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Discussion in 'Electronic Design' started by RichD, Aug 22, 2013.

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  1. RichD

    RichD Guest

    A plastic liter jug, half filled with water. It has
    an inlet valve, with a pump. You pump several
    atmospheres of pressure into the jug.

    Wait for the system to equilibrate to room temp.
    Then open the outlet valve. Let the air escape until
    equilibration pressure.

    What's the interior temp.?
  2. Robert Baer

    Robert Baer Guest

    RichD wrote:
    1) what is "several atmospheres of pressure"?
    2) how strong is that jug? I had one that could BAERly hold 1.1
    atmospheres at STP.
  3. Syd Rumpo

    Syd Rumpo Guest

    Lower than ambient, although the outlet valve will heat up through
    friction. But what's the water for?

  4. Bill Sloman

    Bill Sloman Guest

    Air saturated with water vapour has slightly different thermodynamics to dry air - the cooling as it expands causes some of the water vapour to condense, releasing its latent heat of vaporisation.
  5. Correct - this is one of the thermodynamic effects I don't calculate in
    my (1997) water rocket simulator at


    Bryan Holt duplicated my math independently, and correlated it with some
    high-speed video measurements taken by Prof Dean Wheeler at UCB.

    Bryan found a couple percent variation in the velocity curves which was
    only corrected when he added the thermodynamic effect of vapour

    Just thought you'd like to know that Bill's assertion is confirmed by
    real physical experiments.

    Oh, and regarding strength, our tests on PET soft-drink bottles say that
    typical 2L bottles are safe for 120psi, 1.25L bottles for 160psi, and
    600/660ml bottles for 200psi. That's assuming new bottles with the
    typical wall thickness of about 0.3mm. Smaller diameter bottles are
    under less hoop stress for the same pressure.

    As to the original question, google for the adiabatic expansion formula,
    and adjust for condensation.

    Clifford Heath.
  6. Bill Sloman

    Bill Sloman Guest

    Some experiment. Physicists think big, but calling a hurrican "an experiment" is a trifle meglomaniac. Then again, our current exercise in digging up and burning every last scrap of fossil carbon we can find to raise the CO2 level in the atmosphere is an even larger scale experiment.

    It's a pity that our right-wing nitwits haven't been able to get their heads around the preliminary results, to the extent that they claim that it's not working.
  7. Fun! (I've always wanted to build a water/ bottle rocket.)

    George H.
  8. Phil Hobbs

    Phil Hobbs Guest

    Wow, Bill, your trip back to Holland does seem to have refilled your
    Europeon scorn-and-derision tanks. Go try that stuff down at the local
    pub in Sydney, and I bet it drains out again pretty fast.


    Phil Hobbs

    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC
    Optics, Electro-optics, Photonics, Analog Electronics

    160 North State Road #203
    Briarcliff Manor NY 10510 USA
    +1 845 480 2058

    hobbs at electrooptical dot net
  9. Bill Sloman

    Bill Sloman Guest

    "Scorn and derision"? Check out Phil Allison for what you've got to producefor a Sydney audience to register scorn or derision.

    What I posted was benign fair comment.

    Or have you actually created your own hurricane in the course of one of your experiments? That really would have been a Clausewitzian extension of diplomacy by other means. You'd have been lucky to get off with mere scorn andderision if you'd been responsible for such a pontentially destructive phenomenon.
  10. Robert Baer

    Robert Baer Guest

    To absorb the CO2 and make fizzy water at high pressures (OT baybe
    assumes bottle cannot bust).
  11. There's a Yahoo ML/forum that acts as a support group for addicts. I
    created it when I closed our majordomo server a decade ago.

    Personally I haven't launched in over two years, and even then it was
    only to scare the crows away - I found they don't like things going
    booom and then flying close by at 300km/hr. It's a great way to teach
    physics, however, as there's enough complexity to tickle NASA but an 8yo
    can still participate. The "Clark Cable" cable tie launcher is pretty
    easy to make.
  12. RichD

    RichD Guest

    No. Why should it drop?
    Sort of a distraction.

    But also motivated by reality. A friend believes
    he's going to develop a water cooler this way.
    For bicyclists and hikers -
  13. Bill Sloman

    Bill Sloman Guest

    Joule-Thompson effect.–Thomson_effect
    Not a very effective one. Run the numbers.
  14. Syd Rumpo

    Syd Rumpo Guest

    You'd certainly need a cool drink after putting in the effort to cool a
    drink. Best plan is to carry a few sachets of dehydrated ice to drop in
    as required.

  15. Roger

    Roger Guest

    "RichD" wrote in message

    A plastic liter jug, half filled with water. It has
    an inlet valve, with a pump. You pump several
    atmospheres of pressure into the jug.

    Wait for the system to equilibrate to room temp.
    Then open the outlet valve. Let the air escape until
    equilibration pressure.

    What's the interior temp.?


    Since you did not specified how many atmospheres of pressure was pumped into
    the jug, all I can give you is a general answer: The interior temp. will be
    lower that room temp.


  16. If you fill a canvas bag with water, the water slowly wicks to the outside,where it evaporates, this cools the rest of the water inside. Not sure what they are called, but I say them in Australia many moons ago. (I guess they work best in a dry climate.)

    George H.
  17. Hey near the first line the wiki link says that,
    "At room temperature, all gases except hydrogen, helium and neon cool upon expansion by the Joule–Thomson process."

    Why not those gases too? In grad school I use to drain 2000 psi helium tanks into the helium liquifier... they got plenty cold on the outside.

    George H.
  18. Hi Mike, thanks, more thought will be required. (It's been way too long since I did any Thermo.) I don't think the pressurized bottle is best described as the Joule-Thompson (J-T) process.. it's a free expansion.

    The above mention helium liquifier did have a J-T throttling valve, but as you say it would only start working down around ~10K where some fraction ofthe throttled helium would turn into liquid. I spent many nights as a grad student baby sitting the liquifer and tweaking the J-T valve.

    George H.
  19. RichD

    RichD Guest

    That's a new one on me, I don't keep up wioth the
    latest chemistry research.

    But anyhow, that effect refers to the gas
    EXTERIOR to the container. The question
    here involves the gas remaining inside.

    Though the Wikipedia article is poorly written,
    typical of that site. Which means you cited
    an incomprehensible reference, one which YOU

    The quiz is motivated by the ideal gas law.
    I alwways thought, one equation, multiple
    degrees of freedom, how the heck can you
    determine anything?

    So with this pressurized water bottle,
    if we examine the formula:
    pv = nRT
    Considering the initial and final conditions,
    which variable(s) vary, on each side?

    As expected, everyone believes a pressure
    drop must imply a temperature drop -

    Perhaps the nozzle makes a difference,
    I don't know -
  20. Guest

    Hi Rich, So if you take two bottles, one full of gas and the other empty.
    And then you connect them and let the gas flow from one to other, then the gas filled bottle gets cold and the bottle that started empty gets hot. And if you take both bottles as the whole system, then it looks like thermo predicts that the temperature of the total system doesn't change. It seems like the exact temperature difference will depend on how the gas is transfered.... after all I could always let the gas spin some generator and get energy out of the system.

    George H.
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