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Vacuum in Torricelli Barometer and it's force?

  1. Nov 29, 2015 #1
    Hello! When a torricelli barometer is made there is a vacuum at the end of the top of the tube, does this vacuum exert a force to the water or mercury to go up? and how is that it can be a bigger vacuum depending on the air preassure? How does it exactly work? Thanks.
     
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  3. Nov 29, 2015 #2

    SteamKing

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    How can a vacuum exert a force? A vacuum means there's nothing there.

    It's not clear what you mean here. All vacuums have the same absolute pressure: zero.

    What would happen to your barometer if there were no vacuum in the end of the tube, and the liquid (water, mercury, whatever) tried to climb into the top of the tube?
     
  4. Nov 29, 2015 #3
    I mean, this force as an absorbent force like the contrast of natura force(?
     
  5. Nov 29, 2015 #4

    SteamKing

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    I'm not sure I understand the distinction. A force is a force. What is the liquid in the barometer tube being absorbed into?
     
  6. Nov 29, 2015 #5
    the top of the tube which is closed is empty, so there is a vacuum there, below there is water, but when the atmospheric preassure decrease the vacuum becomes bigger and the water decrease its level in the tube.
    I've seen videos with vacuum chambers that absorb the air and it produce like a "force" that suctions X thing.
    So why the vacuum in a barometer is produced and why its force is weak(? :P
     
    Last edited: Nov 29, 2015
  7. Nov 30, 2015 #6

    SteamKing

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    If nothing else, please learn to spell 'pressure' correctly.

    It's not clear what you mean when you say the vacuum becomes 'bigger'. Since a vacuum is already nothing, does this mean it becomes more 'nothing'?

    A simple barometer is made by taking a tube of sufficient length and then filling it completely with liquid, so that no air or other gas remains trapped inside. Once the tube is filled and no gas is left trapped, the open end of the tube is closed and the tube is then inverted. The end of the tube is placed in a dish or bowl which contains additional fluid, which will mix with the liquid in the tube once the lower end is opened.

    https://en.wikipedia.org/wiki/Evangelista_Torricelli

    The reason the fluid level in the tube drops and creates a vacuum in the tip is that atmospheric pressure from the outside of the tube is trying to balance the pressure created inside the tube by the column of liquid remaining there. If the tube is filled with mercury and there is 1 atmosphere of pressure on the outside, the top of the mercury column inside the tube will be exactly 760 mm higher than the level of the fluid in the dish or bowl. This is an illustration of Pascal's law:

    https://en.wikipedia.org/wiki/Pascal's_law

    You've been looking at the barometer all wrong. The vacuum in the tip is created when the filled tube is inverted. The fluid isn't pulled up by some mysterious force located in the top of the tube. The outside atmospheric pressure pushes the fluid up into the vacuum in the tip, because there is nothing in the tip to resist the fluid being pushed up by the atmosphere outside.
     
  8. Nov 30, 2015 #7
    Pressure! Sorry, English is not my mother tongue and I'm learning it! I'm from Argentina hehe..
    I understand what you are saying but what I mean is:
    I see too realities, the one that you have just said, that there is nothing there and that nothing doesn't do anything indeed. And a reality that there is a nothing that does do something, and I saw it in some videos for ejample "video". So why is it that in the video the nothing has an "absorbing force", but in the vacuum of the barometer nothing seems to do that vacuum, and even though it can be bigger or not (yeah to become more nothing) :). Thank you
     
  9. Nov 30, 2015 #8

    SteamKing

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    I don't know about these videos you are viewing. I do know that there is no such thing as an 'absorbing' force.
     
  10. Nov 30, 2015 #9
    i left the link u just have to press where it says video in blue colour
     
  11. Nov 30, 2015 #10

    DrClaude

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    I haven't looked at the entire video, but it looks very-well made. The presenter clearly explains, for instance in the sequence staring at 2:17, that is is the air left at the top of the vial that is pushing the liquid out.

    Vacuums don't suck.
     
  12. Nov 30, 2015 #11

    Sorry, I still can't see it :P
     
  13. Nov 30, 2015 #12

    DrClaude

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    That second video has a horrible explanation. What happens is that, when the flame extinguishes, the remaining gas inside the glass cools down, which lowers the pressure (see, e.g., the ideal gas law) above the liquid inside the glass. The atmospheric pressure is now higher than the residual pressure, so the atmosphere pushes the liquid that is outside the glass into the glass. This stops when the pressure of the now-compressed gas inside the glass is the same as atmospheric pressure.
     
  14. Dec 1, 2015 #13
    Hallo

    Maybe it helps to define the vacuum better. At the vacuum end of the tube filled with mercury there is a pressure: the vapor pressure of mercury at the tube temperature.
    The force that this presurre exerts is very small compared with the atmspheric pressure at the other, open end of the tube. A perfect vacuum does not practically exist. In the mercury tube the vacuum volume is filled with mercury atoms exerting the very small pressure. What is left is a force balance where this very small pressure contributes practically zero force. The atmospheric pressure to be measured carries the mercury colummn on its own.

    Greetings

    Onno
     
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