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Air pressure stays the same or increase?

  1. Nov 17, 2014 #1
    the barrel has a lid at the top...sealed. I make a hole at the bottom to attach water hose to it. Then I turn on the water hose. The water volume in the barrel increases, no doubt about that. When It takes 7/8 of the barrel. Will the air in the top 1/8 of the barrel be compressed? I ask because the air has no way out. It has to be somewhere in the barrel.

    Needless to say that the barrel is empty but air to begin with.
  2. jcsd
  3. Nov 17, 2014 #2


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    Yes, of course it will. And its pressure will be much above the atmospheric. But that's in theory, because tap water is rarely available at pressures that high...
  4. Nov 17, 2014 #3
    I think I understand. Since you say tap water is rarely available at high pressure, therefore water level in the barrel will go as far as the air pressure in the top of the barrel is equal to the tap water pressure. Then it stops. Maybe the barrel can only go as far as half.
  5. Nov 17, 2014 #4


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    welcome to PF :)

    what do you think would happen ?

    if you think it would stay the same ... give a reason for that thought
    likewise, if you think it would increase ... give a reason
    and we will see how you do

  6. Nov 17, 2014 #5
    I was not sure.... I thought air molecules may seep into water as water level increases like air bubbles in water. In other words, dissolve in water. If it does happen, then air pressure will be about the same. But I got reply earlier that pressure does increase in theory. It confirms my hunch.
    Last edited: Nov 17, 2014
  7. Nov 17, 2014 #6
    Air is not very soluble in water. Only a tiny fraction would dissolve.

  8. Nov 17, 2014 #7
    Thanks for the confirmation. I thought so.
  9. Nov 17, 2014 #8


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    there ya go :)

    when you just think about it simply ... you are squeezing the air into a smaller and smaller volume inside the
    container. It can do nothing but increase in pressure

  10. Nov 17, 2014 #9
    Ok. By now I may go a step further by putting an air pressure gauge into the top barrel to get readings. I believe I can simply plug it in provided sealed.
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