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Homework Help: Round bubbles in water

  1. Dec 8, 2012 #1
    1. The problem statement, all variables and given/known data
    1)In a bubble the air molecules spread themselves out equally. However, outside where there is liquid, the pressure on the higher part is greater than below. So why would the bubble be perfectly round?

    Also, would the pressure in the bubble be calculated via the hpg of the bubble as demonstrated in this image:
    http://postimage.org/image/67myy06gl/full/ [Broken]

    2. Relevant equations

    3. The attempt at a solution
    1)I am guessing that the air molecules in the bubble aren't spreaded out equally and on top the pressure exerted on the membrane of the bubble is greater than below. So this makes up for the higher pressure of water acting on the membrane on top than below. Hence the bubble is perfectly spherical. But still the air pressure cannot provide so much difference such that it should be equal right?

    2) i think so because there is no formula for find the pressure of the air alone (usually when we use hpg, it would be of the fluid covering the whole system). But is it possible to find the pressure of the air bubble at point A by using hpg? If so how will it be done?

    Thanks for the help! :smile:
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Dec 8, 2012 #2


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    Are bubbles round under water?
  4. Dec 8, 2012 #3
    i think so? Or they are oval shaped? If that is so then it explains the pressure difference on the different height of the bubble. But still, is it possible to use hρg on the inside of the bubble to find the pressure? Because in most cases the h we use is the 'covering' fluid and not the one immersed like the bubble.
  5. Dec 8, 2012 #4
    I think the answer is to do with surface tension playing a greater role than pressure differences. It is after all surface tension that keeps the air molecules together.
  6. Dec 9, 2012 #5
    oh okay i understand now. but for the second question, if we have a gas bubble in water, we can find the pressure exerted on it by using hpg where h is the height of the liquid at various points on the bubble. But is it possible to find the pressure exerted onto the water molecules by the bubble using hpg?

    I'm pretty confused because in most cases the height, h in hpg is usually the main covering fluid but still is it possible to use hpg for the air bubble if so how?

    Thanks for the help :smile:
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