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Thermodynamics Entropy state function?

  1. Aug 16, 2015 #1
    Entropy of any system is state dependent. I just read about thermodynamics and I got this question. I first set my system which contained my classroom, the hallway connecting my physics teacher's cabin and the classroom. Then let the entropy of the system be S when I was going to the classroom in the hall way.(The system conatins me and my teacher in the cabin who is reading a book.) I went to the classroom and took out my book from some bag, scribbled something and put it back in the bag again. Then I just sat there and after sometime took out the book when my teacher came in. He started teaching and I started taking notes. Let the entropy be now S'.
    Nowe let us take another case. I never put my book inside the bag again. My teacher came in the classroom and I started taking notes. So now we again have the same entropy S'.
    This means that even if danced around like a fool and then sat back, there should not be any change in the final entropy. So this means that if I calculate the change in entropy between two states, then I cannot tell what happend in between. But things should occur in only one manner or things should change in the system. I am not able to understand where my mistake is. Please help me out on this one.
     
  2. jcsd
  3. Aug 16, 2015 #2
    If you put your book back in your back you're decreasing the entropy of the system, after you take it out again, you increase it to S'. With work, you can decrease the entropy of a system.

    While dancing around your body increased its temperature and you sweated, so you transferred (more) heat from your body to the air by convection and to the wall by radiation and water of your body is evaporated (so changed from liquid to gaseous state) and spread in the air. All effects which increase the entropy of the system.
     
  4. Aug 16, 2015 #3
    I think what stockzahn is saying is that, for the various situations you described, the final entropy of your system is not the same. Maybe you think it is macroscopically, but microscopically (which is what really counts), no.

    Chet
     
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