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23

What I fail to see is how moving too quickly could also impair cooling performance as stated in a lot of online forums. One argument I clearly remember from reading about this a while back was: and: You shouldn't crank the pump speed too fast or the water won't have time to pick up the heat from the waterblock as well. The latter statement you quoted ...


10

You have to be careful to distinguish between microstates and macrostates. Thermodynamic equilibrium is a macrostate which consists of a mixture of all possible microstates of energy $E$ weighted by a Boltzmann weight $e^{- \beta E} / Z$. A state in macroscopic thermal equilibrium can be thought of as "moving through phase space" ergodically (i.e. the ...


6

The temperature appearing the the Clausius inequality is definitely the temperature of the "boundary interface (with the surroundings)", or simply the temperature of the sources. One of the best places I have seen this discussion is in Fermi's book, chapter 5, section 11. He is explicit about it. To see this you have to recapitulate the steps in obtaining ...


6

There are two definitions of entropy, which physicists believe to be the same (modulo the dimensional Boltzman scaling constant) and a postulate of their sameness has so far yielded agreement between what is theoretically foretold and what is experimentally observed. There are theoretical grounds, namely most of the subject of statistical mechanics, for our ...


4

The Earth+windmill system has conserved angular momentum. When the windmill starts spinning the angular momentum of Earth must change in response. However this change is marginal. Furthermore the windmill system will stop spinning when the wind dies down and this will restore the original angular momentum of Earth (when I say Earth I mean everything inside ...


4

$1$ litre of water will remain almost $1$ litre as long as it is in the liquid state, no matter what the temperature is. The following formula gives you an order of magnitude estimate of the expansion: $$\Delta V=V_0\ \Delta T \ \beta$$ where $\beta$ is the coefficient of thermal expansion and $V_0$ is the initial volume. For water, $\beta \approx 10^{-...


4

Real world assumptions: No temperature gradients over the cross-section of the pipe. Plug flow (turbulent flow). Water heat capacity $c_p$ and density $\rho$ are temperature invariant. Consider an infinitesimal mass element $dm$ at temperature $T(x)$ travelling down the pipe at speed $v$. We apply Newton's law of cooling to it: $$\frac{dQ}{dt}=hdA\big(...


4

It can't fall slower as the first cosmical speed (7.8 km/s), which is still very high. Although it would cause much smaller destruction as it would hit directly with the mean speed of the meteors (10-70km/s). The lower angle of the hit doesn't play a significant role, because considering its mass, the interaction with the atmosphere will be probably ...


3

This is an answer adapted from Rococo and Wolphram jonny's comments plus a little googling. Thermodynamics in the presence of gravity is no longer extensive (even classical gravity) due to the long range nature of gravity. This is one of the reasons why people developed non-extensive thermodynamics, like Tsallis statistics. Tsallis Statistics Tsallis ...


3

Water boils at $100 ^\circ~\mathrm C$. $30$-$45^\circ~ \mathrm{C}$ is not nearly enough to boil water, but the constant heat of the sun is enough to evaporate water over time. It takes about $4.314~\mathrm{J/K}$ to evaporate 1 kg of water (under normal atmospheric pressure). The can slowly evaporate the water on the ground, but $30$-$45 ^\circ~\mathrm{C}$ is ...


3

Whether an answer exists depends on your definition of "near" compared to STP. There are a few fluids that have their critical point at a temperature close to STP, but higher pressure. For example, (see http://www.engineeringtoolbox.com/critical-point-d_997.html) material Tc(K) Pc(atm) acetylene 309.5 61.6 ethylene 283.1 50.5 ethane ...


3

The 'go to' partial differential equation here is surely the Heat equation (Fourier), here in one dimension: $$\frac{\partial T}{\partial t}=\kappa \frac{\partial^2T}{\partial x^2}+\frac{\dot{Q}(x,t)}{c_p\rho }$$ It can be easily expanded into three dimensions or expressed in polar, spherical or cylindrical coordinates. It's not clear from your question ...


2

Gold will compress to about half of its volume at atmospheric pressure if you compress it to 2 million atmospheres at room temperature, which is something that I'm sure has been done with diamond anvil cells. For many metals, the atomic lattice will also undergo structural phase transitions from one lattice type to another at certain pressures, but I don't ...


2

The critical pressure is given by $$P_c=\frac{a}{27b^2},$$ while the critical temperature is $$T_c=\frac{8a}{27bR}=\frac{8bP_c}{R}.$$ The parameter $b$ is related to to the effective volume occupied by the molecules, $$b=4N_0V_0,$$ where $V_0$ is the volume of the molecule and $N_0$ is the Avogadro number. So at least theoretically you can chose $P_c=1\, \...


2

Since my contribution is not valued and appreciated. This will be my last post here. What really entropy is? Answer: Information or more precisely the inverse of information What is a major characteristic of entropy? Answer: The more even, the higher entropy (the less information). Now, let's get to the rigorous part. This definition of entropy will ...


2

The relationship between thermodynamic and information entropy is still a work in progress. However, it is clear that he thermodynamic entropy is a rough measure of disorder, in the sense of not knowing in which specific microstate, all compatible with a given macrostate, the system is. Thus there is at least a loose relationship between thermodynamic ...


1

It is not true in general that liquids expand linearly with the temperature but it might be true for a particular liquid and a particular temperature scale. Let us say you have mercury. You put that Mercury inside your fridge and after some time you measure its volume and define this Mercury is at $0\, \mathrm{X}$, where $\mathrm{X}$ is the scale you are ...


1

Well, no, nobody has any idea how it happened. But personally that sounds kind of nuts. Entropy is not like a Pac-Man game, where if you go off the left side of the screen you pop up on the right. One way to check a theory is to look at its implications. This idea raises all kinds of problems. If (as suggested by Boltzmann, and is behind the Boltzmann's ...


1

Volume is not a meaningful measure of quantity, for the reason you hint at in your question. You can say how many moles (or grams) of water you drank - more useful if you want to know about the impact on your body chemistry. This is related to my answer about scales measuring in grams rather than Newtons. Can you see how?


1

Coriolis force on a moving object originates when you look at its motion from a non-inertial reference frame. Earth is a non-inertial reference frame, but that fact alone is not sufficient to cause Coriolis force on air. Air must also be set into radial motion by some agency, that agency being our Sun, as pointed out by @James Rowland and @CuriousOne.


1

To calculate the amount of heat energy needed to heat a pot with its content of water we can use a simple formula: $$\Delta H=mc_p(T_2-T_1)$$ Where $\Delta H$ is the heat energy needed to heat an object of mass $m$ and specific heat capacity $c_p$ from $T_1$ to $T_2$. For a pot with water these energies would need to be calculated for both separately and ...


1

This is an interesting question and one that probably needs detailed simulation to settle. But one can make the following broad prediction: the shape of the meteorite would have minimal effect on the outcome, for the following reasons: At the kinds energies let slip in the moments of impact and the kinds of pressures and temperatures that prevail, all ...


1

You can wait three or four minutes and let the pressure drop or..... If you feel lucky you can try tapping the can a dozen times, as is shown on this video https://youtu.be/NQYO3Dp8lCA


1

I tried to look around but I couldn't find anything. It does seem like supercritical $\mathrm{CO_2}$ is very popular in applications because the critical temperature is just a little over $30^{\circ}\mathrm{C}$, but it still requires 73 atmospheres of pressure. An interesting thing mentioned on the Wiki page is that Venus may have had supercritical $\mathrm{...


1

From a biochemical point of view, heat detection is achieved by proteins at the surface of nerve cells. They basically just trigger a nerve signal above a given temperature. So they DO detect temperature and not a "heat flux". It may seem surprising that nerve cells react so quickly but the increase/decrease in temperature does not need to go all through the ...


1

What is entropy really? I want to answer(!) this question from a different point of view. First off, I focus on your title and the phrase “really”. We don’t know what entropy is really. We don’t know what energy is really also, and any thing or concept else too. Entropy, like all other concepts created by humans, is a convention between some people to ...



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