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Water all around when air stays upside down

  1. Oct 13, 2015 #1
    I'm not sure I understand exactly how or why an upside down container filled with air does not then become filled with water when submerged in it. I make the following inquiry:

    assuming the integrity of the material that the container is made out of, and the water itself not being deep enough to undergo any strange or radical physical changes under tremendous pressure [which, from my limited understanding, can happen to substances at certain amounts of pressure], then a) does the air in the container itself become more highly pressurized, or does some of it escape, as it goes deeper underwater, and b) would moving an object into and/or out of the container significantly perturb or alter the state of the air in some way?

    I know this seems an ignorant question, but I greatly appreciate any help given in assisting my understanding.
     
  2. jcsd
  3. Oct 13, 2015 #2

    sophiecentaur

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    HA. Good question and welcome to PF

    Your upside down bucket was full of air before it touched the water. As you carefully lower the bucket into the water, if you don't disturb the water surface, where could the air go? The air that was actually in the bucket has to stay under the bucket. The pressure across the horizonal surface of the water will be equal all over. That means the air will not be 'squirted out' sideways (once the bucket has been lowered far enough so that the whole rim is under water with no gaps for air to escape). When you press harder, the bucket will go deeper and the pressure will increase in the space inside. The air is very compressible so it will take up less and less volume as the bucket descends further down. At a depth of about 10m (pressure about 2 Atmospheres) , the volume of the air will have become half what it was at the surface.
    At great depth, the gases in the air could dissolve slowly in the water until it all disappeared. If you did this in an atmosphere of CO2, you would find that the bucket would fill with water farily quickly because CO2 is relatively soluble (fizzy drinks). Nitrogen is soluble enough to give divers problems when they spend a long time breathing compressed air (compression sickness / the bends) whan they come to the surface and bubbles of N2 form in their blood.
    Puting an "object" up into the bucket space could displace some of the air and send bubbles up to the surface. Are you looking into what happens in a diving bell?
     
  4. Oct 13, 2015 #3

    phinds

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    The air in the container will get compressed as the vessel gets deeper in the water. As for the vessel getting filled with water, where is it that you think the air will go and how will it get there?

    EDIT: I see the centaur beat me to it.
     
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