A important property of all systems in thermodynamics and statistical mechanics. Entropy characterizes the degree to which the energy of the system is *not* available to do useful work
1
vote
1answer
39 views
Sackur-Tetrode equation - clarification required - problem with units
I'm a 2nd year physics undergraduate and recently I've volunteered to give a short presentation on the Sackur-Tetrode equation derivation and its use at removing the Gibbs paradox. I've looked on the ...
2
votes
0answers
55 views
Entanglement and Black holes
If you have two entangled quantum states, One state falls into a black hole and you measure the other state, What can you say about the state that has fallen into the black hole?
If you have billions ...
0
votes
0answers
54 views
The most general entropy
What is the most general (likely non-extensive) entropy allowed/realized in Nature and its symmetry group of transformations?
We know Shannon-Gibbs-Von Neumann entropy, Tsallis entropy, ...
4
votes
3answers
175 views
Definition of entropy
In physics, the word entropy has important physical implications as the amount of "disorder" of a system. In mathematics, a more abstract definition is used. The (Shannon) entropy of a variable $X$ is ...
3
votes
4answers
204 views
If particles can find themselves spontaneously arranged, isn't entropy actually decreasing?
Take a box of gas particles. At $t = 0$, the distribution of particles is homogeneous. There is a small probability that at $t = 1$, all particles go to the left side of the box. In this case, entropy ...
4
votes
2answers
71 views
What is the entropy of a system when its volume tends to zero?
Say that a closed system has $n$ dimensions and is in the shape of a $n$-ball with a radius of 1, it's volume will be
$$\frac{\pi^\frac{n}{2}}{\Gamma(\frac{n}{2}+1)}$$
which tends to 0 yet is not ...
1
vote
1answer
98 views
Mathematical proof of non-negative change of entropy $\Delta S\geq0$
I understand that we can prove that for any process that occurs in an isolated and closed system it must hold that
$$\Delta S\geq0$$
via Clausius' theorem. My question is, how can I prove this in a ...
-2
votes
1answer
98 views
Could entropy explain dark energy?
This was 3rd beer idea, so please bear with me. What if the universe was not actually expanding but the speed of light was slowing? Wouldn't that be indistinguishable to our observations? Either way ...
1
vote
0answers
30 views
Calculating the change in entropy in a melting process
I have a homework question that I'm completely stumped on and need help solving it.
I have a $50\, \mathrm{g}$ ice cube at $-15\, \mathrm{C}$ that is in a container of $200\, \mathrm{g}$ of water at ...
0
votes
1answer
47 views
Negative temperature and Absolute hot
This video explains that heat at negative temperatures flows from the negative object to the normal object. If the temperature of the normal object is absolute hot, what happens with the heat? The ...
1
vote
1answer
47 views
Would the universe get consumed by blackholes because of entropy?
Since the total entropy of the universe is increasing because of spontaneous processes, black holes form because of entropy (correct me if I'm wrong), and the universe is always expanding, would the ...
1
vote
2answers
39 views
Decrease in entropy in a fluid flow
Let's imagine a section of a pipe through which a fluid, gas for example, flows. When there is no pressure gradient, there is no flow. However, that does not mean that the molecules are at rest. They ...
-1
votes
0answers
42 views
Entropy / speed relationship in vehicle
I'm trying to figure out if the speed of a motorized vehicle (a car of instance) varies with the 'age' of the vehicle.
If we suppose a vehicle of a given 'age', considering the yield loss of the ...
1
vote
3answers
132 views
Integrating factor $1/T$ in 2nd Law of Thermodynamics
How would you prove that $1/T$ is the most suitable integrating factor to transform $\delta Q$ to an exact differential in the second law of thermodynamics:
$$dS = \frac{\delta Q}{T}$$
Where $dS$ is ...
1
vote
1answer
66 views
Basic energy calculation for N identical spin system
We have a system that has N identical spins $n_i$, and each spin can be in state 1 or 0. The overall energy for the system is $\epsilon\sum_{i=1}^{N}n_i$.
My understanding: There is only one ...
3
votes
3answers
137 views
Does entropy alter the probability of independent events?
So I have taken an introductory level quantum physics and am currently taking an introductory level probability class. Then this simple scenario came up:
Given a fair coin that has been tossed 100 ...
5
votes
1answer
67 views
What happens at the interface between two universes with opposite thermodynamic arrows of time?
I was trying to think but cannot figure it out. For instance, if the interaction is small, for instance limited to a windows, the observers in each universe will see that the other goes in reverse. ...
0
votes
1answer
76 views
Uncertainty and Thermodynamics
Dilemma
The uncertainty principle of energy and the 2nd law of thermodynamics don't add up : the uncertainty principle of energy says that
$\Delta \tau \cdot \Delta E \ge \frac{h}{4\pi} = ...
1
vote
1answer
49 views
Why does a temperature increase on a fixed volume increase entropy?
I heard that this statement is correct. However, it seems odd to me. The number of possible microstates is still the same, so isn't the entropy constant?
2
votes
2answers
184 views
Does entropy really always increase (or stay the same)?
Consider this image. If the big (grey) molecules were all to spontaneously move to the left, and the small ones were to move to the right, there would be an increase in order.
While unlikely, ...
4
votes
3answers
320 views
Is there a mechanism for time symmetry breaking?
Excluding Thermodynamic's arrow of time, all mathematical descriptions of time are symmetric. We know the arrow of time is real and we know the equations describing physics are real so is there any ...
2
votes
2answers
85 views
Von Neumann Entropy: varying definitions
I have seen different authors define von Neumann entropy in different ways. In particular, some use the natural logarithm and others log to base 2. What is the reasoning for this? Does it make any ...
1
vote
0answers
97 views
Difficulties with understanding total entropy change and unavailabillty
Of course, I know the fact that the entropy of an isolated system never decreases. Neverthless what makes me confused about the entropy(or change of entropy) of an isolated system is the explanation ...
0
votes
1answer
58 views
Why is the equation for Entropy of an ideal gas that undergoes reversible change in T at constant Pressure like this?
Why is the equation for change in Entropy for a reversible change in $T$ at constant $P$ described as
$$\Delta S = n C_p \ln\frac{T_f}{T_i}$$
0
votes
1answer
75 views
Is there a relation between supersymmetry and entropy?
Considering that entropy denotes the level of order/disorder in a system, would it be possible for entropy and supersymmetry to exist at the same time? Or, are they entirely unrelated?
0
votes
3answers
234 views
Why isn't the Bekenstein-Hawking Entropy considered the quantum gravitational unification?
Based on the Bekenstein-Hawking Equation for Entropy, hasn't the relationship between quantum mechanics and gravity already been established.
1
vote
0answers
43 views
How to solve state parameters using these givens for an ideal gas?
In a thermodynamic turbine using air as an ideal gas, given that you have a known inlet temperature value $T_i$, a known exit pressure value $P_e$, a known inlet and exit velocity $V_i$ and $V_e$, a ...
0
votes
1answer
48 views
Do gasses always mix because of their Gibbs free energy?
As far as I know there are no two gasses that don't mix (excluding demixing by gravitational effects). For me, as someone working with fluids and surface tensions a lot, this means that the surface ...
0
votes
0answers
44 views
Entropy change relation to the number of lost bits
Can we use entropy change value to define the (perhaps fractional) number of
bits of information that are lost by and, or, xor gates?
0
votes
0answers
44 views
2
votes
1answer
63 views
How can dQ/T be interpreted as a system's level of disorder?
Long before statistical mechanics, entropy was introduced as:
$dS = \frac{dQ}{T}$
At the time when entropy was introduced in this manner, was it known that entropy represents how "disordered" a ...
3
votes
6answers
226 views
Entropy and Crystal Growth
I was reading about growing single crystals and I'm a little confused about this -
In most crystal growing processes, a "seed crystal" is used, and the rest of the material crystallizes on the seed ...
1
vote
1answer
48 views
What are the units of the Bekenstein bound?
Working with the Wikipedia definition of the Bekenstein bound:
$S \leq \frac{2 \pi R k_bE}{\hbar c}$
$2\pi R \ $ is $m^2$
$k_b$ is $\frac{J}{K}$
$E$ is $J$
$\hbar$ is $J*s$
$c$ is $\frac{m}{s}$
...
8
votes
3answers
205 views
comments on entropy and direction of time in Landau and Lifshitz stat mech
In Landau and Lifshitz's Stat Mech Volume I is the comment:
Thus in quantum mechanics there is a physical non-equivalence of the two diretions of time, and theoretically the law of increase of ...
0
votes
2answers
142 views
How to compute configurations (entropy) of a system?
If we have a system $X$ consisting of subsystems $X_1$ and $X_2$.
We also know that $X_1$ and $X_2$ have eigenstates $H_1 = 1 \times 10^{20}$ and $H_2 = 1 \times 10 ^{22}$.
Can we calculate the ...
0
votes
1answer
118 views
relation between first law of thermodynamics and statistical mechanics definition of entropy
From the definition of entropy as $S= - Tr (\rho\, ln \rho)$ one obtains that
$S = \frac{\langle E \rangle}{T} + \log Z.$
The first law of thermodynamics has
$dS = {dE \over T}$.
Why is there no ...
3
votes
1answer
160 views
Why is (von Neumann) entropy maximized for an ensemble in thermal equilibrium?
Consider a quantum system in thermal equilibrium with a heat bath. In determining the density operator of the system, the usual procedure is to maximize the von Neumann entropy subject to the ...
2
votes
2answers
161 views
Entropy exchange of a free fall
I have a problem in which the tell me that you drop a bag of 50 kg of sand from 10 meters high, and you have to caltulate the entropy difference of the sand, asuming that the speific heat of the sand ...
3
votes
5answers
663 views
For an isolated system, can the entropy decrease or increase?
In any sizable system, the number of equilibrium states are much, much greater then the number of non-equilibrium states. Since each accessible micro state is equally probably, it is overwhelmingly ...
5
votes
2answers
184 views
Order = Energy = Mass?
Here is a following problem I encountered when chatting about physics with my friend:
Let us imagine a classical example of ordered state of matter in thermodynamic sense: let's take a cylinder ...
3
votes
2answers
464 views
Can ice have a higher entropy than water?
I've leant that entropy is a state of randomness, and that solids have a more structured form, therefore having less entropy.
However, I saw a YouTube comment stating the following:
a liquid NOT ...
2
votes
1answer
120 views
The definition of entropy
As history of thermodynamics say, it was a mystery that what is the required condition for a given energy conversion to take place? Like there are two possible events each conserving energy but only ...
3
votes
1answer
59 views
Amount of energy to separate Gases - relationship to concentration
I want to understand the efficiencies of separating mixed gases, and for that I want to understand the thermodynamic limit case. Looking at the wikipedia page for entropy of mixing, I find the ...
1
vote
1answer
174 views
Isentropic Processes
I'm having trouble understanding why reversible adiabatic processes are isentropic.
I understand that in a reversible adiabatic process there is no heat exchange and so $dQ = TdS = 0$. However, if ...
3
votes
1answer
62 views
Relation between Casimir and hydrophobic effects
Background
Some years ago I was studying "Fundamentos de biología" (Biology fundamentals) and learned how the lipids create a bilayer due to the water repulsion. Some time later I learned that this ...
2
votes
2answers
435 views
Entropy Change During Reversible Processes
I'm confused about the Second Law of Thermodynamics. The Second Law of Thermodynamics prohibits a decrease in the entropy of a closed system and states that the entropy is unchanged during a ...
0
votes
0answers
196 views
Entalpy and entropy role in freezing-point depression phenomena
There's this "atomic" explanation of the freezing-point phenomena on Wikipedia that leaves me really intrigued.
Consider the problem in which the solvent freezes to a very nearly pure
crystal, ...
4
votes
1answer
88 views
Where and how is the entropy of a black hole stored?
Where and how is the entropy of a black hole stored?
Is it around the horizon? Most of the entanglement entropy across the event horizon lies within Planck distances of it and are short lived.
Is ...
0
votes
0answers
40 views
Entropy of binding two rigid bodies together in a closed system (e.g. forming a diatomic out of two monatomic gas particles)
When I bind two molecular complexes together, or in the simplest case, form a diatomic out of two atoms in a monatomic gas, the entropy of my (closed) system will decrease. We can perhaps understand ...
2
votes
1answer
112 views
free energy and entropy of 2D soap froth
This is a (exploratory) computational project. The soap froth was created by injecting bubbles into a chamber formed by two rectangular plates which are 0.16cm. From the moment the soap froth was ...
