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B Second law of thermodynamics

  1. May 29, 2017 #1
    From what I know, the law says that disorder increases over time.
    But, when a building is constructed the disordered bricks,cement etc. take form of the ordered building.
    Am I wrong or is this an exception?
     
  2. jcsd
  3. May 29, 2017 #2

    Dale

    Staff: Mentor

    Is the building an isolated system?
     
  4. May 29, 2017 #3
    Well, I think it is. It is not interacting with any other thing.
     
  5. May 29, 2017 #4

    Dale

    Staff: Mentor

    Really? You think that a pile of disordered bricks, cement, etc. with no external interaction, will assemble themselves into a building.

    You probably should visit a building during construction and gather some observations to support or reject your hypothesis.
     
  6. May 29, 2017 #5
    I did not mean that.
    What I meant was that the bricks won't interact with any thing outside the system like the vehicles,pedestrians and such like
    Sorry, if I wasn't clear.
     
  7. May 29, 2017 #6

    Dale

    Staff: Mentor

    I am not sure why you think that is relevant.

    What is the definition of an isolated system, and what is the definition of the second law of thermo?
     
  8. May 29, 2017 #7
    OK, so what do you mean by 'system' ?
    I did not understand your first question.
     
  9. May 29, 2017 #8

    jbriggs444

    User Avatar
    Science Advisor

    In this context, a "system" is a collection of things around which you draw a notional boundary. You watch what comes in or goes out through the boundary over time. You can ask questions about the state of the system.

    So you start with a pile of bricks in the middle of a vacant lot. You decide to consider the property line around the lot as the boundary of your system. Dale asks: if nothing comes in or goes out across the property line, will that pile of bricks assemble itself into a building.

    Edit: Note that Dale spoke of an "isolated" system. That means that no workers are allowed to cross the line. Nor any bags of cement, catered lunches, air or water. Nor any electricity, light, sound or heat. No external forces or interactions of any kind.
     
    Last edited: May 29, 2017
  10. May 29, 2017 #9

    hilbert2

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    Gold Member

    The people and machines, which construct the building, release heat and gaseous metabolism/combustion products that increase the entropy of the combination of the construction site and its immediate surroundings. This entropy increase is larger than the entropy decrease that happens when the construction materials are ordered in the form of a building.
     
  11. May 29, 2017 #10

    Dale

    Staff: Mentor

    You need to know the concept of a system to do any physics. A system is a defined boundary containing some configuration of matter and energy. The configuration of the matter and energy is known as the state of the system. Physics is largely a process of keeping track of the matter and energy that crosses the boundary of a given system and applying laws that allow you to predict how the system changes state.

    So in your case the system would be the building, and we have it in two states: a completed building, and several disorganized piles of building materials.
     
  12. May 30, 2017 #11
    Ok, sorry for the problem, I just got a bit panicky .
    No, the building is not an isolated system.
    Thanks for your help.
     
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