# Tag Info

### Do all solids sublimate at low enough temperature and pressure?

Yes, all condensed matter is expected to spontaneously sublimate/evaporate at any pressure and temperature. (Put another way, all materials have a nonzero vapor pressure.) The standard argument is ...
• 15.5k

### Why isn't the free expansion of a gas in an adiabatic container isentropic?

The free expansion isn’t reversible because the gas flows down a pressure gradient (that arises when you remove the piston). Any energy flow down a gradient generates entropy. In contrast, during the (...
• 15.5k

### Why isn't the free expansion of a gas in an adiabatic container isentropic?

If you expand a gas adiabatically using a piston, the process is isoentropic. An adiabatic process is not isentropic unless it is also reversible. To be reversible, it must be carried out quasi ...
• 58.4k
Accepted

### How does the density of the air affect our perception of temperature?

A 1 meter diameter sphere is not bad, but I think I will skip factors of $4\pi$ and just take the surface area to be $2m^{2}$, which corresponds well with what doctors tend to use. Your body is ...
• 4,537

### Why does sound travel faster while light travels slower in hotter mediums?

I suppose the answer you’re looking for is that light is a wave without a medium, whereas sound is not. The fact that light can propagate in vacuum at all was not originally obvious, and people ...
• 11.1k
Accepted

• 21.2k
1 vote

### Can trains use permanent magnets to be propelled?

Permanent magnets might allow you to furnish the levitation force needed to lift the train out of contact with the rails, but because they are permanent there is no way to switch them on and off and ...
• 76.7k
1 vote

### Energy increasing with spacetime expansion?

Accelerating expansion of the universe means that in our reference frame the kinetic energy of the galaxies (those outside of our local gravitationally bound cluster) would increase with time as those ...
• 13.1k
1 vote

### Why doesn't the phase diagram of water look different?

It is fundamentally related to the probabilistic/statistical nature of thermal motion. Actually, the material will be subliming at any temperature above absolute zero - it's just that the rate ...
1 vote

### How does the adiabatic coefficient $γ$ vary with temperature (200K-20000K)?

One place to start is the NASA Technical Reports Server (ntrs. nasa. gov - take out the spaces) and get Thermodynamic and Transport Properties of Air and the Combustion Products of Natural Gas and ...
• 236
1 vote

### Question regarding Heat transfer in Carnot Engine

First, it is not quite right to say that some other kind of heat engine is practical while the Carnot engine is not. Rather, many kinds of heat engine can be made to work in actual practice, and they ...
• 48.7k
1 vote

### How do you find the rate of heat flow through a multilayered wall?

In Fourrier law you have conductance and Resistance. I suppose your L/k ist the resistance? than Resistances jaust ad up; see wiki https://en.wikipedia.org/wiki/Thermal_conduction
• 4,095
1 vote

### Why is the rate of heat transfer constant throughout different materials?

First principle of thermodynamics, $d E = \delta W + \delta Q$ with no work $\delta W = 0$, and steady conditions $d E = 0$ (no variation of the energy of the system), give you $\delta Q = 0$. If you ...
• 2,273
1 vote

### Why is the rate of heat transfer constant throughout different materials?

The statement Heat flow in = heat flows out is made on the assumption that no heat escapes from the sides, ie the sides have an ideal thermal insulator around them. It is a restatement of the law of ...
• 81k
1 vote
Accepted

### How can flow devices be analyzed by choosing a closed system, i.e. a fixed quantity of matter?

Moran et al have the derivation of the open system 1st law equations, based on a closed system. See section 4.4. I would have drawn Fig.4.5 a little differently, with "before" and "...
• 28.8k
1 vote

### How can flow devices be analyzed by choosing a closed system, i.e. a fixed quantity of matter?

You can use integral balance equations for a material volume (Lagrangian approach, closed system), for a fixed control volume (Eulerian approach) or for a arbitrary control volume. You only need to ...
• 2,273
1 vote

### Why energy fluctuates in canonical ensemble?

There is a connection between the temperature of the system and the average energy. In the canonical ensemble, the temperature and the average energy of the system are constant. Here, the average ...
• 622
1 vote

### Ohm and Fourier's law with Seebeck effect - derivation?

Check Landau & Lifshitz volumes 8 "Electrodynamics of Continuous Media" chapter III ("constant current"), paragraph named "Thermoelectric Phenomena" for the origin of ...
• 244

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