1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

2nd law thermo vs freezer

  1. Aug 4, 2014 #1
    I have read quite a few discussions about this topic of entropy and if this universe has a truly closed or, as I prefer to describe it "undisturbed", system and I have read some discussions about a freezer and chilling ice cubes etc. etc.... To my extent of reading these conversations no one has addressed the actual energy source of the freezer which is the plug in the wall which comes from an external system. Any comments?
  2. jcsd
  3. Aug 4, 2014 #2
    I'm not really sure what you are talking about. Indeed a freezer is not a closed system...
  4. Aug 4, 2014 #3
    Agreed. Maybe you can help me, please. What should be a law of physics...do not trust the internet! ;)
    Anyway, I was just reading on how people say the 2nd law only works for a closed system; yet the Earth is not closed nor anything else in reality, so how can this be a "law"? There is no true vacuum, if you dig string or most quantum theories, and biology seems to go in the opposite direction. And a star's energy from across the galaxy to conveniently set the universe at net zero doesn't float because a universe that is net zero would not be unstable enough to expand. Please respond.
  5. Aug 4, 2014 #4


    User Avatar

    Staff: Mentor

    You are right that there can be no such thing as a truly closed system, because there's always going to some energy leaking into or out of the system. However, that doesn't prevent us from mathematically analyzing how such a system would behave if we could construct one, and that's where the law comes from.

    It is possible to construct systems in which the energy leakage in or out is small enough to ignore. These act so much like the truly closed systems that we cannot build that we can use the closed system math on them.
  6. Aug 4, 2014 #5
    I guess that makes sense. Thank you.
  7. Aug 4, 2014 #6
    A closed system could maybe have a way of a recycle energy method instead of needing an outside source.
  8. Aug 4, 2014 #7
    With respect to the freezer, yes, it is not a closed system. If it were, then it would not be able to freeze (separating heat from not-heat requires work). How ever, since you know that that is true, then you also know that everything that enables that work to get done has to come in through that small black wire, so you can study the relationship between the cold (not temperature, but heat capacity, or lack of it) and what the wire is doing. Without the law, you have no way of knowing there has to be an exclusive relationship. That's how the relationship between a horse working turning a cannon drill and the heat given off by the work were related.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook