# Tag Info

Accepted

### Is sand in a vacuum a good thermal insulator?

Simply put: If sand in vacuum had a heat conductivity close to that of vacuum, i.e., at least much closer to zero than the heat conductivity of the silicon dioxide (aka glass) it consists of, ...
• 6,875
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### Can we use negative temperature for nuclear fusion?

The thermodynamic definition of temperature is based on the way that the entropy of a system changes as we increase the energy. We expect that as we increase the energy of a system it will become more ...
• 331k

### Is sand in a vacuum a good thermal insulator?

Powder filled vacuum (I don't know if sand per se is used for this purpose) is in fact used in cryogenic insulation, and can be better than vacuum alone, because vacuum alone is susceptible to radiant ...
• 2,499

### Is sand in a vacuum a good thermal insulator?

Comparing sand in air to sand in vacuum: The sand in vacuum is less conductive than sand in air in all reasonable conditions. The reason is, the air conducts some heat and vacuum doesn't, air can even ...
• 6,093

### How hot can steam be?

If you can manage to get the steam sample up to about 3000 C, roughly half of the water molecules in it will have split apart into oxygen and hydrogen, and won't be water anymore.
• 73.8k

### Interpretation of temperature in General Relativity

If this is referring to the comment after equation 2.7, the difference is actually coming from the difference between the time coordinate $\tau$ and the local proper time. There is one version of ...
• 171
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### Does standard enthalpy of reaction assume a constant temperature?

According to Wikipedia article Standard enthalpy of reaction (emphasis is mine): The standard enthalpy of a reaction is defined so as to depend simply upon the standard conditions that are specified ...
• 39.3k

### What is an intuitive explanation for $T = \mathrm{const}$ when $\Omega(E) = e^E$?

The basic intuition here is that temperature is not about number of microstates as such. Rather, it is about how the number of microstates varies with the energy---the standard definition of ...
• 47.1k
Accepted

### What is an intuitive explanation for $T = \mathrm{const}$ when $\Omega(E) = e^E$?

I'd say the relevant quantity here is $\omega (E):=\ln\,(\Omega (E)) = - \ln \,(1/\Omega(E))$. You can view $\omega$ as a measure of information or rather of missing information, in the sense of how ...
• 4,132

### Is sand in a vacuum a good thermal insulator?

It sounds reasonable. But there are a few more things to consider. How good a vacuum are you talking about? If you mean just good enough to make a better insulator, likely yes. But if you are pulling ...
• 27.2k
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### How to derive $TV^{1-\gamma}$? I get $\frac{T}{V^{1-\gamma}}$

On the fourth line, you forgot the $-$ sign on the RHS: $$\int_{V_i}^{V_f}-\frac{R}{C_V}\ \frac{dV}{V}=-\frac{R}{C_V}\,\ln\left(\frac{V_f}{V_i}\right)=-(\gamma-1)\ln\left(\frac{V_f}{V_i}\right)$$ ...
• 2,539
Accepted

### How can the Gibbs free energy equation be at constant temperature and pressure?

I have read that the equation $\Delta G = \Delta H-T\Delta S$ is valid only at constant temperature and pressure. Not quite; $G\equiv H-TS$, so $\Delta G\equiv \Delta H-T\Delta S$ requires only ...
Accepted

### How can the temperature of the surroundings be constant during a reversible thermodynamic process?

This is an example of 2+2=5. First of all, not all reversible processes are isothermal. You have reversible isobaric, isochoric and adiabatic processes. For a reversible adiabatic process, the ...
• 56.2k

### Could a window mounted wet cloth sheet chill a room?

Yes, if the humidity of the incoming air is low. This is the "evaporative cooling" (swamp cooler) method used for air conditioning in very dry climates (e.g., Albuquerque, NM). It is not ...
• 6,551
1 vote

### Is sand in a vacuum a good thermal insulator?

"the area of contact between adjacent grains of sand is very small, which means heat would transfer between grains relatively slowly" But there are many small grains so there are many ...
• 7,204
1 vote

### Moments of physical qualities other than density

It would be the "center of delta". Common values of delta are: pressure, buoyancy, gravity (which differs from mass), population (used in demography), and more.
• 23.2k
1 vote

### Calculating time it takes to heat water in a microwave and the steam to evaporate

First we break the question up into different processes, and then determine the time that each took. Phases: A) Water heats up to 100∘ → B) Phase transition Water to Steam → until P hits 10 atm (...
• 614
1 vote

### Pressure in a gas as it becomes a plasma

At some point (probably before ionizing the hydrogen in a plasma) the thermodynamic concept of pressure will stop having a physical meaning. So one can just compute the second velocity moment (e.g., ...
• 14.4k
1 vote

### Shockley diode equation - temperature influence

I was struggling with this same issue and finally found the answer! The first reference is another post: Voltage across diode, Shockley equation. This references Ken Kuhn's Diode Characteristics ...
1 vote

### Can anyone explain the REAL reason why CO$_2$ increases global temperatures (not the simplistic greenhouse analogy provided for public consumption)?

FWIW, here is my answer from the Earth Sciences SE, the key is to focus on how energy leaves the planet, rather than the fate of IR photons emitted from the surface: Borrowing an explanation from one ...

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