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Thermodynamics and Open Systems

  1. Dec 20, 2013 #1
    Two caveats:

    One, I may or may not be posting my questions in the correct section of this forum. If not, then please kindly direct to where I should post it.

    Two, I studied English in college. I am not entirely ignorant of the sciences, but they are certainly not my area of expertise. So, if my questions seem elementary or naive, you can understand why.

    My questions:

    If the laws of thermodynamics are or were formulated in reference to closed systems, with "closed" being the key word, then how do they apply to "open" systems?

    In this context, I take a closed system to be one with a finite amount of energy; there is no energy entering the system and no energy exiting the system.

    Would the Earth then be an example of an open system, because energy in the form of sunlight is constantly entering the system? If not, then what would be a good example of an open system?

    Many thanks ...
  2. jcsd
  3. Dec 20, 2013 #2
    A closed system is one of constant mass, not constant energy.
  4. Dec 22, 2013 #3

    Philip Wood

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    Gold Member

    To illustrate Chestermiller's point, the first law of thermodynamics relates the internal energy of a system to the heat and work entering or leaving it. The internal energy can vary.

    The laws were originally derived for systems of constant mass (such as gases trapped in cylinders), but can be - and have been - adapted for use with open systems (i.e. systems with particle numbers that can vary owing to particles entering or leaving through real or imaginary system boundaries).
  5. Dec 22, 2013 #4


    Staff: Mentor

    A closed system is defined as a system that does not interact with other systems. So, yes you are right, Earth is not a closed system because is absorbs energy from the sun and radiates energy to space.
  6. Dec 22, 2013 #5


    Staff: Mentor

    This is not correct. You are talking about an isolated system, which is different from a closed system. Here is a good starting point.

  7. Dec 22, 2013 #6


    Staff: Mentor

    I stand corrected.
  8. Dec 23, 2013 #7
    My thanks for your responses. I chose to post my question here because I was hoping to stay away from Wikipedia (which is a wonderful resource that I use regularly, but I don't like to rely on it as a single source of information).

    My question arises because of a recent debate between myself and a couple of my friends. The topic of the debate actually concerned metaphysics more than physics. One of my friends invoked the second law of thermodynamics in relation to the subject, and my other friend objected to his doing so, claiming that the subject in question was an open system, not a closed system, and therefore the law would not apply.
  9. Dec 23, 2013 #8


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    Staff: Mentor

    For common and uncontroversial questions, it is usually good. But good to verify.
    Hit him with this: system definition is arbitrary/a matter of convenience. Ultimately, everything is part of one big closed and isolated system: the universe. So there is no escape from the laws of thermodynamics.
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