1. The second law of thermodynamics states that the entropy in a system, such as our universe, always increases. The third law, however, says that entropy reaches zero as a system approaches absolute zero temperature.

Our universe has been cooling off since its origin (because of its expansion), accounting for a myriad of interesting physical bodies and processes, but how is this not at odds with both thermodynamical laws? If the system that is our universe is cooling (and indeed slowly approaching absolute zero), its entropy could be said to decrease. The second law however dictates that it must increase invariably. Which is right?

2.

3. Originally Posted by Maximise24
The third law, however, says that entropy reaches zero as a system approaches absolute zero temperature.
The third law doesn’t say that. The third law refers to a perfect crystal as having zero entropy, meaning all atoms within it are perfectly arranged and do not move. The word “system” that you tossed into the definition is not in the third law. The crystal cannot attain a zero entropy state unless its energy is transferred elsewhere and that elsewhere gains entropy as a result of this transfer.

So while the crystal itself has zero entropy its surroundings have had their entropy increased and the third law is consistent with the second law.

Rereading this, I'm not sure it's quite right - anyone else got something to add?

4. Originally Posted by Bunbury
Originally Posted by Maximise24
The third law, however, says that entropy reaches zero as a system approaches absolute zero temperature.
The third law doesn’t say that. The third law refers to a perfect crystal as having zero entropy, meaning all atoms within it are perfectly arranged and do not move. The word “system” that you tossed into the definition is not in the third law. The crystal cannot attain a zero entropy state unless its energy is transferred elsewhere and that elsewhere gains entropy as a result of this transfer.

So while the crystal itself has zero entropy its surroundings have had their entropy increased and the third law is consistent with the second law.

Rereading this, I'm not sure it's quite right - anyone else got something to add?
A crystal does not have zero entropy when its temperature is above absolute zero. It can and does absorb and transmit energy. Therefore, it has its own energy states.

Quoting Planck: "If a system is completely isolated, and therefore guarded against all external thermal and mechanical actions, then is any ensuing change the entropy of the system will increase: dS > 0. In this equation, dS =-Q/T, if Q denote the hea which is given up in the change by the surroundings to the system." In his equations, S is the entropy.

5. I see. Now, I suppose my confusion comes from the very specific nature of the Third Law. It is not, as I wrongly assumed, applicable to any system but exclusive to perfect crystals. This is quite at odds with the first two laws, which are larger in scope and can be applied to systems as large as the universe. Why the disparity? What is the relevance of this very specific property of crystals in light of the other two, much more fundamental laws (being conservation of energy and entropy)?

6. You're reading Bunbury's statement too specifically. You might want to read the Wiki article on it, but the more general idea is that entropy decreases with temperature, and for a perfect crystal it'd decrease to 0 at absolute 0.

7. Originally Posted by MagiMaster
but the more general idea is that entropy decreases with temperature
Except in the case of the cooling universe itself, that is?

8. Originally Posted by Maximise24
I see. Now, I suppose my confusion comes from the very specific nature of the Third Law. It is not, as I wrongly assumed, applicable to any system but exclusive to perfect crystals. This is quite at odds with the first two laws, which are larger in scope and can be applied to systems as large as the universe. Why the disparity? What is the relevance of this very specific property of crystals in light of the other two, much more fundamental laws (being conservation of energy and entropy)?
The concept of entropy is not limited to crystals. Any system, which is in accordance with the Planck formula above, reaches zero at absolute zero. When the external temperature is absolute zero, no input energy is possible. Therefore, the system radiates energy but does not absorb any energy. Eventually, the temperature of the system also reaches absolute zero.
However, this phenomenon is probably most easily observed in a crystal that emit coherent light.

9. Originally Posted by sharpsword
Any system, which is in accordance with the Planck formula above, reaches zero at absolute zero.
But I don't think this applies to the universe, does it? The expanding universe gradually cools off as energy and matter get spread out more and more. Eventually this "heat death" will bring about a maximum entropy state. Here maximum entropy comes with temperatures close to the absolute zero, as the dissipated energy is unable to produce any work or temperature (a "Big Freeze"). What does that say about the third law?

10. Originally Posted by Maximise24
Originally Posted by MagiMaster
but the more general idea is that entropy decreases with temperature
Except in the case of the cooling universe itself, that is?
The universe is also expanding, which complicates things. I'm not an expert on this subject though, so I won't say that that explains everything.

11. Originally Posted by Maximise24
Originally Posted by sharpsword
Any system, which is in accordance with the Planck formula above, reaches zero at absolute zero.
But I don't think this applies to the universe, does it? The expanding universe gradually cools off as energy and matter get spread out more and more. Eventually this "heat death" will bring about a maximum entropy state. Here maximum entropy comes with temperatures close to the absolute zero, as the dissipated energy is unable to produce any work or temperature (a "Big Freeze"). What does that say about the third law?
The third law refers to a perfect crystal at absolute zero with a unique ground state. Not all systems, even at O degrees K have a unique ground state and O entropy. The mere fact that the universe is expanding against gravitational potential prevents it from being in a ground state.

12. For an example of increasing entropy and decreasing temperature, expand a volume of gas.

13. Originally Posted by Maximise24
Originally Posted by sharpsword
Any system, which is in accordance with the Planck formula above, reaches zero at absolute zero.
But I don't think this applies to the universe, does it? The expanding universe gradually cools off as energy and matter get spread out more and more. Eventually this "heat death" will bring about a maximum entropy state. Here maximum entropy comes with temperatures close to the absolute zero, as the dissipated energy is unable to produce any work or temperature (a "Big Freeze"). What does that say about the third law?
Yes. As our stars and planets radiate energy, they cool off. The sun heats our planet/earth during the day, and it loses energy during the night. If the energy lost exceeds the energy absorbed, our earth with cool off, but will never reach absolute zero as long as it absorbs radiation. However, in the process of cooling off, it will not form a crystal since it is composed of a generally disordered array of molecules. Therefore, why would the third law apply?

 Bookmarks
##### Bookmarks
 Posting Permissions
 You may not post new threads You may not post replies You may not post attachments You may not edit your posts   BB code is On Smilies are On [IMG] code is On [VIDEO] code is On HTML code is Off Trackbacks are Off Pingbacks are Off Refbacks are On Terms of Use Agreement