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Thread: thermodynamics in a box

  1. #1 thermodynamics in a box 
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    Hi to all physicum passionates,

    I was wondering a simple fact,
    Imagine you have a box in the space filled with gas and you have a cube inside the box that you can put in movement with magnet outside the box.
    Imagine the cube is put in movement for a certain duration T to a constant velocity V.
    With the front face In each collision the gas momentum velocity will ingress in the direction of the cube velocity V.
    But with the back face the inverse effect will occure.
    So why does it require energy to make it moving forward ?


    Thank for your replies


    Last edited by manoir; May 1st, 2019 at 12:49 PM.
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  3. #2  
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    Quote Originally Posted by manoir View Post
    Hi to all physicum passionates,

    I was wondering a simple fact,
    Imagine you have a box in the space filled with gas and you have a cube inside the box that you can put in movement with magnet outside the box.
    Imagine the cube is put in movement for a certain duration T to a constant velocity V.
    With the front face In each collision the gas momentum velocity will ingress in the direction of the cube velocity V.
    That is not how things move through air. Pressure builds up in front of the block and the air will flow to the side. This results in a bow wave.
    But with the back face the inverse effect will occure.
    At the back of the block there will be a low pressure area.
    So why does it require energy to make it moving forward ?


    Thank for your replies
    If you stop applying a force to make it move, the high pressure in the direction of travel and the low press in the back will both stop the movement of the block. Not to mention the friction from the ground.


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  4. #3  
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    yes thank you for your help it s reversible only with "an infinity slow deplacement"
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  5. #4  
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    My question was more if you have helium gas in a bottle, when you have a collision of a helium molecule with another one, it s reversible so the displacement of a helium molecule in helium gas is reversible. But the displacement of a macroscopique object in the gas is not. From which dimension the displacement becomes to be non reversible ?
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