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Temperature around a bubble of air in water

  1. Oct 10, 2013 #1
    1/ Like density in water change with depth, the number of chocs (from temperature) is not the same all around the bubble. The bubble have an additional up force I think. But the weight is directly the volume of bubble by the density of water. The weight can be change with temperature ? Maybe it's possible to study easily a vacuum object. How temperature compensate up force in water ?

    2/ In addition, it's possible to have different temperatures in water (like Oceans have), this change the number of chocs, so the up force can be bigger (or down force if hot temperature is at top). Bubble can be an object with vacuum and a shape like we want. If temperature is hot at top, the number of choc at bottom is lower.

    3/ It's possible to study in gas, I think there is the same problem. No gas outside, just gas inside with a vacuum object attached with a rope for example. I consider in this case force from gravity at k/d² not k/d. It's important because if I isolate a part of gas somewhere, the difference of pressure inside object is not the same.
     
    Last edited: Oct 10, 2013
  2. jcsd
  3. Oct 10, 2013 #2
    Your questions are impossible to understand. What's a choc? Why do you have so many truncated phrases? What's a vacuum object? Try asking one question at a time, using full sentences, using words with their standard definitions, better explaining the physical situation at hand, etc...
     
  4. Oct 10, 2013 #3
    OK, sorry if my english is not perfect :thumbs:

    First question: the weight of a column of water is given by the pressure at bottom multiply by surface. If I add an object in it with vacuum in it (theoretical study) and attached with a rope at bottom, the water move up, the pressure increase, the weight must be the same because rope give a force to up. But if water is at 20°C for example, all around the vacuum object water give pressure from static pressure and "chocs" (vibrations if you prefer) from temperature, density of water change with pressure (imagine a big vacuum object for have significant effect), so more pressure is at bottom than at top of the vacuum object, so the object has an additionnal force to up. Like that the weight reduce. So another force must be elsewhere but I don't find it.
     
    Last edited: Oct 10, 2013
  5. Oct 10, 2013 #4

    mfb

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    Only if the pressure at the top is zero.
    If the bottom is fixed.
    There is no pressure from vibrations. Temperature changes the density profile a bit, which changes the pressure profile a bit - you would have to calculate if thermal effects win over pressure effects, but there is an equilibrium in both cases. The forces you imagine do not exist.
     
  6. Oct 10, 2013 #5
    I don't say it's pressure, just "chocs". It's energy from heating, like there are more atoms at bottom than top (per volume because density is different), why temperature don't give a force to up ?
     
  7. Oct 10, 2013 #6

    mfb

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    There is no reason why temperature should give any force anywhere.
    That does not make sense at all.
     
  8. Oct 10, 2013 #7
    May be we would understand your question better if we knew what you mean by "choc". That's not a word. May be you mean shock? And by that you really mean collisions between the air molecules and the container walls? Can you confirm that interpretations before we proceed any further?
     
  9. Oct 10, 2013 #8
    yeah, shock :thumbs:

    no air, but water molecule and the container walls (it's vibrations)
     
  10. Oct 10, 2013 #9
    Vibrations in the water will not increase the net pressure. These vibrations are a form of sound and part of the time there will be a temporary increase of pressure at any given point, but part of the time there will be a decrease in pressure (when the molecules start moving back - during oscillations particles move back and forth). On average over time there is no net change in pressure.
     
  11. Oct 10, 2013 #10
    but there are not the same number of molecule at top than at bottom (density is different), ok molecules move back and forth but it's quantity of movement I think ?
     
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