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Sum of energy in a system like barometer

  1. Oct 16, 2012 #1

    lba

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    The drawing show a barometer with water inside. If I move up object with air inside I recover PV energy (P=external pressure, V=volume of object). If I want to move out object when it is at top, I need PV, is that ?

    So if it's that. If I replace:

    1/ gravity by balls attracted with springs (for have law of gravity in blue column)
    2/ atoms with empty balls with very thin walls
    3/ pressure with velocity of empty balls
    4/ object to move up with some empty balls

    I can disappear empty balls at top: at top we can recover energy from velocity and let at top only the small walls of balls. If walls are very very thin, I let virtually nothing. With friction = 0 where the energy is lost ?
     

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  3. Oct 16, 2012 #2

    CWatters

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    It takes energy to force the object into the bottom of the tube against 10m head of water (about 1 bar).

    I'd be surprised if the idea wasn't in the museum somewhere..

    http://www.lhup.edu/~dsimanek/museum/unwork.htm

    Oh look it is..

    http://www.lhup.edu/~dsimanek/museum/unwork.htm#buoy4

     
    Last edited: Oct 16, 2012
  4. Oct 16, 2012 #3

    lba

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    thanks for your reply :)

    but it's like a barometer not a simple column of water, no need energy to enter in water because outside pressure = 1 bar.
     
  5. Oct 16, 2012 #4

    CWatters

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    That just means the problem is elsewhere. If the thing is 10m high the pressure at the top will be close to a vacuum so work has to be done on the object to get it out to atmospheric pressure.

    Personally I don't find it productive looking for reasons why a perpetual motion machine won't work.
     
  6. Oct 16, 2012 #5

    lba

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    I'm agree with that, the sum of energy is 0.

    But I would like to know with a macroscopic scale system. Imagine the system with balls, velocity, springs, ..., like that it's possible to recover energy from velocity. And if balls are empty, they disappear at top, let only walls and walls can be very small in theory.
     
  7. Oct 17, 2012 #6

    lba

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    I put a drawing for show an example. Sure the system would be complex but it's only a technological problem.
     

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  8. Oct 17, 2012 #7

    CWatters

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    I'm not sure what you are trying to achieve? It's easy to make a system that appears to have a cycle over which the energy sums to zero. Here are three examples..

    1) Planets orbiting the sun. PE and KE exchanged in eliptical orbits.

    2) Magnetic spring..Imagine three magnets arranged...

    NS SN NS

    with the outer pair fixed to the desk and the middle one free to slide left and right. If you pull it to the left and let go it will oscillate back and forth.

    3) Imagine a mass on a spring. If you press the mass down energy is stored in the spring. Let it go and that stored energy is converted to PE. The mass bounces up and down.

    In an ideal world the these systems keep going forever because the energy sums to zero. How well they work in practice is also "only a technological problem".

    There is no excess energy to be harvested from the velocity the objects have in your system. That energy will have come from somewhere.

    After awhile most people stop playing noughts and crosses (aka Tic Tac Toe) because they realise you can only ever draw no matter how well you play.
     
  9. Oct 18, 2012 #8

    lba

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    I'm trying to understand where energy is lost. First, with a system with water+gravity+vacuum+pressure (a barometer), the sum of energy is 0, we agree with that. Now, take the hypothesis we can recover energy from temperature without lost energy. With a barometer when you put a ball with air inside at bottom (air come from atoms around), when the ball move up in water, we recover energy PV (p=external pressure and V=volume of ball). At top, recover energy Wth from temperature. And put atoms of balls outside (small volume because it's a liquid). Move out atoms of ball and give the energy Wth of atoms for have the same temperature. I don't lost energy of heating but I have recover energy from mouvment of the ball. And even it's not possible to recover energy from temperature I think it's not possible, so where the energy is lost ?
    Second: I take a macroscopic system and try to find where the energy is lost. I don't find...
     
  10. Oct 18, 2012 #9

    CWatters

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    I don't follow what you mean by..

    You need to state any assumptions made such as

    The walls are thin/rubber so the volume can change in response to changing pressure
    The temperature is changing/constant?

    If the pressure falls and the volume increases P*V might be constant.

    PV=nKT

    if temperature is constant the P*V is constant.
     
  11. Oct 18, 2012 #10

    lba

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    Why energy is PV ? because at top if I want to return ball (without change its temperature) I need P*V energy. Where the pressure falls ? At top the ball is in 0 pressure (vacuum)

    Temperature changing/constant ? For me the internal temperature of ball don't change during move up. The external temperature change because water move up but after water move down so for me all temperature is constant. And this is for that I have a problem
     
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