# Questions tagged [thermodynamics]

Covers the study of (primarily homogeneous) macroscopic systems from a heat/energy/entropy point of view. Consider also using the tag: [statistical-mechanics].

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### Cooling a cup of coffee with help of a spoon

During 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 be ...
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### Did the Big Bang happen at a point?

TV documentaries invariably show the Big Bang as an exploding ball of fire expanding outwards. Did the Big Bang really explode outwards from a point like this? If not, what did happen?
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### Why are the wet patches on these floor tiles circular?

My friend's 3-year-old daughter asked "Why are there circles there?" It had either rained the night before or frost had thawed. What explains the circles? Follow-up question: Ideally, are these ...
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### 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 ...
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### Why is my hand not burned by the air in an oven at 200 °C?

I have this problem from University Physics with Modern Physics (13th Edition): The inside of an oven is at a temperature of 200 °C (392 °F). You can put your hand in the oven without injury as ...
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### Why doesn't water boil in the oven?

I put a pot of water in the oven at $\mathrm{500^\circ F}$ ($\mathrm{260^\circ C}$ , $\mathrm{533 K}$). Over time most of the water evaporated away but it never boiled. Why doesn't it boil?
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### Why does ice cream get harder when colder?

What would seem to be a silly question actually does have some depth to it. I was trying to scoop out some of my favorite soft name-brand ice cream when I noticed it was frozen solid, rather than its ...
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### Why can I touch aluminum foil in the oven and not get burned?

I cook frequently with aluminum foil as a cover in the oven. When it's time to remove the foil and cook uncovered, I find I can handle it with my bare hands, and it's barely warm. What are the ...
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### 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 ...
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### 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 ...
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### Is it possible to start fire using moonlight?

You can start fire by focusing the sunlight using the magnifying glass. I searched the web whether you can do the same using moonlight. And found this and this - the first two in Google search ...
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### What is time, does it flow, and if so what defines its direction?

This is an attempt to gather together the various questions about time that have been asked on this site and provide a single set of hopefully authoritative answers. Specifically we attempt to address ...
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### Explain it to me like I'm a physics grad: Greenhouse Effect

What is the mechanism by which increasing $\rm CO_2$ (or other greenhouse gases) ends up increasing the temperature at (near) the surface of the Earth? Mostly what I'm looking for is a big-picture ...
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### Would a pin head heated to 15 million degrees Celsius kill everyone in a 1000 mile radius?

The YouTube video How Hot Can it Get? contains, at the 2:33 mark, the following claim: A pin head heated to 15 million degrees will kill everyone in a 1000 miles radius. On what basis can this ...
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### What is the speed of sound in space?

Given that space is not a perfect vacuum, what is the speed of sound therein? Google was not very helpful in this regard, as the only answer I found was $300\,{\rm km}\,{\rm s}^{-1}$, from Astronomy ...
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### Where is the flaw in this machine that decreases the entropy of a closed system?

I was thinking about a completely unrelated problem (Quantum Field Theory Peskin & Schroeder kind of unrelated!) when the diagram below sprang into my mind for no apparent reason. After some ...
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### Why does the shower curtain move towards me when I am taking a hot shower?

When I am taking a shower, the shower curtain slowly moves towards my legs. Also, it seems that the hotter the water, the faster it gets to my skin. Why is that?
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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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### 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 ...
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### Why does matter exist in 3 states (liquids, solid, gas)?

Why does matter on the earth exist in three states? Why cannot all matter exist in only one state (i.e. solid/liquid/gas)?
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### Will a hole cut into a metal disk expand or shrink when the disc is heated?

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? What will happen to the diameter of disc?
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### Why does fire make very little sound?

Sound is air particles vibrating (thus hitting each other to make longitudinal waves) and heat is the vibration of air molecules. Because we can only assume that heat made from fire is a higher ...
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### Why does water stop boiling immediately after turning off the heat?

When I am heating water on a gas stove, it begins to boil after some time and bubbles of air can be seen escaping out. However, as soon as I increase the amount of heat in the stove, the rate of ...
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### Why is there no absolute maximum temperature?

If temperature makes particles vibrate faster, and movement is limited by the speed of light, then I would assume that temperature must be limited as well. Why is there no limit?
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### If I stood next to a piece of metal heated to a million degrees, but in a perfect vacuum, would I feel hot?

A friend of mine told me that if you were to stand beside plate of metal that is millions of degrees hot, inside a 100% vacuum, you would not feel its heat. Is this true? I understand the reasoning ...
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### 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 the ...
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### Why is it cold on the sea floor if pressure heats things?

I was reading this and it says that Microsoft put a server farm at the bottom of the ocean because it's cooler there. Particularly it seems to imply that it get's colder as you go deeper, "Since ocean ...
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### Is the Boltzmann constant really that important?

I read a book in which one chapter gave a speech about the fundamental constants of the Universe, and I remember it stated this: If the mass of an electron, the Planck constant, the speed of light, ...
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### 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 extinguish the flame by cooling it as it is itself warmer ...
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### Could a candle theoretically melt iron?

The title question is rather illustrative. I suppose the real question would be: Is heat cumulative? Put back into an example: If I have a lit candle right beneath an iron bar, assuming the candle ...
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### When I boil a kettle, what stops all the water from turning (exploding!) in to steam in one go once it reaches 100°C?

While making a cup of tea in the office kitchen, a colleague asked me this question and neither of us could answer with any certainty. We're guessing it has something to do with the pressure of the ...
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### Why does a lot of water vapour come suddenly after the heat source of boiling water is removed? [duplicate]

I have noticed this several times. When I am boiling water, a few seconds before its boiling point, vapours are formed as usual. But if I turn the gas off before boiling, the moment it turns off, I ...
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### What makes cheese so effective at absorbing microwaves?

Whenever I put a meal in the microwave which contains cheese, why does the cheese get hot before the rest of the meal is heated through?
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### Why was water freezing almost instantaneously when shaking a bottle that spent the night outside during a frosty night?

Due to the forecasted frost last night, I placed yesterday evening, some 1.5l standard PET bottles filled up to 90% with warm tap water(+60°C) close to some vegetables that I wanted to protect in my ...
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### What does Enthalpy mean?

What is meant by enthalpy? My professor tells me "heat content". That literally makes no sense. Heat content, to me, means internal energy. But clearly, that is not what enthalpy is, considering: \$H=U+...
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### 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 ...
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### Why don't you get burned by the wood benches in a sauna?

When you go to the sauna you may sit in a room with 90°C+. If it is a "commercial" sauna it will be on for the whole day. How does it come that when you sit on the wood you don't get burned? I ...
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### Have researchers managed to "reverse time"? If so, what does that mean for physics?

According to press releases, researchers have reversed time in a quantum computer and violated the second law of thermodynamics. What does that mean for physics? Will it allow time travel? Further ...
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### The Sun is giving us a low entropy, not energy

While I was watching a popular science lecture on YouTube, I came across this sentence "Sun is giving us a low entropy, not energy" which was said by Prof. Krzysztof Meissner. I am not a ...
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### Why is it so inefficient to generate electricity by absorbing heat?

When I turn on a heater, it's supposed to be roughly 100% efficient. So it converts electricity to heat with great efficiency, but why can't we do the reverse: generate electricity by absorbing heat? ...
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### How do different definitions of entropy connect with each other?

In many places over the Internet, I have tried to understand entropy. Many definitions are presented, among which I can formulate three (please correct me if any definition is wrong): Entropy = ...
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### Why does pressure in a thermos increase after shaking up hot water and soap?

Whenever I wash my thermos, I put hot water and then some soap in; then I seal the one end with my hand or use the lid. After shaking it up, if I slowly remove the lid or my hand, it expels a little ...
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### How did my candle wax crawl up the sides of the jar?

I have an Ikea candle which has sat on my bookshelf in the sun for >5 years. Aside from an hour or two shortly after I bought the candle, I have not burned the candle regularly (in fact, the wick is ...
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### 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 ...
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### How strong is the force of ice expanding when freezing?

Why does water contract on melting whereas gold, lead, etc. expand on melting? reminded me about something I've been wondering myself for some time. We know that water expands as it freezes. The force ...
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### Does entropy depend on the observer?

Entropy as it is explained on this site is a Lorentz invariant. But, we can define it as a measure of information hidden from an observer in a physical system. In that sense, is entropy a relative ...
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### 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 ...
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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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