# Questions tagged [entropy]

An important extensive property of all systems in thermodynamics, statistical mechanics, and information theory, quantifying their disorder (randomness), i.e., our lack of information about them. It characterizes the degree to which the energy of the system is *not* available to do useful work.

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### Alternative interpretation of the second law of thermo [closed]

I have had this thought for a while and I was wondering whether it's valid. Entropy is often described as the amount of "useable" energy in a system at a micro level, entropy is related to ...
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### Why is the holographic encoding from Bekenstein-Hawking entropy remarkable?

The holographic encoding interpretation from the Bekenstein-Hawking entropy is due to it scaling with the area $S_{BH} = A/4$ of the surface, rather than the volume $V$. While I am aware that most ...
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### Can increase in entropy increases the probability of finding something specific [closed]

See if we take a bookshelf with 10 books in it and drop all of them in a room and repeat this process infinite times. So we can say that entropy of room increases and now we have to find a specific ...
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### Is the concept of entropy a result of limited technology?

I know that entropy is the energy within a system that is unable to do useful work. However, there was a time in which we were unable to harness the energy of the wind or the sun, and now we can, ...
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### Are von Neumann entropies of complementary but physically distinct subsystems in time-dependent settings identical?

We assume a quantum system AB with subsystems A and B and take the Schmidt decomposition of a state $\vert\psi_{AB}\rangle=\sum_i\lambda_i \vert a_i\rangle\vert b_i\rangle$ defined on the compsite ...
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### Free energy change in reversible/irreversible processes

Yet another follow up to this question, I am struggling to understand the example provided in Chet Miller's answer: An example of this is expansion of an ideal gas in contact with an ideal constant ...
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### Difference between Reversible and Irreversible processes in Physics vs. Chemistry

In Physics a reversible process is defined as one in which the system can be returned to its initial conditions via the same path (along the PV Diagram), and every point along the path is an ...
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### Why do we put factors of zero in a Lagrangian that is to be extremized?

According to the Wikipedia page on Lagrange multipliers under the section - Example 3: Entropy, it is written that: $$f(p_1,p_2,\ldots,p_n) = -\sum_{j=1}^n p_j\log_2 p_j$$ For this to be a ...
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### How the temperature used in second law of thermodynamics and ideal gas law equivalent? [closed]

I was reading Theory and Problems of Thermodynamics by Schaum's Outline series and there the author starts from some axioms (which are the thermodynamic laws, excluding the zeroth and third). Then he ...
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### Second law of thermodynamics and Unitarity of quantum mechanics [duplicate]

From the second law of thermodynamics, we know the entropy must be increasing in an isolated system, such as our Universe. On the other hand, we have quantum mechanics which, I think, somehow tells us ...
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### How a thermodynamical system relaxes to equilibrium?

For a system in contact with a reservoir with well-defined constant temperature $T$, its change in Helmholtz energy satisfies the following inequality: $$\Delta F \le -W_{by}$$ where $W_{by}$ denotes ...
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### Equivalence of mixed states

I have questions regarding equivalence of mixed states. Consider a single photon and its polarization as an example. If I know that some kind of process creates a single photon in an exact ...
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### Why is all matter in the universe, not found at its lowest state of potential energy?

Preface: it may be of interest that I am a second year Biology student, with no experience in studying Physics and a very basic understanding of Mathematics. ...
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### Does the Sun increase entropy?

The Sun generates heat via fusion. The heat from this reaction gets distributed around the solar system and beyond. This process of spewing heat and radiation all over the place doesn't immediately ...
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### Is entropy in the third law of thermodynamics a continuous quantity?

In the third law of thermodynamics, entropy goes to zero or to a constant value at vanishing absolute temperature. The change of entropy also goes to zero. The third law is valid in the thermodynamic ...
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### What is the average increase in entropy per unit space?

The second law suggests that averaged over a large enough space and time, entropy always increases. The rate of increase obviously depends on local conditions. The increase in Entropy in a year in ...
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### How to calculate the total entropy of dS black holes?

In several articles where the thermodynamics of dS black holes have been investigated, the entropy part of the model or the total entropy has been analyzed based on the entropy of the black hole ...
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### Is there entropic force in a black hole?

A black hole is regarded as a thermodynamic system that has entropy. Its horizon has a temperature and corresponds to a macroscopic system, thus occupying a large number of states. In this perspective,...
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### Reverse entropy? [closed]

I don't know much about entropy but all I know is the energy of a system tends to spread, more generally things tend to chaos. And entropy is not reversible, you can not stick a broken egg together. ...
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### How can I derive the second law of thermodynamics from Newton's second law of motion? [duplicate]

Do you know of an elementary proof for the second law of thermodynamics, for example, from the Newton laws or perhaps some particular model in which it is equivalent/reduces to it? My naive concept of ...
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