Originally posted by JSK333
So, what makes our definition of what "order" and "disorder" are objective? What enables us to say that "disorder" is something that is objetively measureable?
Entropy is a measurable quantity. It is NOT an arbitrary definition.
Think of it this way: the light we get from the sun is a color we call "yellow". Thats an arbitrary name for a specific physical thing corresponding to the wavelength of light. If you prefer, you can call it "canary" but what you can't say about it is that it is at a shorter wavelength than blue.
Similarly, you can call entropy "Bob" if you want to, but what you can't say about "Bob" is that it decreases with time. The value of "Bob" in a closed system increases with time. Period. Its like describing the wavelenght, not the popular name for it.
Now, there is general agreement that a shattered glass is in a disordered state when compared to the glass before it was shattered. If you subtract the value of "Bob" for the shattered glass from the value of "Bob" for the whole glass (or any other similar scenario), you find that the answer is ALWAYS a positive number. So it makes sense that instead of calling this number "Bob" we call it something that describes what we see occurring: disorder. We could just have easily defined disorder to be a negative quantity, but the end result is the same - the number itself increases with time. So its just more convenient to use a positive number.
In this context, all entropy really means is that it will take more energy to turn that broken glass back into a whole glass than it took to break it in the first place.
Maybe you are just wondering if scientists observe something, arbitrarily decide it looks like disorder increasing, then assign it an entropy? No, they MEASURE the entropy, find it to be increasing, and then describe it as such.
If what you really are asking is "How is entropy measured?", I'll explain: Take a chemical reaction - about the simplest one is splitting water into hydrogen and oxygen. It takes a specific quantity of energy (electricity usually) to do this. Now let's assume conditions are perfect - no loss in the electral wiring or in the power source, no heat lost from the container, etc. So you know EXACTLY how much energy it really took to split that water into hydrogen and oxygen. Now mix the two together and ignite. Again, assuming perfect conditions - the container is perfectly insulated to capture all of the heat generated, the ignition source's energy is subtracted out, etc. Using a termometer, you measure the temperature of the water in your container and find calculate the energy you got back by re-combining the hydrogen and oxygen. What you find is that it took MORE energy to split the water into hydrogen and oxygen than you get back by burning it. This difference is the entropy in the reaction.
http://www.howe.k12.ok.us/~jimaskew/cheat.htm is a site with a problem like I describe. Actually, there is an even simpler one - simply boiling water.
