What is Entropy?

[JMJones0424]

I hope we don't get into the argument from that other thread, but wouldn't an object the size of, say, earth possess enough potential energy to generate--even without an outside energy source--a number of fairly dynamic systems?

 

Based on my understanding of current theories on abiogenesis, not only does the earth have enough potential energy, but it in fact was required to end up with the kind of life that dominates the earth today.

 

Without diverting too terribly from the intended topic of this thread, chemotrophs are organisms that use chemical energy to produce their own food. Their waste (oxygen) is what provided the blanket in the atmosphere that shields enough solar radiation to enable DNA to persist at the surface of the earth, enabling phototrophs to exist.

 

coldcreation- In the absence of modest, and since I am too impatient to wait, I found this answer to How is entropy effected during the formation of a star that may help to answer your question. Unfortunately, the math is just beyond my grasp, but the explanation seems valid. Perhaps you could help translate :eek_big:

[Qfwfq]

I think Q answered that question pretty well, albeit I did not understand exactly what he wrote.

This appears to be a request for clarification, but I would need to know more exactly which difficulty you encountered.

[coldcreation]

This appears to be a request for clarification, but I would need to know more exactly which difficulty you encountered.

 

Yes Q, thanks for asking. I've been meaning to compile a list of things that (to me) remain unclear from your earlier post.

 

Also, I use the example of the solar system (and it's evolution in time) because it seems to have gone from simple (a gaseous disc) to complex, from an disordered state to an orderly state: in apparent violation of the 2nd law. Other examples could just as well have been used, where evolution has transpired in a similar fashion, from simple to complex, from disorderly to ordered.

 

 

Ok, but what about the classical box experiment where half is filled with gas, the other empty. A separation is removed the gas fills the entire space, increasing the entropy inside the system.

 

[Edit: in relation to a proto-planetary disc evolving into the solar system as it looks today. After all entropy is often described with the example of a gas spreading from one side of a compartmentalized container (with a hole to allow gas to pass through) to fill the entire container. The diffused result is more randomly scattered than the original configuration. Yet this restricted view is too simplistic. It seems related, albeit from afar, to the solar system, then compared to now.]

 

Entropy is a measure of the number of unique states a system can have which share some common attribute or, if you like, meaning. In thermodynamics, the former are often called the microstates and the latter the macrostates: the state of motion of each particle vs. a macroscopic quantification such as temperature-pressure-volume.

 

This is called adiabatic expansion. The glaring question of course is: how can it correspond to an increase in entropy, defined as [imath]\frac{dQ}{T}[/imath], when it is adiabatic? The answer is: if a hot boilerplate and a pot of cold water are placed inside an excellent thermos, the whole thing is fairly adiabatic as well. If this isn't clear, in either case the system is not in thermal equilibrium.

 

Likewise, planet formation did not occur in thermal equilibrium and further, once the matter is not all gaseous, you have macroscopic kinetic energy which thermalizes with inelastic collisions, So, not all the [imath]\frac{dQ}{T}[/imath] must come in from outside and this goes for things such as chemical reactions too.

 

Are you saying that the formation of the solar system (with all it's properties and internal energy, along with the effects of gravitation) from it's planetary disc epoch to now does not represent a net increase of entropy? That the entropy has actually decreased, or remained on average the same?

 

Are you saying that the formation of the solar system in not an adiabatic process, or that adiabatic processes are not subject to the second law when gravity enters the equation?

 

How does volume and temperature play a role in the entropy problem (again related to the solar system)?

 

Do you consider the solar system to be an open or closed system? Is it tending toward equilibrium, and if not, how is the net entropy related to equilibrium in this case scenario?

 

 

Entropy has to do with information or, as I said earlier, some kind of meaning.

 

I'm not sure if this is your definition of entropy or not.

 

Isn't this like saying cows have to do with milk, or... some kind of white stuff?

 

 

CC

[coldcreation]

[...]

coldcreation- In the absence of modest, and since I am too impatient to wait, I found this answer to How is entropy effected during the formation of a star that may help to answer your question. Unfortunately, the math is just beyond my grasp, but the explanation seems valid. Perhaps you could help translate :eek_big:

 

Yes, this link of yours is very interesting, and indeed related to the question I just asked Qfwfq.

 

What follows is from Re: how is entropy effected during the formation of a star?

 

In general, the material of which the forming star is composed undergoes a large decrease in entropy during the formation process, and the rest of the universe (since total entropy must never decrease) endures an even larger increase. [...]

 

This is, of course, simply another example of the entropy change which occurs when a hot body loses energy to a cooler one. The finite difference in temperature is the tip-off that the process is irreversible, and that the total entropy must be decreasing. [...]

 

What we are interested in here is that in its collapse, the proto-star changes from a size of ~1018 cm in its space dimension to something like 1011 cm, so the coordinate volume factor V decreases by an enormous factor of roughly 1021 per particle. The momentum factor Vp in the phase space volume increases by a factor of perhaps 403/2 per particle (based on T changing from 25 K to 1000 K, our previous example), or a few hundred at most. So the volume factor wins out, and the total entropy of the material of the star decreases by a large factor, as we concluded from thermodynamics.

[Pyrotex]

Actually that is not at all what I was doing with the example of the solar system. I merely ask whether the formation and evolution of the solar system, from it's original protoplanetary disc (say) to the actual configuration, represent an increase in entropy,

No. As I said in my post, this represents a clear decrease in entropy. (Increased ORDER)

...I'm not sure I agree with this assessment. I would consider life as a reflection of the second law (not a violation of it), So too the formation of the solar system would be in accord with the 2nd law. This latter is the point I try, without apparent success, to understand.

If, within an arbitrarily defined boundary, ORDER increases, then entropy decreases. This is not a violation of the 2nd law, since we allow free energy to cross the boundary, and the boundary defines a local region.

We've discussed this "boundary conditions" before. I would say there is no "boundary conditions" relative to the solar system. You say there is one, but it can be arbitrarily chosen. But if the former is true, then the solar system is an open system, where entropy is still a nondecreasing property.

You're looking for the solar system to tell you where the "boundary" is. :eek_big: It doesn't work that way. Being a physicist is like being an umpire at the World Series. The umpire must take upon himself full responsibility for calling the balls: it's not a "strike" until he calls it a "strike". The physicist is looking to understand a natural phenomenon. He knows it has something to do with the 2nd law, and so he makes the calls: Does this problem require a "boundary"? And where shall I choose it? If the problem can only be understood as a "closed system" then he must arbitrarily choose a boundary that makes the "system" as closed as necessary for him to be able to state that entropy must increase. This may require some assumptions, generalizations or approximations.

...In other words, I wouldn't expect the solar system ...to be an exception to the 2nd law. Quite the contrary.

Correct. It's not.