New answers tagged thermodynamics
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Understanding entropy and its connection to probability distributions
The difference comes from the distinction between microstates and macrostates. It is the distribution over the microstates that is uniform. However, macrostates encompass many microstates with varying ...
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Heating object in airless environment
The evacuated container is very similar to a thermos flask (or a cryostat) so the object in there would (under most scenarios) heat up slower and end up melting later.
By removing the air you disabled ...
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Heating object in airless environment
... there is no heat transfer in a vacuum other than radiation ...
This is not really correct. Heat can also transfer by conduction if two surfaces are in contact, even though they are surrounded by ...
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Heating object in airless environment
The answer is yes,- there would be heating dynamics difference. For simplicity, let's consider cube-shaped container placed in some weightless environment, say in the Earth orbital station, so that ...
- 14.9k
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Heating object in airless environment
It will depend mostly on the vapor pressure of the sample at the melting point.
A sample with a high vapor pressure will vaporize as the container is evacuated, so the temperature will decrease as ...
- 14.1k
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Clarification of notions regarding kinetic energy and kinetic theory of matter
For the sake of increased clarity, we can expand the concept of "kinetic energy" to include not just the kinetic energy associated with the body's center of mass motion (call that $T$), but ...
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How are heat capacities of substances measured?
Specific heat capacity can be determined by Differential Scanning Calorimetry. A standardized test for making the determination is ASTM E1269.
The benefit of using a single standardized test is one ...
- 77.9k
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How are heat capacities of substances measured?
You could put your substance at a specified temperature into contact with a reference substance (e.g., water) at a different temperature and allow them to equilibrate. If you know the heat capacity ...
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How are heat capacities of substances measured?
Place an electric heater element inside the substance. Insulate the substance from its surrounding as best as you can (still air is a pretty good thermal insulator). Calculate the power from $P = UI$ ...
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Can heat radiate from a cold to hot body?
There is a net flow of heat froma hot body to a cold body. However there can be local fluctuations where heat flows from a cold body to a hot body. The emphasis is on the net flow.
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Complete ionization temperature of an atom
If we don't over interpret OP and take the question very literally, the desired answer is the temperature scale that is comparable to the energy required to remove the innermost electron:
$$
T \sim Z(...
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Complete ionization temperature of an atom
I think the question is slightly misposed: For hydrogen the Saha equation gives (Ionization energy $I_e$=13.6eV)
\begin{align}
\frac{n_\text{e} n_\text{p}}{n_\text{H}}=\frac{n^2_\text{Ion}}{n-n_\text{...
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Accepted
Does viscosity increase temperature of a fluid at the cost of macroscopic kinetic energy?
Yes, the loss of macroscopic kinetic energy due to viscosity is transferred to microscopic averaged motion and thus heats up the fluid – this increases the thermal energy of the fluid (and thus the ...
- 12.2k
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Entropy change in the free expansion of a gas
For the Entropy change on a free gas expansion.
I like to imagine a kind of "Maxwell demon" holding a movable virtual wall separating a Volume V1 at atmosferic pressure from a volume V2 ...
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Mean-field theory and coarse-grained modelling: Are these the same methodology?
The two are closely related, but refer to different aspects of the issue: Coarse-graining is a procedure by which the continuous field is introduced, whereas mean-field theory is description of a ...
- 65k
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Accepted
Exercise about entropy
$\Delta S_{surr} = 0$ because the system is thermally insulated.
$\Delta S_{sys} > 0$, and your work to calculate it appears correct. The process is thus not reversible: $\Delta S_{universe} = \...
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Accepted
Work in a two-compartment system
The problem is that if the piston is free to move under a finite initial pressure difference, then the resulting process is not quasi-static; which is to say, the pressures aren't well-defined ...
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Confusion about reversibility of a carnot engine
The Carnot operates reversibly between only two reservoirs by using adiabatic expansion and compression steps—not heat transfer—to swing between the temperature extremes.
During the isothermal steps, ...
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Confusion about reversibility of a carnot engine
for a transformation to be revesible it is needed that the (say 2) objects are constantly in thermal equilibrium
This is related to reversible exchange of heat. In order to two systems to exchange ...
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What does breaking or overcoming intermolecular forces mean?
This contradicts the latent heat, which implies some chain has to be broken.
If you think of intermolecular bonds as "chains", you are wrong, and it is clear that this is where your ...
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Optimisation of pop pop boat?
This is not going to work with a three-port valve (as in your second sketch) since for the water to flow in you need a connection to the inside which is at a lower pressure than the outside at the end ...
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Accepted
Internal energy of a gas and $p$-$V$ curve
If the initial and final final points of 2 different curves are same
then the internal energy is same .
The change in internal energy is the same because the change in internal energy (a property of ...
- 77.9k
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Nusselt number for developing turbulent flow
You're correct that the standard Dittus-Boelter equation, $Nu = 0.023Re^{0.8}Pr^{0.3}$, applies only to fully developed turbulent flow, which is not the case in your entrance region. To address this, ...
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Zero Kelvin and Negative Kelvin
Temperature is not the same thing as kinetic energy. Temperature is, by definition, the reciprocal of the derivative of the entropy of the system with respect to energy. In some cases the only ...
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Zero Kelvin and Negative Kelvin
Zero Kelvin is understandable that all molecules kinetic energy is Zero
But what is Negative Kelvin, means negative kinetic energy ? How can a body will have negative energy ?
As explained on this ...
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Zero Kelvin and Negative Kelvin
You can't have negative kinetic energy. Kinetic energy is half times the mass times the velocity squared. As a squared quantity cannot be negative, the mass would have to be negative in order for ...
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Zero Kelvin and Negative Kelvin
You have arrived at the expected result: negative temperature means negative kinetic energy, which is impossible; therefore negative temperature is impossible.
This is why $0\,\text{K}$ is considered ...
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What if gas molecules collide inelastically?
I know that inelastic collision (e=0) of gas molecules does not make sense...
They do.
..., but I wonder, if somehow hypothetically gas molecules made inelastic collision with the container in which ...
- 41.2k
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Taking derivative with respect to quantum canonical ensemble expectation value
IMHO the OP is mislead by a recourse to imaginary time... which is not warranted here. Indeed, let us assume that we know the exact eigenstates of $H$:
$$
H|n\rangle = E_n|n\rangle,
$$
(these coule be ...
- 65k
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Taking derivative with respect to quantum canonical ensemble expectation value
On a side note, you can avoid the Trotter formula by using instead the standard interaction picture (if you are already familiar with it from previous QM courses). I will write $\partial_{x_i} = \...
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Why does the motion of a gas never stop?
The problem is that, with real gases, the internal area of the container is not a closed system, and the container walls themselves are neither fully stationary nor opaque.
What I mean by "not ...
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What if gas molecules collide inelastically?
In addition to the good answer of @Roger V. :
When you heat your gas to a temperature high enough, the electronic degrees of freedom start to participate in the energy distribution as well.
Electronic ...
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Taking derivative with respect to quantum canonical ensemble expectation value
$$\partial_{x_b} \exp(-\beta H/N)\approx -\exp(-\beta H/N) (\frac{\beta}{N})(\partial_{x_b} H) $$
My question is, what is the strict justification for the last step? How to prove that the error ...
- 23.3k
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Cryostage solid state active insulation
The cooling power of Peltier effect stages drops to virtually nothing once you get to a 70K temperature difference between the hot and the cold sides of the device.
This means that a single-stage ...
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Why Gifford-Mcmahon cycle's Refrigeration Effect is $Q=V(P_1-P_2)$?
It's from the first law of thermodynamics! This says that ideal refrigeration is $$\Delta U = Q_\mathrm{c}-W$$
Where $\Delta U$ is the change in internal energy, $Q_\mathrm{c}$ is the cooling energy ...
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Confusion while deriving kinetic-molecular theory of gases
Projections of the velocities of gas molecules follow Maxwell distribution (i.e., Gaussian/normal distribution):
$$
f(v_x)dv_x=\sqrt{\frac{m}{2\pi k_B T}}e^{-\frac{mv_x^2}{2k_BT}}dv_x,
$$
that is ...
- 65k
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Thermodynamic equilibrium state of constant $(p,S)$ system
From this, we can follow that under constant volume and entropy, there should be a minimum of the internal energy!
When we write $$dU=TdS-pdV$$
we imply that $U$ is a function of variables $S, V$: $$...
- 65k
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Effect of movement of container on pressure of gas
In figure below, first case is a fluid container moving with constant horizontal speed. The second case is where the container has some non-zero acceleration in the horizontal direction.
Consider an ...
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Effect of movement of container on pressure of gas
EDIT: Judging by the low number of upvotes, I guess not many people are convinced by the logical argument I presented earlier, so I have decided to present a case based on calculations instead:
...
- 10.1k
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Why does the motion of a gas never stop?
As the molecules of the gas collide with the container do they lose some energy (kinetic)? - as no collision is perfectly elastic in nature.
They can, but not because of elasticity of collision. ...
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Effect of movement of container on pressure of gas
In case of constant acceleration the pressure in the moving container is not uniform it is increasing to the rear like the pressure increasing in the air of earth.
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Effect of movement of container on pressure of gas
Now, will there be pressure difference between the two containers? I
know it's zero, but can't comprehend it. Can somebody please explain.
If the motion is in a straight line as well as constant ...
- 77.9k
16
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What if gas molecules collide inelastically?
I know that inelastic collision(e=0) of gas molecules does not make sense, but I wonder, if somehow hypothetically gas molecules made inelastic collision with the container in which it is filled and ...
- 65k
2
votes
Why does the motion of a gas never stop?
Why does the motion of a gas never stop?
It does stop, eventually. Well almost. Over an infinite period of time, the motion will approach zero. Consider a closed vessel in space containing a gas ...
- 10.1k
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Why does the motion of a gas never stop?
As the molecules of the gas collide with the container do they lose
some energy (kinetic)? - as no collision is perfectly elastic in
nature.
In a microscopic system collisions are elastic. Energy is ...
0
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Why is work done in compressing a gas in piston but not when a gas freely expands?
I would like to add some of my ideas about the edit part.
Assume that a molecule is approaching the piston with speed 'u' in ground frame. Since the piston is compressing, it moves towards the ...
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Why does the motion of a gas never stop?
The real answer is that entropy always increases in a system by fundamental laws of thermodynamics. As entropy decreases within the container, it increases in the container itself and/or in the ...
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Why does the motion of a gas never stop?
Gas molecules collide not only with the container walls but also frequently with one another. The energy exchange during these collisions depends on the temperature of the walls. If the walls are ...
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Why does the partition function require an integral?
One way to look at it is the following: The sum is the correct representation for a regularized system with a countable Hilbert space (by e.g. working with periodic boundary conditions).
The ...
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Why does the motion of a gas never stop?
It actually stops. https://en.wikipedia.org/wiki/Heat_death_of_the_universe
But not anywhere close to our lifetimes.
Probability of emitting a photon from collisions at room temperature is so small, ...
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