Originally posted by: f95toli
True, but that still does not explain what entropy IS.
I undestand the mathematical definition and I can use the concept to calculate things, but still I can not explain what it is, it is a strange concept.
I have the same problem with the concept of temperature, this is another concept which is almost impossible to define (as far as I know there IS no definition of temperaure so I am not alone).
Temperature is the ratio of excited QM states to unexcited states. If 100% of your electrons are in unexcited states, that corresponds to absolute zero.
Think of it this way: you have a can full of marbles. At first you shake it gently (low temperature), so pretty much all the marbles are at the bottom, with a couple of them bouncing up a bit, every once in a while. As you shake it harder and harder, more and more marbles start to bounce to higher points in the can. If you shake the can as hard as possible (actually, harder than possible for T->infinity), the marbles are equally likely to be at any height in the can. Infinite temperature corresponds to all states being equally occupied.
Entropy really isn't that strange a concept once you realize that it really IS just the amount of order/disorder, and that the trend is to increase OVERALL disorder (so entropy can go down in some processes, but it has to go up by a higher amount in a different process)