# Tag Info

38

While I agree in principle with David Lynch's answer, I think it's good to take a closer look at the phase diagram (adapted from http://upload.wikimedia.org/wikipedia/commons/4/46/Carbon_basic_phase_diagram.png): I added the arrows to show possible paths you might follow. Red path: diamond would become graphite before melting; the molten carbon becomes ...

14

What follows is certainly not a comprehensive answer addressing all of your concerns. It is an answer to the question is there a way to see something clearly pathological like superluminal signals in the heat equation? I would argue that yes, there is. The general solution to the initial value problem $T(x,0) = T_0(x)$ for the heat equation on the ...

5

It may be worth pointing out that blankets also (surprisingly) act as (thermal) radiation shields. This is the reason that "emergency blankets" can sometimes be found in survival kits that appear to be nothing more than thin shiny plastic. But they really make a difference in the amount of heat lost by a warm (37 °C) body on a cold night (cloudless sky - ...

4

Is the vacuum a required part of the problem? The available action time is increased if it were only oxygen suddenly removed, baring panic. The best case scenario with planning and specific conditions met is about 20 minutes, http://www.guinnessworldrecords.com/world-records/1000/longest-time-breath-held-voluntarily-(male) (Out of water the max time is ...

4

The key here is the air curtain. You can be certain that if it didn't save Costco money, they wouldn't bother with it! It takes a bit of power to push air that much. Two very helpful diagrams are in the youtube video Powered Aire - Cold Storage Air Curtain: In the first case, the air can mix and change temperature through convection, and it does so in a ...

2

The rate of temperature change will be the power per unit mass times the specific heat. So if you have a certain mass of water $M$ flowing per second, at a velocity $v$, losing $\Delta P$ pressure per second, then work done is $v\Delta P A$ and $A = \frac{M}{\rho v}$ . Then with a heat capacity $c$ (about 4.2 kJ/kg/K for water), and the relationship between ...

2

How can I calculate the gas pressure given particles per cubic centimeter, and its temperature in Kelvin? as pointed out in comment by KyleKanos $PV=Nk_BT$ where $P$ is pressure, $V$ is volume (in $m^3$), $N$ is the number of particles, $k_B$ is Botzmann's constant and $T$ is temperature in Kelvin. If you rearrange it $P= {N \over V}~k_BT$ so ...

2

The line itself does not change much over years. What changes and therefore needs maintenance on power transmission lines is insulators, connectors and spacers. Insulators get dirty or simply break, connectors work loose due to thermal expansion and contraction, mechanical stresses and oxidation, and spacers can be damaged by wear due to these same ...

1

Some corrosion always takes place (pure gold is not used for transmission lines, AFAIK:-) ), so the conductivity decreases with time, although for some materials this effect can be very small ...

1

If you think about the infinite square well problem, the states with higher energy have higher momentum, (and also a higher velocity). However, it is better to think of the higher energy states as higher frequency standing waves. Because they have a higher frequency, the have to "travel faster", which is where the large velocity comes from in the Fermi ...

1

In a Fermi-Dirac distribution, the relationship between temperature and the speed of particles is not intuitive. Even at cold temperatures, fermions can have high speeds simply because of degeneracy - the lower momentum states "fill up", leaving only states with large momentum available, and this is true even at very cold temperatures. However, the heat ...

1

Normally your body heat would dissipate in the air, so when it's cold, your outer body cools down, because you are losing your body heat to the air near you. So when you cover yourself in a blanket, you stop your body heat from escaping, and as it is trapped, and your body continues to produce heat, you feel warmer and warmer under the blanket. Overall, the ...

1

The thermal conductivities of a range of materials is given here. I can't find figures for the thermal conductivity of solid wool or cotton (i.e. a solid block with no air voids in) but the thermal conductivities of organic materials seem to be around $0.25$ Wm$^{-1}$K$^{-1}$. By contrast the thermal conductivity of air is $0.024$ Wm$^{-1}$K$^{-1}$, so given ...

1

You don't have to discretize your problem (XY model). For each step, just take some value as the new $\theta$, and calculate the transition rate accordingly. Of course, when choosing the new value of $\theta$, better don't do it in a completely random way, otherwise your transition rate might be usually too small and you are just wasting time. Having said ...

1

The gas is expanded adiabatically and then isothermally. Thus the temperature it has at the end of adiabatic expansion stays the same even after the isothermal process. Ideal Gas equation after adiabatic expansion: $p_aV_a=nRT_a$, where index "a" shows after. You do not have $V_a, T_a$ in this equation. However, another equation you can write down is the ...

Only top voted, non community-wiki answers of a minimum length are eligible