Entropy

[RichIsnang]

second law of thermodynamics states that entropy of a closed system will most probably increase over time.

people say you see a vase smashing but never unsmashing. but if i just leave a vase on the work top the probability of it smashing of its own accord is also extremely low.

 

but thats a human example which i think isnt extremely relevant with entropy.

water in space freezes and goes into a sort of ordered state of its own accord, and stays there?

gas in space will come together to form a star if not a planet, which are pretty oredered, we can be more certain of the arrangement of molecules of a planet than of a nebula.

 

whats the relation between brownian motion and entropy? it seems to me they are pretty much the same thing, and brownian motion describes the situation alot better than entropy.

I struggle to find examples of the second law in action that arent brownian motion.

 

also it seems to me like there is always something stopping the entropy increasing, gravity, cold, electrostatic attraction etc.

 

i may have mistaken the law, does it mean you need to put work in to stop entropy decreasing?

[juanrga]

second law of thermodynamics states that entropy of a closed system will most probably increase over time.

people say you see a vase smashing but never unsmashing. but if i just leave a vase on the work top the probability of it smashing of its own accord is also extremely low.

 

but thats a human example which i think isnt extremely relevant with entropy.

water in space freezes and goes into a sort of ordered state of its own accord, and stays there?

gas in space will come together to form a star if not a planet, which are pretty oredered, we can be more certain of the arrangement of molecules of a planet than of a nebula.

 

whats the relation between brownian motion and entropy? it seems to me they are pretty much the same thing, and brownian motion describes the situation alot better than entropy.

I struggle to find examples of the second law in action that arent brownian motion.

 

also it seems to me like there is always something stopping the entropy increasing, gravity, cold, electrostatic attraction etc.

 

i may have mistaken the law, does it mean you need to put work in to stop entropy decreasing?

 

The second law of thermodynamics states that the average entropy of an isolated system cannot decrease over time.

 

Brownian motion is a kind of dissipative motion in complete agreement with entropy and the second law. In fact the Brownian motion equations

can be obtained from an entropic analysis.

[RichIsnang]

What if I have some gas in space, enough to form a small planet. It comes together to form said planet, and stays like that. Hasn't the average entropy decreased over time?

[juanrga]

What if I have some gas in space, enough to form a small planet. It comes together to form said planet, and stays like that. Hasn't the average entropy decreased over time?

 

I repeat

 

The second law of thermodynamics states that the average entropy of an isolated system cannot decrease over time.

 

For an open system entropy can increase, decrease, or remain constant.

[studiot]

Hello Rich,

 

It would be to your advantage to review your ideas of 'order' and probability before trying to mix them up with entropy.

 

It is very common to think of say a crystal lattice as 'order' because it is formed into a pretty pattern.

 

This is nowhere near the truth.

 

You can take any (possible) state whatsoever of a system and call it 'order'.

 

Probability theory then makes the comparison with the totality of other possible states.

 

Since most systems have (very) many possible states the possibility of finding the particular one you have called order, when sampling the system at random, is very low.

 

Further distinction is possible by clumping states into sets of similar ones.

 

This difficulty in understanding how random processes can result in a directed outcome is very common and appears in many areas of the application of probability.

A particular example is Darwinianism.

 

A further profitable consideration would be to apply Newton's laws (they are sufficient for qualitative understanding) to your cloud of gas.

 

Some force makes the gas coalesce. What are the classical thermodynamic implications of that statement?

Edited May 29, 2012 by studiot

[swansont]

What if I have some gas in space, enough to form a small planet. It comes together to form said planet, and stays like that. Hasn't the average entropy decreased over time?

 

The planet will have less gravitational potential energy than the gas. Gravity is not a dissipative force, so the only way to form a planet is via a bunch of inelastic collisions. So forming a planet requires the creation of a bunch of photons. This increases the entropy.

[RichIsnang]

Okay, thanks guys