water transfer heat to the ethanol and, since ethanol has a lower boiling point, ...
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Can entropy be equal to zero?

I've searched for it but I only found contradicting answers from "scientists": Dr. David Balson, Ph.D. states: "entropy in a system can never be equal to zero". Sam Bowen does not refutes the ...
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580 views

How much does increased world population contribute to global warming?

In 1974 there where 4 billion people on earth. Now in 2013 we passed 7 billion people. So the world population is nearly doubled in 40 years. Every living human being also haves a body temperature of ...
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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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649 views

Intuitively, why is a reversible process one in which the system is always at equilibrium?

A process is reversible if and only if it's always at equilibrium during the process. Why? I have heard several specific example of this, such as adding weight gradually to a piston to compress the ...
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Would this pump water up? and if so, how far?

I had this idea of an osmotic pump way back in high school and I never got a satisfactory answer if it would work. If I had this configuration: Would it continually pump water up given ambient ...
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Is thermal noise “quantum random”?

Is the randomness that can be extracted from thermal noise "as random" (that is, even theoretically inaccessible to measurement according to our knowledge of quantum mechanics, and not just random for ...
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319 views

Is “equilibrium state” equivalent to “well-defined state variables”?

Follow up to Intuitively, why is a reversible process one in which the system is always at equilibrium? and How slow is a reversible adiabatic expansion of an ideal gas? Suppose you have a ...
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819 views

Second Law of Black Hole Thermodynamics

I've been looking for a satisfying proof of this, and can't quite find it. I read the brief proof of the black hole area theorem in Wald, which is similar, but doesn't quite come down to the actual ...
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594 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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How do greenhouse gases trap heat?

I am looking for a molecular-level understanding of the greenhouse effect. What is it about the carbon-dioxide molecule (and methane, and water, etc) that is different from other gasses ...
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260 views

Surface energy as thermodynamic potential

Consider free energy of sharp interface $\Gamma$ $$ \int_\Gamma \sigma\;\mathrm{d}S $$ or also free energy of diffuse interface of characteristic width $\epsilon$ given by Cahn-Hilliard/Allen-Cahn ...
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What are the implications for the AdS/Cft program if AdS is unstable?

To my understanding recent progress in the study of the non linear stability of AdS spacetime suggest that $AdS$ might be unstable. If this is true, what are the physical and mathematical ...
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283 views

Nonequilibrium thermodynamics in a Boltzmann picture

The Boltzman approach to statistical mechanics explains the fact that systems equilibriate by the idea that the equillibrium macrostate is associated with an overwhelming number of microstates, so ...
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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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538 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 the low entropy at the big bang require an explanation? (cosmological arrow of time)

I have read Sean Carrol's book. I have listened to Roger Penrose talk on "Before the Big Bang". Both are offering to explain the mystery of low entropy, highly ordered state, at the Big Bang. Since ...
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Is “dark clothes for winter, light for summer” relevant?

We are told to wear light clothes in summer as they are better at reflecting sunshine and keeping us cool. And dark clothes absorb sunshine and keep us warm. But is it really relavent? If I buy ...
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LED Thermal Modeling (How to solve heat equation with constant heat source)

I have a mechanical design with LEDs that generate heat. I want to estimate the temperature at the LED junction vs. time, but especially at steady state. Knowing the LED voltage drop and current, I ...
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Definition of stress at the microscale

Take, for simplicity, a Lennard-Jones fluid below the critical temperature, which is to say that there is a phase separation into fluid and gas and thus an interface is formed. The macroscale picture ...
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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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Can we detect whether food has previously been heated in a microwave oven?

An acquaintance told me that she refuses to eat microwaved food because she read that the microwave changes the molecules of the food somehow. Her statement is vague and her sources are dubious but I ...
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What is entropy really?

On this site, change in entropy is defined as the amount of energy dispersed divided by the absolute temperature. But I want to know: What is the definition of entropy? Here, entropy is defined as ...
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Does a cooling mass lose mass as it radiates?

Assume we have a red hot cannonball in space. It starts off with mass M at 1000K. Later it has cooled by radiation to 100K. Has the mass decreased?
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Black and white matters. But why and how?

I know black conducts heat while white reflects it. But they are colors after all. If a metal is painted black, it conducts more heat or at a rapid speed than it would do before it was coated. But, ...
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Why must the particles of an ideal gas be point-like?

Why is a gas of elastically colliding hard balls of finite size not ideal? Respectively: Why is it essential that the particles of an ideal gas are point-like? Especially: Which ...
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Where does the joules come from (ideal gas constant)?

I have not done any physics in ages and have recently started studying it. The first chapter in my book deals with the ideal gas constant: $$pV=nRT$$ It is rewritten as: $$R=\frac{pV}{nT}$$ When I ...
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Why does pizza cheese seem hotter than the crust?

When I eat hot pizza or a melted cheese sandwich, the cheese feels a lot hotter than the crust or bread: in particular, the cheese might scald the roof of my mouth. but the crust will not. Is this ...
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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 ...
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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 ...
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927 views

What's the best strategy to fully fill the fridge with beer bottles and have them all cooled?

I'm having a party. Suppose I'd like to have a fridge full of cold ($6~^\circ\text{C}$ or below) beer bottles, in as short a time frame as possible. The fridge indicates that it is targeting (and ...
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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 ...
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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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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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Existence of negative temperatures and the definition of entropy

How negative temperatures can be possible has been treated on StackExchange before (several times in fact), but in light of some recent academic discussion, most of these answers seem to be possibly ...
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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 ...
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Why isn't in counting the no. of degrees of freedom, rotation about the axis running down the length of the molecule counted?

I was reading about the Equipartition Theorem. Then I got the following quotations from my books: A diatomic molecule like oxygen can rotate about two different axes. But rotation about the axis ...