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Perpetuum Mobile

  1. Aug 2, 2006 #1
    i had an idea about a perpetual motion machine.
    i have read and saw that my idea is just like Robert Boyle's self-flowing flask.
    from what i read this machine who by my idea should let the water move for eternity is not working because it doesn't follow the rules of thermodinamics.
    I didn't really understand this explanation,does anyone knows what's the problem?
  2. jcsd
  3. Aug 2, 2006 #2


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    Both ends of the "device" are at the same pressure. There can not be flow without a pressure differential. Examone what the static pressure will be in the tube and the vessel. Also take a look at how liquid levels work. You'll see some similarity there.
  4. Aug 2, 2006 #3
    Since both ends are open to air pressure, the only thing that can make water flow through the tube is a difference in heights. Only when the output nozzle is lower than the flask water level will the water flow. This is not the case in the drawing, and what will really happen is that the water will fall back down the tube until it is at the same level as the flask water. If the nozzle is held lower than the flask water level, water will flow out the tube, but there will be no way to get it back into the flask without doing work. While this concept can be used to steal gasoline from people's cars, sadly it can't be used to make a perpetual motion machine.
  5. Aug 2, 2006 #4


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    Actually, it's also possible to get this to happen if there's a heat difference. Water has weird thermal properties, but any gas will flow up the warm side, and down the cold side of a looped vertical tube if the temperature difference is maintained.
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