Naked Science Forum

General Science => General Science => Topic started by: Locke on 03/02/2011 04:13:10

Title: The Principle and Nuances of Entropy
Post by: Locke on 03/02/2011 04:13:10

Entropy, the measure of disorder in the universe is a somewhat confusing subject...
For example, if a star forms, isn't that a direct violation of the second law of thermodynamics, as it is a completely natural process.
The only thing I can see doing work is gravitational force, but, then again, that's included in the system, I should think.

Title: The Principle and Nuances of Entropy
Post by: lightarrow on 03/02/2011 12:26:05

Quote from: Locke on 03/02/2011 04:13:10

Entropy, the measure of disorder in the universe is a somewhat confusing subject...
For example, if a star forms, isn't that a direct violation of the second law of thermodynamics

Why? Are you sure the star's entropy is lower than the initial gas' entropy?

Title: The Principle and Nuances of Entropy
Post by: Locke on 03/02/2011 15:40:52

How can it not be? The stars are the only semblance of order we have in the universe... That is, if you count the situation 'before' the big bang as ordered.

Title: The Principle and Nuances of Entropy
Post by: lightarrow on 03/02/2011 18:29:20

Quote from: Locke on 03/02/2011 15:40:52

How can it not be? The stars are the only semblance of order we have in the universe... That is, if you count the situation 'before' the big bang as ordered.

You have a cloud of hydrogen gas spreaded out many millions of km, then, after it has collapsed gravitationally, you have a star. Which has less entropy, the star or the cloud of gas? If you say the star, remember that the stars' temperature is much higher then the cloud's temperature.

Title: The Principle and Nuances of Entropy
Post by: Locke on 04/02/2011 03:27:05

Quote from: lightarrow on 03/02/2011 18:29:20

Quote from: Locke on 03/02/2011 15:40:52

How can it not be? The stars are the only semblance of order we have in the universe... That is, if you count the situation 'before' the big bang as ordered.

You have a cloud of hydrogen gas spreaded out many millions of km, then, after it has collapsed gravitationally, you have a star. Which has less entropy, the star or the cloud of gas? If you say the star, remember that the stars' temperature is much higher then the cloud's temperature.

Here's a thought: objects that transition from solid to liquid and from liquid to gas gain entropy, so the nebula/cloud's phase shift from gas to plasma, and the internal energy change that came with it, explains the gain or entropy, right?

Title: The Principle and Nuances of Entropy
Post by: lightarrow on 04/02/2011 16:13:57

Yes, however the simple fact that a body's temperature has increased, means that the entropy has increased, if the internal energy has increased as well: if you put a block of iron on the flame, its temperature and its entropy increases.
In the case of the star the situation is a bit more complicated because you have to consider gravitational potential energy (in the non relativistic description) so, after the cloud's collapse, some gravitational potential energy has converted into kinetic energy of the star's particles.

However, it's not very clear to me if this means that internal energy has increased or hasn't varied at all, because I don't know if to consider potential gravitational energy as part of the system's internal energy (intended in thermodynamical sense) or not; if then one consider that, in GR, gravitational potential energy doesn't exist at all, it's even more complicated.

So, is the star's entropy lower than that of the hydrogen cloud from which it formed? Or it's greater? Or it didn't vary at all?