# Tag Info

76

Does any component in an incandescent lightbulb actually reach temperatures in the thousands of degrees? Yes, the filament. This is why the filaments are made of tungsten, which has a melting point of 3695K and can comfortably tolerate the temperature. The actual limitation is not the melting point but the evaporation rate of tungsten which becomes ...

70

Think about a frozen-over lake in the winter. The water underneath is liquid, but it doesn't melt the ice. In fact, it wasn't even able to stop the ice from freezing as the weather got colder in the winter. The surface of the lake was losing heat faster than it could soak up heat from the warmer water below, so it froze while the deeper water was still ...

66

When you pour the hot water in, the air inside the thermos is still quite cold (ambient temperature, approx.) But then when you shake it up the cold air is heated by the hot liquid. Gases expand considerably when heated, approximately acc. the Ideal Gas Law: $$pV=nRT$$ This causes a modest (and harmless) pressure increase in the flask, which is what you ...

56

Yes. A simpler way to look at this is that because freezing, as well as resublimation (turning a gas directly into a solid) emits heat. It may seem strange but consider this: you need to put in heat to make water turn from solid into liquid, so the inverse process should transfer the heat in the opposite direction. And that's what it does. In cold air the ...

54

TL;DR: You have probably not been exposed to 100°C water in either phase and even if you had, you could not have reasonably felt its temperature on account of receiving a third-degree burn. Hot water and steam are both dangerous but fundamentally different, so comparing them is like gorilla vs. shark. I am not exactly sure what you are comparing here, but if ...

52

There is another effect here which is significant, as follows. Warm water wants to evaporate, but in a flask-shaped container, the evaporation can take place only at the free surface of the water in the flask. Furthermore, as soon as the boundary layer of air right next to the warm water becomes saturated with vapor, the diffusion of water vapor into the air ...

50

First thing to notice is that the heat flow is limited, so the heat from the core does not flow to the surface instantaneously. Second point is that the surface of the Earth radiates energy to the space. The combination of these effects makes it possible to have a molten core but a cold surface.

49

The thermodynamic phase of a material is never a function of temperature only. The correct statement is that helium remains in a liquid state at whatever small temperature achievable in a laboratory at normal pressure. It is well known that $^4$He freezes into a crystalline solid at about 25 bar. Such peculiar behavior (helium is the only element remaining ...

48

What we define as "hot" or "cold" is the transfer of energy -- how much (quantity) and how fast (rate of transfer) -- and how it raises our temperature. The more energy that is transferred from the object quickly, the hotter the object feels. First, steam is in a vaporized phase -- which is why it has more energy. At 100 Celsius, water ...

42

Technically, it all is the same motion. The difference is magnitude and direction and how you separate out the superposition of them. Temperature is a result of the components of motion (vectors) of each individual air molecule with a net translational movement of zero over time. This motion is random and all over the place in many directions as they move ...

41

Here is another scenario where the thing that you describe does happen: A tube is filled with a gas, for example plain air. The tube fits nicely around a finger. The fit is so precise that there is a sufficient seal, so the air cannot escape, but there is only just enough friction between the tube wall and your finger to prevent the tube from sliding off ...

38

Not necessarily. Glass is transparent and it can be heated with convection, conduction and some radiation. It will maintain any temperature around it.

37

It's because of the crystal structure of the solids. When water freezes, the molecules form various structures of crystals which have empty gaps that cause the solid to be about 9% larger in volume than the liquid was. Metals usually form crystals when they freeze too, but they're often simpler crystals, if you will, and often don't have as much empty space ...

31

My counterargument: Intermolecular forces between molecules are either intact or broken. There is no in-between. Therefore, the change from intact to broken is instantaneous. I don't think this is a great argument, as bonds are forming and breaking all the time in real materials at finite temperatures. Liquids have some fraction of broken bonds compared to ...

29

Weather is... complex. Turning water vapor to solid releases heat, but this happens up in the sky. You end up with somewhat hotter and drier air somewhere at altitude and some amount of snow that falls down. (If "down" is hot enough, you get rain instead. The rain almost always starts as a snow.) The point is, you may, or more often may not get the ...

29

Although the premise of this question is, well, questionable, I’ll answer because invisibility cloaks do exist. Of course, as of now, they work imperfectly, in impractical ways, and for only a narrow spectrum of light. And this last part is the key. Just because something bends light of one color does not mean it will bend light of another. When one ...

28

This behavior is basically described by Newton cooling with heat generation, using the equation: $$MC\frac{dT}{dt}=G-k(T-T_{\infty})$$where T is the temperature, t is time, M is the mass, C is the heat capacity, G is the heating rate, k is the Newton cooling coefficient (convective heat transfer coefficient times surface area), and $T_{\infty}$ is the ...

26

Temperature is interesting in a logarithmic way. There is as much potential for interesting physics between $1\rm\,K$ and $10^{-6}\rm\,K$ as there is between $1\rm\,K$ and $10^{+6}\rm\,K$. The difference is that we live in the upper interval, so we have lots of experience and a finely-honed intuition which we call "classical thermodynamics." To ...

26

The melting phase transition transforms the long-range ordered-crystalline solid structure into the short-range-ordered average liquid structure. Looking at the process from the solid side, melting can be seen as the dramatic effect of a collective building-up of defects in the solid over a limited interval of temperature, eventually destroying the long-...

22

It's actually an ingenious, but relatively simple bit of physics and engineering. It works by compensating for the linear thermal expansion of the pendulum rod, utilizing the thermal expansion of the mercury but in the opposite direction and thus preserving the position of the center of gravity. Note that the period of the (compound) pendulum is given by T=...

18

You may try to find similarities between water and bismuth, both expand when solidify. Most other materials contract when solidify. It mostly depends on crystalline structure. Water in form of ice happens to have crystalline structure that takes more space than liquid water.

17

You are right, the answer you get this way is the temperature of the sun's photosphere (what we call the "surface" of the sun even though it is not a solid surface). Remember also that the temperature of an object is the mean of a distribution of energies possessed by all the atoms in it, so in this sense the "averaging" has already been ...

16

In order to undergo a phase change from a solid to liquid, or liquid to gas, the heat added needs to cause molecules to move farther apart from one another, in opposition to the intermolecular attraction forces, which in turn means it increases the internal potential energy of the substance. Thus the heat added increases internal potential energy and not ...

16

Why can't the Earth's core melt the whole planet? In other words, what is stopping Earth from being melted up to its surface? I'll rhetorically ask the reverse question: Why can't radiation to empty space freeze the whole planet? In other words, what is stopping Earth from being solid all the way down to its center? The answer to this reversed question is ...

16

Temperature is related to motion, yes. And wind is motion. But numbers matter. A particle with temperature $T$ (in absolute units, like Kelvin) will typically have a kinetic energy of approximately $kT$, where $k$ is Boltzmann’s constant. For nitrogen and oxygen molecules at room temperature, the typical speed associated with this typical kinetic energy is a ...

15

From the Wikipedia article on helium: Unlike any other element, helium will remain liquid down to absolute zero at normal pressures. This is a direct effect of quantum mechanics: specifically, the zero point energy of the system is too high to allow freezing. Solid helium requires a temperature of 1–1.5 K (about −272 °C or −457 °F) at about 25 bar (2.5 MPa) ...

14

One geometric way to look at it is as follows: Imagine that the solid sphere is in fact a collection of concentric shells (like a spherical Russian doll). The hollow sphere on the other hand obviously is exactly the same thing, except that it has only one shell, namely the outermost one. Let’s ignore the contribution of the inner shells in a first ...

13

It is not true that only the kinetic energy of translational motion of the molecules contributes to temperature. If it was true, there could be no difference between specific heats of mono- or diatomic molecules. The reason two other answers appeared at the moment I ended writing my answer say the opposite is related to the possibility of defining the ...

12

We can most definitely feel heat in space. As @aystack said, on earth heat transfer is through contact, convection or radiation. Convection relies on the movement of air or some other fluid, so that will only work inside a spacecraft or spacesuit. Transfer of heat through contact means that, if you touch something that is hot or cold, you will definitely ...

12

You have correctly deduced that the rate of heat transfer to the system is negative. This means that heat is leaving the system (that is leaving the filament). What is heat? Heat is energy in transit from a higher temperature region to a lower temperature region due to the temperature difference. In this case the filament is very hot and the surroundings are ...

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