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How do gasses in a bottle have the atmospheric pressure

  1. Sep 17, 2013 #1
    when you close a bottle , why doesn't it collapse on itself ? shouldn't it since the pressure outside the bottle is greater than the pressure inside
    i know this is not the case , but how do gasses inside the bottle have the same pressure as the atmospheric ?
     
  2. jcsd
  3. Sep 17, 2013 #2
    The gas pressure equalizes while the cap is open. If there was a pressure imbalance then there would be flow until the pressure is balanced. One the pressure has equalized putting a cap on isnt going to change that.
     
  4. Sep 17, 2013 #3
    Before you put the lid on, the air pressure inside is equal to the air pressure outside. Why do you think that placing the lid on the bottle would substantially change that situation?

    Chet
     
  5. Sep 17, 2013 #4
    There are quite a few "bottles" we come across daily whose internal pressure is lower than external. For example, certain light bulbs are that way. CRT computer/TV screens. When we dive a few meters deep, we are roughly in the same condition, too.
     
  6. Sep 17, 2013 #5
    I hope this response doesn't confuse the OP. Maybe you can elaborate a little more on how, in these instances, the lower internal pressure comes about, and how this differs from the situation of simply putting a lid on a bottle at atmospheric pressure and constant temperature.
     
  7. Sep 17, 2013 #6
    I merely wanted to illustrate that it is perfectly fine to have a "bottle" with some low pressure inside, without its collapsing. I hope the OP can figure out what keeps it from collapsing. If not, we can discuss that.
     
  8. Sep 17, 2013 #7

    davenn

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    as others have said ... It doesn't collapse, or for that matter expand. UNLESS there is a change of pressure on the outside
    If the outside pressure drops, the internal pressure will be higher and visa versa

    some years back when in Hawaii, I did this experiment with an empty plastic coca cola bottle.
    On the "Big Island" is the mountain of Mauna Kea one of the highest places in the world you can drive to, just shy of 14,000 ft ( I went up to see all the telescopes at the top).
    There is a large change in atmospheric air pressure between sea level and the top of the mountain. At the bottom of the mountain, I uncapped and recapped the empty coke bottle so that the pressure inside and outside were close to that of sea level pressure. The sides of the bottle would flex happily as I tried to squash it. But by the time I got to the top of the mountain, the sides of the bottle were VERY tight as the inside air pressure was now much higher that the outside air pressure.!!

    I then repeated the experiment in reverse. I uncapped and recapped the bottle at the top of the mountain and by the time I got to the bottom, the bottle had virtually totally crushed itself cuz now the air pressure inside was much less than the air pressure outside.

    A real cool experiment :smile:

    cheers
    Dave
     
    Last edited: Sep 17, 2013
  9. Sep 17, 2013 #8
    But that is the wrong answer to the question - the bottle in the question doesn't have a lower pressure inside.
     
  10. Sep 17, 2013 #9
    I was not answering the question. I was addressing this: "why doesn't it collapse on itself ? shouldn't it since the pressure outside the bottle is greater than the pressure inside", where "since" is replaced with the less suggestive "if".
     
  11. Sep 17, 2013 #10

    davenn

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    Here's an experiment you can do at home
    Make your own barometer and note the changes in air pressure

    Take a glass jar and stretch a sheet of rubber across the opening and rubber band or tape it so it seals the jar. Take a drinking straw and glue it to the rubber top and so it sticks out past the edge of the jar for some inches
    set it against a cardboard backing and have a line marked off in say 1/4 inch increments and watch how the end of the straw changes its position as the weather changes

    attachment.php?attachmentid=61916&stc=1&d=1379469965.jpg


    cheers
    Dave
     

    Attached Files:

  12. Sep 18, 2013 #11
    thanks everyone for your answers , i thought that once the gas is isolated it lost pressure , but now i understand the the atmospheric pressure already pressurized enough gas in the bottle such that they actually keep pushing on themselves with pressure = to that of the atmospheric gas , am i right ?
     
  13. Sep 18, 2013 #12

    davenn

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    You are basically right, just your wording is a little oddball :wink:

    As you have discovered ... there is no logical reason why that would happen

    The gas in the bottle may have depressurised when you took off the cap ....
    consider what happens when you unscrew the cap of a new bottle of coca cola .... what do you hear ? .... the hiss of the sudden depressurisation of the gas inside ( some of it which was disolved in the drink till you uncapped the bottle)

    you would have done better by just saying ...
    When the cap was removed from your bottle, the pressure inside and outside equalised.

    Screwing the cap back on keeps the equal pressure UNTIL something occurs that changes the pressure either outside or inside the bottle.

    hope that helps :)
    its all fun learning .... I really suggest you try making one of those barometers as I showed above.

    cheers
    Dave
     
  14. Sep 18, 2013 #13
    yup thanks alot for answering my questions :D i will do it once i get the rubber :D thanks alot again
     
  15. Sep 18, 2013 #14

    jtbell

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    I think you're referring to the relative pressure inside the bottle, that is, the difference between the internal and external pressure. If the volume of the bottle doesn't change, nor the temperature, then the absolute pressure inside doesn't change either.

    Right, the difference in pressure between inside and outside had increased, because the outside pressure had decreased, but the inside pressure stayed the same (more or less; if the bottle expanded a bit, that would have reduced the internal pressure a bit).
     
  16. Sep 18, 2013 #15

    davenn

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    QUOTE=jtbell;4507103]I think you're referring to the relative pressure inside the bottle, that is, the difference between the internal and external pressure. If the volume of the bottle doesn't change, nor the temperature, then the absolute pressure inside doesn't change either........
    [/QUOTE]

    yes the relative pressure difference betwen inside and outside :smile:

    hence my following comment....
    I just, at this stage, didnt go into what might cause those changes



    D
     
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