Negative Changes in Entropy?

Re: Negative Changes in Entropy??

If you are talking at the micro-level, there are lots of examples where you have a temporary local decrease in entropy.

At the level of the observable universe, the answer turns out to be no. You can calculate the chances of something weird happening in something several tens of billions of light years across over several tens of billions of years, and the odds are very low. The observable universe just isn't that large.

Now if the universe were infinite in either space or time, then you'd come up with a different result.

 

Re: Negative Changes in Entropy??

The simplest way to tackle issues in thermodynamics is this: Imagine a system enclosed in a boundary that separates the system from the surroundings. Energy exchanges in the form of heat and work between the system and surroundings are allowed. The second law says that it is impossible for the sum of the entropy changes of the system and surroundings to decrease. However, it is possible for the entropy change of the system to be negative and that of the surroundings to be positive (and vice-versa) so that the their sum is not a negative quantity.
By way of example of negative entropy changes of a system we may quote a simple example:
1. Bring a closed system say, m grams of an ideal gas at a temperature T into thermal contact with a heat reservoir(HR) at temperature T. Allow heat to flow from the system to the HR by reversibly expanding the gas from an initial state of equilibrium A to final state of equilibrium B. Now the entropy change of the system is negative and that of the HR positive their sum being zero. If, instead of HR being at temperature T it is at a temperature less than T, then the expansion of the gas (irreversible) leads to a decrease of entropy of the system and increase of entropy of the HR, the sum being a positive quantity. HR is the surroundings in this example.
Finally, the entropy change at a location in the universe can be negative with a simultaneous entropy change at another location being positive their sum being a non negative quantity, but it is impossible for entropy change at a location to be negative with no other change (or a change that entails the sum of the entropy changes to be a negative quantity) else where in the universe is impossible. The probabilities of the results of these experiments is 1 (100%) with no uncertainties.
Statistical thermodynamics (ST) does not give results that are 100% certain, It gives probabilities of occurrence of processes, such as the one quoted by you, leaving scope to raise questions such as the one you raised. ST results are never 'yes' or 'no' type; while results of (equilibrium) thermodynamics are always 'yes' or 'no' type.

 

Re: Negative Changes in Entropy??

That only works if you assume the universe has a boundary. There is no evidence supporting that premise.

 

Re: Negative Changes in Entropy??

Start here and look at the references

http://en.wikipedia.org/wiki/Fluctuation_theorem

 

Re: Negative Changes in Entropy??

We don't assume universe has a boundary; we only assume that the system has a boundary. What is not system is surroundings. System and surroundings make the universe.

 

Re: Negative Changes in Entropy??

So, you are saying the universe is a system, and, by your logic, it necessarily has a boundary?

 

Re: Negative Changes in Entropy??

I wrote, 'System and surroundings make the universe.'

 

Re: Negative Changes in Entropy??

I believe the universe as a whole is considered one system, so there would be no surroundings.

 

Re: Negative Changes in Entropy??

If we try to treat the universe as our system it would not help us much. We must then be able to specify its state by giving definite values of the thermodynamic variables such as pressure, volume, temperature, internal energy and so on.

 

Re: Negative Changes in Entropy??

Why? What's incorrect about not knowing what those variables are?

 

Re: Negative Changes in Entropy??

Not knowing the values of those variables is to confess that we do not know the state of our system.

To get issues in thermodynamics resolved, it is necessary that we have systems in well defined states or else thermodynamics does not give us the results that we seek from it.

 

Re: Negative Changes in Entropy??

Hold on, can you just define part of the universe to be your system and make it an open system instead of closed?

 

Re: Negative Changes in Entropy??

We define open system as one in which exchange of matter (besides energy in the form of heat and work) is allowed across the boundary that separates the system and surroundings. To distinguish it from a closed system, we define closed system as one in which exchange energy (in the form of heat and work) is allowed across the boundary that separates the system and surroundings.

 

Re: Negative Changes in Entropy??

Ok, but if you say the system and it's surroundings is the universe, then you can't have anything else outside of this, as the universe isn't believed to have a boundary.

 

Re: Negative Changes in Entropy??

It is not clear if you have something in mind and trying to drive at it.

Once you define your system then what all that you can imagine or want to have in the universe, can be included in the surroundings. the system and the surroundings now make the universe. You will then be left with nothing more to think of than the universe, which encompasses all that you could think of.

 

Re: Negative Changes in Entropy??

I'm so confused lol. Ok, all I was saying was that if you have the system AND it's surroundings as the universe, you can't have another boundary somewhere that would be another system and it's surroundings. You could absolutely have part of the universe as the system and the rest as it's surroundings.