# Tag Info

## Hot answers tagged thermodynamics

45

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 ...

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 ...

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 ...

16

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 ...

12

At the radius of the earth, the solar irradiance is approximately $1.412\;\mathrm{kW/m^2}$, giving a total power hitting the foil sheet (assuming normal incidence) of $\sim7.06\;\mathrm{MW}$. The average human in America is around $1.7\;\mathrm{m}$ tall, and somewhere around $0.5\;\mathrm{m}$ wide, making his cross sectional area around $0.85\;\mathrm{m^2}$. ...

11

I expect that there is no minimal pressure. A sound wave is a density wave. If the particles are close to each other they will interact due to strong forces like van der Waals force and Coulomb force. Reducing the pressure in constant volume leads to long distances between the particles. Lets assume the particles have a huge distance and we ignore even ...

11

The heat energy being moved does not contribute to any global warming. The A/C is simply moving energy from inside to outside; the heat just turns around and flows back into the room, and the total stays the same. It's like someone trying to keep a leaky boat from sinking; you dump some water over the side, and it (or some just like it) leaks back in. A ...

9

On your figures, it's going to depend on conditions, and also critically on what the referee was wearing; but in general it would certainly put the hapless referee in very dangerous position. It probably wouldn't be quite so dramatic as in the tale. On a typical day, let's say the Sun delivers $750{\rm\;W\;m^{-2}}$ intensity when straight overhead. Scale it ...

9

You have it backwards. You're coming from the point of view that being black makes something good at absorbing radiation and being white makes it bad and asking why this should be so. It's exactly the other way around: being good at absorbing radiation (in particular, visible light) makes something black; being bad at absorbing radiation (i.e., good at ...

6

The metal rod will become hotter. Only not very much for a large rod. The energy will flow from your fingers to the metal until the temperature of the metal reaches the temperature of your fingers. For a large metal object this will never happen for all practical purposes. For a small object, though, it does happen. If you pick up a dime it will ...

5

A lens can burn badly at the focal point, one can easily start a fire and get a bad burn if one is stupid enough to focus for long on skin. A $5 \ {\rm cm}$ diameter lens concentrates the power from about $2 \times 10^{-3}$ meter square. Taking the conservative $750 \ W/m^2$ (it can be $1200$ in my area) a power of $750\times (2 \times 10^{-3})$ on $2 \rm ... 5 Heat is a thermodynamic quantity defined in the framework of thermodynamics. Heat, in the strict use in physics, is characteristic only of a process, i.e. it is absorbed or produced as an energy exchange, always as a result of a temperature difference. Heat is thermal energy in the process of transfer or conversion across a boundary of one region of ... 5 If you stick to gases then things are relatively straightforward because the temperature is related to the relative velocity of the gas molecules, that is the velocity of the gas molecules relative to each other. If you put your canister of gas in a fast moving (but non-relativistic) rocket moving at some velocity$v$then you add the same velocity$v$to ... 5 First question: Is this equation applicable for irreversible processes? From the first law, we have: $$\mathrm{d} U = \mathrm{d}Q + \mathrm{d}W$$ where$\mathrm{d} U$is an exact differential, and$\mathrm{d}Q$and$\mathrm{d}W$are inexact differentials. It is thus remarkable to see that the sum of of two inexact differentials makes an exact ... 5 When a short circuit happens, the resistance is low ($R < 1\Omega$). Therefore, by Ohm's law, the current drawn is high (for example, with$V_\mathrm{rms} = 230\mathrm{V}$,$I_\mathrm{rms} > 230\mathrm{A}$). In this situation, the Joule effect is predominant and it follows$\mathcal{P} = UI = RI^2= \frac{V^{2}}{R} > 52\mathrm{kW}$. It also happens ... 4 As everyone else is saying, if you assume Newton's law of cooling: $$\dot Q = m c_p \dot T = h A \Delta T$$ The equation for how you heat or cool is an exponential $$T(t) = T_\infty + \Delta T e^{ -\frac{hA}{mc_p} t }$$ The rate constant for growth (or dying) of temperature is the same (assuming other details of the material don't change much), so ... 4 When a cup of coffee is hot, the air molecules directly above it get hot as well. After some time, they reach equilibrium and no heat transfer (or maybe very little transfer) occurs. By blowing, you disturb that equilibrium and replace the hot air molecules directly above the cup with colder air and therefore create once again a steeper temperature gradient. ... 4 To start with, "water freezes faster when it starts out hot" is not terribly precise. There are lots of different experiments you could try, over a huge range of initial conditions, that could all give different results. Wikipedia quotes an article Hot Water Can Freeze Faster Than Cold by Jeng which reviews approaches to the problem up to 2006 and proposes a ... 4 This happens due to cooling affect of evapourisation. As you must be knowing, the temperature of the lquid is a factor of evapourisation. So as the temperature of hot water is more, the rate of evapourisation is also more. Now this is where thwe cooling effect of evapourisation takes place. As the water evapourates, it takes away some heat thus cooling ... 4 There's a solar furnace in France that can melt steel. I don't know the collection area of it, but it's probably larger than 5000 m${}^2. Tin foil tablets ... I don't know. But it sounds like the sci fi story is not outlandish. 4 It's nothing else than Landau's 1941 two-fluid model of superfluids (similar to helium-4) that won him the 1962 Nobel prize in physics. Landau, L. D., The Theory of Superfluidity of Helium II, J. Phys. 5, 71 (1941) At temperatures near absolute zero, the fluids are composed of two components, the normal fluid we know from room temperatures (and whose ... 4 I agree it looks like a transverse wave - like the ripples on a pond. But I believe you are fooled by a simple thing: the waves you are looking at look like "illuminated ripples" but are in reality just changes in temperature (changes in brightness of the sun's surface). If you have a shock wave traveling out across the surface of the sun, what happens? The ... 4 Assuming the tube is insulated along its whole length (which I think is the intent of the question although it's not stated), I think a flow in either direction is stable and sustaining, but there is no particular reason it will form in either direction if the initial conditions are that the air in the tube is still. It will then depend on the average air ... 4 It's the differential relationship between internal energy and entropy: \begin{align} dU &= T\,dS + \cdots \\ \frac{\partial S}{\partial U} &= \frac 1T \end{align} As energy is added to a system, its internal entropy changes. Remember that the (total) entropy is $$S = k \ln\Omega,$$ where\Omega$is the number of available microscopic states that ... 3 Effectively zero, but it takes a mental stretch to get there. When you're dealing with a gas, lower pressure means that there is a longer mean free path, meaning the atoms/molecules can be expected to go longer and longer between collisions. You can get this either by spacing out the particles more (lower density) or by slowing them down (lower ... 3 Actually, in your rotating torus your are mimicking gravity, which is point outward. I.e. the outside of the torus acts as a floor. The centrifugal acceleration would be$g\approx\omega^2 R$. This$g$plays the same role as the gravitational acceleration on liquid or gas pressures under normal gravity conditions, so you can say$\Delta p = \rho g h$, or in ... 3 I believe it was Boltzmann who first made the connection between entropy and micro states. chapter 12 of "Classical and Statistical Thermodynamics" by Ashley H. Carter discusses Boltzmann's arguments. To summarize from that book: Entropy ($S\$) corresponds to a particular configuration of an ensemble of particles called a macro state. A macro state can be ...

3

If the air in the bubble is expanding, the gas should be doing work against the ocean, and the temperature should drop(in the case of adiabatic expansion). Since the temperature is said to remain constant, heat must be added to the bubble.

3

To greatly simplify John's answer: the temperature gained by friction and velocity is insignificant compared to the current temperature of the liquid. I expect frictional heating would kick in at higher pressures, but once the water had enough kinetic energy to heat up noticeably on impact with your hands, it would also have enough energy to strip the ...

3

But, as far as I know, colors don't have any special "substance" in them, which might trigger the sudden absorption of heat or reflection of the same. This statement appears to be the crux of your confusion, and it is false. When you paint an object black, or white, or any other color, you coat that object with a thin layer of a substance which, in ...

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