I'm speaking with regard to different pressures and different temperatures seeking equilibrium with each other when exposed to each other.
If it has something to do with entropy, why is entropic increase the favored state?
Posted May 24, 2013, 4:14 PM UTC
Every physical object responds to the forces around it. Atoms of gas bouncing around between hundreds of other atoms will tend to move over time in the direction with the fewest atoms to bounce against. In a pressure gradient, that means the gas tends to move toward the area of lowest pressure.
It's not anything weird happening at the subatomic level. You can demonstrate it with a few hundred ball bearings on a smooth surface, if you get them moving fast enough.
Entropy. The simplest explanation I can think of to describe why that is the favored state is that entropy is the measure of usable energy in a system. If there is usable energy then it is going to be used to do work. Or, rather, something at a lower energy state is going to take that energy until it reaches equilibrium and it can't take anymore.
Why wouldn't it? Otherwise, there must be something to stop it. Whatever that is would require additional energy from another source. Which seems like the default or ground state, a situation where everything is free to interact naturally, or one where there are artificial barriers in place to prevent that from happening? Maximal entropy is the ground state, because all energy has been used up. There is no concentration of energy in one place that strangely exists in contrast to regions of lower energy.
Think about pressure. You have a high pressure container (could just be above 1 atm) and a low pressure container (could just be 1atm). Who made that container? A human did, one way or another. They mined or collected and refined a bunch of materials and constructed this container to store more of the gas in a smaller volume than it would normally occupy at 1atm of pressure. But that isn't its natural state. And that container doesn't naturally form due to some antithermodynamic laws that want all energy to be sequestered away in usable form, yet unable to be used. We do that so we can transport it or store it efficiently or use its energy. So once that barrier is removed, say, by a valve that is turned or fails, the gas is going to expand. But it only has so much energy. Once it equalizes with its environment, neither the gas, nor the environment, have energy that can be used by the other. The environment won't repressurize the gas, and the gas can't pressurize the environment further once they are the same pressure. So once that equilibrium is reached, there is nothing more that can be done. And the pressurized gas is the one in the higher energy state, so it has the energy to transfer to its environment.
Does that answer your question or were you asking more about why entropy works the way it does at a much lower level? That is harder to answer. The short answer is "logic", but a longer answer involving why our universe is structured in a way that calls for that and could it be structured any differently in that regard would need to be a lot more comprehensive.
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One way to understand why entropic increase is favored is that there are many more ways to be disordered than there are to be ordered. So when random change is applied to any system with order, chances are that the result will be less ordered.
Say you have a box with 99 steel balls on the left side of a line drawn down the middle, and 1 ball on the right. Assume they're all buzzing around randomly like the molecules of a gas.
Even ignoring collisions and assuming that every ball can switch sides randomly with equal frequency, the chance that the next ball to cross the line will come from the right (thus increasing your order by putting all 100 balls on the left side) is 1/100 while the chance that the next ball will come from the left (thus decreasing your order and leaving you with 98 on the left and 2 on the right) is 99/100. Over time you are likely to end up with about 50 on each side, because there are many more ways to have 50 on each side and only two ways to have 100 on a single side.
The only way to reliably create order out of disorder is to apply energy to a system, such as when a newly-sprouted acorn harnesses energy from the sun and uses it to turn carbon dioxide and nitrates into sugars, fats, and proteins with specific functions so that it can grow into an oak tree.
Brilliant and concise response. I am not sure it could be said better.