Difference between revisions of "Second Law of Thermodynamics"

Revision as of 20:03, January 8, 2008

The Second Law of Thermodynamics states that heat can never flow from a cold body to a warmer one. An equivalent definition is that the entropy of an isolated system tends to increase. ^[1]

According to this law, it is impossible to build a perpetual motion machine, and entropy in a completely closed system must either increase or remain constant.

The three forms of the Second Law of Thermodynamics

There are three different types of systems that the Second Law of Thermodynamics can apply to:

• Isolated system - Does not exchange matter or energy with its surroundings
• Closed system - Exchanges energy, but not matter, with its surroundings
• Open system - Exchanges both matter and energy with its surroundings

Interpretation

A simplification caused by misinterpretation says: disorder will increase within a closed system.

Actually entropy is a little more abstract and the second law of thermodynamics means something like this: the universe will always become increasingly uniform, that is: heat will spread until the entire universe has the temperature and energy level (in a closed system heat will always spread from a place where there is a lot of heat to a place where there is less until balance is achieved), forces will continue to work until a universal balance has been achieved.

In this final state the universe is one uniform space where nothing happens and no work (moving something) can be done since there are no above average concentrations of energy left. This state is called maximum entropy and is said to be in perfect disorder (although intuitively its uniformity would seem to be a state of perfect order) because it has become impossible to determine what happened in the past. i.e. There are an infinite number of ways (histories of the universe) maximum entropy could have been reached.

Entropy and disorder for dummies

In this context "increasing disorder" means "that which happens if you let nature take its course". Imagine your old room at your parent's house. Remember how easy it was to let the room turn into a uniform mess (disorder) and remember how hard it was to clean it up until it fit a specific, non-uniform design (order). Not cleaning up would always result in an increase of entropy in your room!

Now, in nature there is no one to clean up the universe, only chances. The chance of something becoming orderly is a lot smaller than the chance of something becoming disorderly (since there are far more possible disorderly states to choose from).

Alternatively one could think of how difficult it is to construct a house of cards, while almost everything that happens in nature will result in its collapse.

On a universal scale a tidy room would be a universe which has pockets of above average concentrations of energy (remember that E=mc^2 so this includes matter as well.) A messy room would be a universe in which all energy is equally spread out.

The flow of energy (by heat exchange) to places with lower concentrations is called the "heat flow".

The often heard argument that this law disproves an eternal universe is true, since in that case maximum entropy would have been reached already.

References

1. ↑ “Another way of stating the second law then is: ‘The universe is constantly getting more disorderly!’ Viewed that way, we can see the second law all about us. We have to work hard to straighten a room, but left to itself it becomes a mess again very quickly and very easily. Even if we never enter it, it becomes dusty and musty. How difficult to maintain houses, and machinery, and our bodies in perfect working order: how easy to let them deteriorate. In fact, all we have to do is nothing, and everything deteriorates, collapses, breaks down, wears out, all by itself -- and that is what the second law is all about.” Isaac Asimov, Smithsonian Institute Journal, June 1970, p. 6