# 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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### Can an actual physical system have zero entropy? [duplicate]

Obviously, a pure quantum state has zero entropy. It appears that in theory, this is correct; but in practice, it is not, because there is always some mixing with the environment. So, zero entropy is ...
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### Example of entropy and information

I was trying to write up a basic, intuitive explanation of entropy, and I wanted to see if this example I made up makes sense. Imagine a cell membrane separating our two compartments. It can contain ...
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### Is there a minimum energy content of information, other than 0 Joules?

Lets say I want to send the bit string 010110 to someone. Is there a theoretical lower bound on the energy needed to do this?
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### Why do we need an explanation for the arrow of time?

I'm reading Sabine Hossenfelder's new book Existential Physics, where she explains that because fundamental laws are time-reversible, we need an explanation for the arrow of time. Why is that exactly? ...
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### Entropy change during adiabatic expansion of ideal gas

I was told that the entropy change is $0$ during adiabatic expansion. However, according to $\mathrm dS=\frac{C}{T}\,\mathrm dT$, $\delta S$ is not zero because temperature is not constant during ...
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### Why doesn't the entropy increase when two similar gases mix with each other?

Entropy increases when two substances mix with each other. For example, the entropy of mixing of two different gases are given by $$\Delta S= 2Nk\ln\frac{V_f}{V_i}\;.$$ But, the entropy doesn't ...
1 vote
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### What does the no-hair theorem state about the entropy of black holes?

So does the no hair theorem say that all classical black holes will have zero entropy? If yes, why?
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### Vapor Compression Refrigeration Cycle

My question here is, why do we need point 2 to be on the saturated liquid line, i.e. why do we need a saturated liquid for the throttling process? Why can we not use it in the liquid region or ...
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### Maximum Efficiency of Thermodynamic Cycle [closed]

I am a non-engineering student about to start an MSc on Sustainable Energy and I'm trying to get a head start so I don't fall behind. I would really appreciate some guidance on the following past ...
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### Is metaphysics a subject that should replace physics and philosophy? [closed]

I'm really seeking some insight and the following may seem opinionated, but it's really my observations and I could be wrong and I'm completely open to hear thoughts and correction from anyone. If ...
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### Boltzmann or Gibbs entropy for the canonical ensemble?

For the microcanonical ensemble the entropy, is given by the Boltzmann entropy which equals: $$S = k_\mathrm{B} \ln(\Omega(E))$$ where $k_\mathrm{B}$ is Boltzmann's constant and $\Omega(E)$ the number ...
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### Convert ambient heat into electricity without violating laws?

This is an idea I've been playing around with for a while, even though many people tell me it's impossible. However, as far as I can tell, it does not actually violate the laws of physics. I am ...
1 vote
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### How to understand the difference between standard entropy and generated entropy

Coming from the definition of entropy variation given as follows: $$\Delta S=\int ^{2}_{1}\left( \dfrac {\delta Q}{T}\right) _{r}+\sigma$$ I don't understand the difference between the two parts. ...
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### Would Maxwell's demon having infinite memory storage break the second law of thermodynamics?

Apologies if my understanding is wrong, I am literally a child. Maxwell's demon is meant to break the second law of thermodynamics by making a disorderly system orderly (see image below). But the ...
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### Conservation of information in expanding universe

As the observable universe expands the amount of information is increasing, at least locally. How is this compatible with the conservation of information in quantum mechanics?
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### What would "break" in reality if I had a perpetual motion device?

Let's say aliens came and gave me a small box that outputs more energy than is input. As one example, the box can take current and voltage $I$ and $V$ as input, and output $I$ and $2\cdot V$. Or I can ...
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### Entropy of a deterministic reversible system

Suppose a deterministic reversible system evolving from state A of gas located in a small bottle in an otherwise empty room, to state B where the gas is dispersed throughout the room. Why is the ...
1 vote
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