19 votes

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 ...
12 votes

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 (...
7 votes

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
4 votes
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
3 votes

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
3 votes
Accepted

Name for a certain 1D lattice model

The limiting case $k_{\rm max}\to\infty$ is well known. When the interaction is $\sum_{k=1}^{n-1} |k_i - k_{i-1}|$, this is known as the (one-dimensional) SOS model; when the interaction is $\sum_{k=1}...
  • 8,276
3 votes
Accepted

Is it safe to say that temperature is a measure of molecular activity?

You can say that but it would be rather vague, because there are other physical concepts which might also be called a measure of molecular activity, such as energy, speed, fugacity, etc. Temperature ...
2 votes

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

There are only two mechanisms by which the entropy of a closed system can change: By heat transfer between the surroundings and the system at the location and temperature of the boundary between the ...
  • 28.8k
2 votes

Is enthalpy relevant to aerodynamics?

For subsonic flow, the pressure changes in a flow field over a wing for example are small and transient and do not involve heat flow between adjacent parcels of the fluid. This means we need not ...
2 votes

Work in thermodynamics and work in mechanics

work in mechanics $w=\vec{f} \cdot \vec{s}$ work in thermodynamics $w=-p \Delta v$ I don't understand why $ \vec{f} \cdot \vec{s}= -p \Delta v$ ? If the work is performed by compressing a volume ...
  • 9,521
2 votes
Accepted

Does pressure decay exponentially for a mixture of gases?

Yes; the sum of exponential functions with different coefficients is not an exponential function but in this case looks approximately like an exponential function, considering that most of the ...
1 vote

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

You can also reason from the formula for the entropy change of an ideal gas. \begin{align} \Delta S &= C_V \ln\left(\frac{T}{T_0}\right) + Nk \ln\left(\frac{V}{V_0}\right), \end{align} where $...
  • 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 ...
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 ...
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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