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 (1)

340
votes
19answers
128k views

Cooling a cup of coffee with help of a spoon

During the breakfast with my colleagues, a question popped into my head: What is the fastest method to cool a cup of coffee, if your only available instrument is a spoon? A qualitative answer would ...
72
votes
5answers
8k views

Why am I not burned by a strong wind?

So I was thinking... If heat I feel is just lots of particles going wild and transferring their energy to other bodies, why am I not burned by the wind? When I thought about it more I figured out ...
65
votes
16answers
123k views

How Does Mass Leave the Body When you Lose Weight?

When your body burns calories and you lose weight, obviously mass is leaving your body. In what form does it leave? In other words, what is the physical process by which the body loses weight when ...
60
votes
12answers
7k views

Why are radiators always placed under windows?

I don't know if anyone else has noticed this but in most buildings and most rooms, radiators are predominantly placed under a window. Now, in my eyes, that is the worst place to put them; hot air ...
42
votes
5answers
7k views

Why does a candle blow out when we blow on it? Our breath is 16% oxygen and only 4% CO2

Don't say that a layer of carbon dioxide covers the flame, because our breath has more oxygen than carbon dioxide. Also, our breath does not cool the flame as it is itself warm. So what is happening ...
41
votes
3answers
1k views

Is temperature a Lorentz invariant in relativity?

If an observer starts moving at relativistic speeds will he observe the temperature of objects to change as compared to their rest temperatures? Suppose the rest temperature measured is $T$ and the ...
36
votes
4answers
4k views

Why does cold metal seem colder than cold air?

(I apologize for this elementary question. I don't know much about physics.) Let's say that I put a metal pot in the refrigerator for several hours. At this point, I guess, the pot and the air (in ...
36
votes
1answer
7k views

Rubber band stretched produces heat and when released absorbs heat.. Why?

I always used to wonder why this happens.. when one stretches a rubberband to nearly it snapping point holding it close to your skin - preferably cheek(helps feel the heat), it emits heat. While ...
34
votes
5answers
7k views

Is fire plasma?

Is Fire a Plasma? If not, what is it then? If yes why, don't we teach kids this basic example? UPDATE: I probably meant a regular commonplace fire of the usual temperature. That should simplify ...
31
votes
8answers
22k views

Will a hole cut into a metal disk expand or shrink when the disc is heated?

Suppose you take a metal disc and cut a small, circular hole in the center. When you heat the whole thing, will the hole's diameter increase or decrease? and why?
31
votes
1answer
342 views

$(\mu,P,T)$ pseudo-ensemble: why is it not a proper thermodynamic ensemble?

While teaching statistical mechanics, and describing the common thermodynamic ensembles (microcanonical, canonical, grand canonical), I usually give a line on why there can be no $(\mu, P, T)$ ...
29
votes
3answers
2k views

What is the minimum pressure of a medium for which a sound wave can exist?

At what pressure will be particles in a medium be unable to form a sound wave when disturbed? How can this pressure be described mathematically? My guess is that this would correspond to the point at ...
29
votes
7answers
3k views

Homemade salad dressing separates into layers after it sits for a while. Why doesn't this violate the 2nd law of thermodynamics?

The oil, vinegar and other liquids in homemade salad dressing separate into layers after sitting for a while, making the mixture become more organized as time evolves. Why doesn't this violate the ...
27
votes
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 ...
27
votes
10answers
2k views

Why are the laws of thermodynamics “supreme among the laws of Nature”?

Eddington wrote The law that entropy always increases holds, I think, the supreme position among the laws of Nature. If someone points out to you that your pet theory of the universe is in ...
27
votes
8answers
3k views

Why is absolute zero considered to be asymptotical? Wouldn't regions such as massive gaps between galaxy clusters have temperatures of absolute zero?

Why is absolute zero considered to be asymptotical? Wouldn't regions such as massive gaps between galaxy clusters have temperatures of absolute zero? I just do not see why our model must work the way ...
25
votes
6answers
6k views

How do whisky stones keep your drink cold?

From a discussion in the DMZ (security stack exchange's chat room - a place where food and drink are important topics) we began to question the difference between how ice and whisky stones work to ...
25
votes
4answers
3k views

Why does wet skin sunburn faster?

There is a popular belief that wet skin burns or tans faster. However, I've never heard a believable explanation of why this happens. The best explanation I've heard is that the water droplets on the ...
23
votes
4answers
1k views

How do we know that heat is a differential form?

In thermodynamics, the first law can be written in differential form as $$dU = \delta Q - \delta W$$ Here, $dU$ is the differential $1$-form of the internal energy but $\delta Q$ and $\delta W$ are ...
22
votes
1answer
2k views

Energy in bolognese reduction - lid on or off?

Generally, to let my bolognese thicken, I leave the lid off in order to "let water vapor escape." I am however distracted from enjoying the taste because I'm having doubts that my physical reasoning ...
22
votes
3answers
7k views

Ice skating, how does it really work?

Okay, some textbooks I came across, and a homework assignment I had to do several years ago, suggested that the reason we can skate on ice is the peculiar $p(T)$-curve of the ice-water boundary. The ...
21
votes
3answers
234 views

Does entropy measure extractable work?

Entropy has two definitions, which come from two different branches of science: thermodynamics and information theory. Yet, they both are thought to agree. Is it true? Entropy, as seen from ...
20
votes
3answers
2k views

Prove that negative absolute temperatures are actually hotter than positive absolute temperatures

Could someone provide me with a mathematical proof of why, a system with an absolute negative Kelvin temperature (such that of a spin system) is hotter than any system with a positive temperature (in ...
20
votes
7answers
939 views

How is $\frac{dQ}{T}$ measure of randomness of system?

I am studying entropy and its hard for me to catch up what exactly is entropy. Many articles and books write that entropy is the measure of randomness or disorder of the system. They say when a gas ...
19
votes
6answers
2k views

Why isn't temperature measured in Joules?

If we set the Boltzmann constant to $1$, then entropy would just be $\ln \Omega$, temperature would be measured in $\text{joules}$ ($\,\text{J}\,$), and average kinetic energy would be an integer ...
19
votes
4answers
3k views

Why does boiling water in the microwave make a cup of tea go weird?

When I boil water in the kettle, it makes a nice cup of tea. Sometimes I need to use a microwave because a kettle isn't available. I boil the water in the mug and it looks pretty normal, but when I ...
18
votes
2answers
410 views

Why are expressions such as $\operatorname{ln}T$ used in thermodynamics where $T$ is not dimensionless?

In all thermodynamics texts that I have seen, expressions such as $\operatorname{ln}T$ and $\operatorname{ln}S$ are used, where $T$ is temperature and $S$ is entropy, and also with other thermodynamic ...
18
votes
2answers
727 views

How hot does the tip of a pencil get while writing?

When writing with a pencil, there seems to be quite a lot of friction - which seems like it would induce heat. How hot would the tip of a #2 pencil get writing on normal copy paper?
18
votes
4answers
13k views

Why does the air we blow/exhale out from our mouths change from hot to cold depending on the size of the opening we make with our mouth?

Why does the air we blow/exhale out from our mouths change from hot to cold depending on the size of the opening we make with our mouth? It's not just a subtle difference, but significant in my ...
18
votes
2answers
441 views

Why aren't gas planets and stars fuzzy? [duplicate]

The edge of Jupiter looks very sharp. Even more bothersome, the edge of the sun looks sharp, aside from kind of a soup of particles floating above it. The sun's surface has an incredibly low ...
17
votes
6answers
2k views

What exactly is heat?

Is it energy? Is it energy per unit volume? Is it energy per unit time i.e power? What is it?
17
votes
6answers
3k views

Why does ice form on bridges even if the temperature is above freezing?

So with this "arctic blast" continuing, I've noticed that for my area, the temperature drops below freezing just long enough to cause freezing rain, but then the sun comes out and the temperature ...
17
votes
6answers
1k views

What are the various physical mechanisms for energy transfer to the photon during blackbody emission?

By conservation of energy, the solid is left in a lower energy state following emission of a photon. Clearly absorption and emission balance at thermal equilibrium, however, thermodynamic equilibrium ...
17
votes
4answers
1k views

Home experiment to estimate Avogadro's number?

How to get an approximation of Avogadro or Boltzmann constant through experimental means accessible by an hobbyist ?
17
votes
4answers
1k views

Does the Moon's core still contain significant heat?

On earth, using earth-sheltering techniques can significantly reduce the temperature fluctuations on a structure. Would the same statement be true as well on the Moon? Does the Moon's core still ...
17
votes
3answers
508 views

Intuitive understanding of the entropy equation

In thermodynamics, entropy is defined as $ d S = \dfrac{\delta q_{\rm }}{T}$. This definition guarantees that heat will transfer from hot to cold, which is the second law of thermodynamics. But, why ...
16
votes
7answers
6k views

Does an empty refrigerator require more power to stay cold than a full one?

Given that everything else is equal (model of fridge, temperature settings, external temperature, altitude), over a given duration of having the door closed, does it require more electricity to cool ...
16
votes
4answers
2k views

How do you prove $S=-\sum p\ln p$?

How does one prove the formula for entropy $S=-\sum p\ln p$? Obviously systems on the microscopic level are fully determined by the microscopic equations of motion. So if you want to introduce a law ...
16
votes
6answers
981 views

Do all massive bodies emit Hawking radiation?

It is known that any accelerated observer is subject to a heat bath due to Unruh radiation. The principle of equivalence suggests that any stationary observer on the surface of a massive body should ...
15
votes
5answers
2k views

Why isn't absolute $0 K$ temperature possible?

So $T$ is defined as $$T = \left(\frac{\partial E}{\partial S}\right)$$ and $S$ is defined as $$S = k_B \ln \Omega$$ where $\Omega$ is the number of accessible states of the system for a given ...
15
votes
4answers
2k views

Where does deleted information go?

I've heard that, in classical and quantum mechanics, the law of conservation of information holds. I always wonder where my deleted files and folders have gone on my computer. It must be somewhere I ...
15
votes
4answers
3k views

Why does the nature always prefer low energy and maximum entropy?

Why does the nature always prefer low energy and maximum entropy? I've just learned electrostatics and I still have no idea why like charges repel each other. ...
15
votes
1answer
813 views

Why doesn't breath condense around poop of flies on the window's glass?

One day it was raining outside and I was breathing on the glass of my window. I noticed that around the spots of fly poop there is a circle where vapor does not condense. With time the circle becomes ...
15
votes
2answers
1k views

Is this Landau's other critical phenomena mistake?

There was an old argument by Landau that while the liquid gas transition can have a critical point, the solid-liquid transition cannot. This argument says that the solid breaks translational symmetry, ...
15
votes
6answers
1k views

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 ...
14
votes
3answers
3k views

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?
14
votes
1answer
332 views

Why does a water drop on a hot plate at $150^o C$ evaporate faster than on a plate at $200^o C$?

I recently read that: "A drop of water landing on a hot plate at $150^o C \:(300 F)$ evaporates in a few seconds. A drop of water landing on a hot plate at $200^o C \:(400 F)$ survives a whole ...
14
votes
3answers
3k views

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?
14
votes
3answers
469 views

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. ...
14
votes
1answer
1k views

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 ...