# Tag Info

84

Air molecules $(\require{mhchem}\ce{N2_}$ and $\ce{O_2})$ have an average velocity of around $500\text{ m/s}$, varying some depending on the temperature. This means that a nice $5\text{ m/s}$ wind is a hundred times slower, and the energy represented by wind is 10,000 times smaller than the thermal energy. Therefore, wind does not have considerably more ...

60

Short answer: The thermometer measures actual temperature (which is the same for both), while your hand measures the transfer of energy (heat), which is higher for the pot than the air. Long answer: Keyword: Thermal Conductivity The difference is a material-specific parameter called thermal conductivity. If you are in contact with some material (gas, ...

56

The reason is because the heat loss occurs mostly in the windows and the fenestration. The idea is that you would like the incoming air to be heated up. Also, it creates an air curtain that prevents more heat from being lost through this exposed areas. The final reason is to make the temperature of the room more or less uniform. If the heaters were placed at ...

51

You blow away the flame from its fuel source. If you would blow less hard the flame might burn harder because more air is supplied to the flame (similar to a Bunsen burner). Because normally the flame of a candle gets its oxygen through a convectional airflow generated by the heat of the flame. The reason why the flame is blown away from the candle is ...

49

Partly practical, the wall under the windows isn't useful for anything else. We had a house where the heaters were placed in the middle of the only empty walls, so nowhere you could put furniture, bookcases, etc. Before double glazing there would be a draft from the windows so the idea was to heat this incoming air by having a radiator immediately below the ...

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No. Boiling itself doesn't mean that the water will cook anything. If you have boiling water at 30°C you could touch it (if we forget that it's at really low pressure) and nothing would happen. Boiling is not what cooks, but temperature. In fact, if you want to purify water at high altitudes, you need to boil water for a longer time because it will be at a ...

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It's obviously not a sharp cut-off, but as a general guide sound waves cannot propagate if their wavelength is equal to or less than the mean free path of the gas molecules. This means that even for arbitrarily low pressures sound will still propagate provided the wavelength is long enough. Possibly this is stretching a point, but even in interstellar gas ...

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This is a very good question. Einstein himself, in a 1907 review (available in translation as Am. J. Phys. 45, 512 (1977), e.g. here), and Planck, one year later, assumed the first and second law of thermodynamics to be covariant, and derived from that the following transformation rule for the temperature: $$T' = T/\gamma, \quad \gamma = ... 39 Since this is a physics forum I assume the OP is interested in a quantitative answer in terms of the efficiency of the system and how it differs based on the relative positioning of heat sources and heat sinks. The math required to analyzed such a system is too much for me to manage right now, but I believe the following principles apply and are objectively ... 32 We can only approach absolute zero asymptotically because we can't suck heat out of a system. The only way we can get heat out is to place our system in contact with something cooler and let the heat flow from hot to cold as it usually does. Since there is nothing colder than absolute zero, we can never get all the heat to flow out of a system. We can ... 27 Short Answer The information is contained in the heat given off by erasing the information. Landauer' Principle states that erasing information in a computation, being a thermodynamically irreversible process, must give off heat proportional to the amount of information erased in order to satisfy the second law of thermodynamics. The emitted information is ... 27 The separation does not violate the 2nd law of thermodynamics, because the oil and water phases being separate is a lower energy state. The water molecules strongly interact with each other, forming hydrogen bonds. The protons of water are shared between two oxygen atoms of two different water molecules, forming a constantly changing network of molecules. ... 24 We want to show that "infinitesimal" changes in heat along a given path in thermodynamic state space can be modeled via a differential 1-form conventionally called \delta Q. The strategy. We introduce a certain kind mathematical object called a cochain. We argue that in thermodynamics, heat can naturally be modeled by a cochain. We note a mathematical ... 22 The answer is a combination of physics and physiology. The warm water in the shower very quickly heats up the air in the shower, and warms up your skin. It also drives up the humidity of the air in the shower. You acclimate very quickly to the temperature/humidity conditions in the shower as being "normal". With the door left open a crack, you allow ... 21 Actually, temperature is defined as$$\frac{1}{T} = \frac{\partial S}{\partial E} = \frac{k_B}{\Omega}\frac{\partial\Omega}{\partial E} So in order to have zero temperature, you would need a system with either zero multiplicity, which you can't have by definition, or an infinite derivative $\partial\Omega/\partial E$ even though the multiplicity itself ...

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Ahem, I come to you from Seasoned Advice (cooking). As Beta suggested, questions like this one would not be uncommon there. The agitation of boiling water has nothing to do with cooking pasta except in that it helps keep the pasta from sticking. Whether it makes good pasta to hydrate it without heat (or at least a lot of heat) is a source of some debate, ...

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First, strictly speaking a neutron star is not a nucleus since it is bound together by gravity rather than the strong force. Measuring a surface temperature for any star is deceptively simple. All that is needed is a spectrum, which gives the luminous flux (or similar quantity) as a function of photon wavelength. There will be a broad thermal peak somewhere ...

19

It's not poop. It's fly barf. A fly spends about 25% of its time re-digesting and it only can eat liquids. It mixes the eaten food with the appropriate enzyme for digestion. The fly does this by retrieving the eaten food from its digestive system (a vomit of sorts), and drop by drop it is placed on the surface on which the fly is sitting. Only then is it ...

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Combustion is a gas phase reaction. There are two requirements to generate a stable flame. Firstly the temperature must be high enough to vapourise the combustible material (wax in this case), and secondly the temperature must be high enough to generate the activation energy needed for the reaction. Heat is needed because gas phase molecules of wax and ...

17

If the particles are not point-like, they will take up some volume. As the gas is compressed, the collision frequency will rise more quickly, which will make the pressure-volume curve change. The corrections in the Van der Waals model of a real gas accounts for the volume of the particles. Also if they have internal structure, that structure can have ...

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There are ways to define "temperatures" for single molecules or atoms, but they are subtly different than the way in which temperature is defined for bulk material. Temperature is essentially a statistical quantity as averaged over the various internal states of the material, and for large composite systems at equilibrium, the internal energy levels are ...

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As a supplement to @tpg2114's answer, it also depends on the "wetness" of an object. As most people should know the evaporation of water requires energy and this lowers the temperature. The lowest temperature a wet object can reach is what is called the "wet-bulb temperature." This can be several degrees lower than the "dry-bulb temperature," the amount ...

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It can't be from the moisture in the air. If there was enough moisture in the air to produce condensation then it would be condensing on everything. There would actually be less of it condensing on the tailpipe, because the tailpipe is quite warm. In fact the water is generated by the combustion of the fuel in the car. It comes from the hydrogen in the ...

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Your statement is somewhat subjective, so really can only be answered by trying to put together what thoughts about physics such great physicists were thinking when they made their statements. Firstly, the laws of thermodynamics have very different origins and putative theoretical justifications and indeed the Eddington quote only talks about the second. I ...

17

I know black conducts heat while white reflects it. The correct term is "black absorbs light while white reflects it". We have named colors of light we see in the visible spectrum . White reflects most of the energy falling from the visible spectrum, black absorbs it. When the energy of light is absorbed it turns into heat . Any material painted ...

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That is the Leidenfrost effect. If the surface is hot enough, a layer of vapor exists between the hot surface and the droplet, insulating the droplet from the full heat. The droplet levitates above the hot surface.

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The water gets colder the longer you run it (in the UK at least) because the water mains pipes buried in the ground are colder than the ones in your house, so sadly this isn't evidence for any fundamental physical effect. In principle any fluid flowing in a pipe gets hotter because energy is dissipated in viscous flow. You could in principle calculate the ...

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Well, I can share with you one experience from my high school. I wanted to boil coffee in my caffetier without a cooker. We had the vacuum pump in the physics room so there was the way to "boil" the water without getting it in 100°C. I did it... and coffee tasted horrible. Never try it again.

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The other answers address your question quite well. Just as a reminder of the ability to be burned by a strong enough wind, the image below shows the Chelyabinsk meteor during entry into Earth's atmosphere last year over Russia. :)

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Typically freezing rain falls as water because the air is warm enough that it won't be ice, but then it freezes when it's on the surface of things because those surfaces are below freezing. This usually happens with roads and the ground when it's been a long time of very cold temperatures. Bridges also tend to get colder, faster, than other roads because ...

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