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Vacuum submerged in vacuum

  1. Sep 10, 2012 #1
    hi to all,
    this one may sound silly but i just can't seem to understand how it will work out.
    this is the setup - in a vertical water column (state of vacuum) a hollow box is attached to the base. all air from the box has been sucked out and the box contains a vacuum(fig A).

    if the lid of the box is opened when inside the column, which of the following will take place?
    - the vacuum in the box will automatically be taken by the surrounding water, in this way lowering the level of the water in the basin (fig B)


    -the vacuum will release an ""empty space"" that will go to the top of the column and lead to the water level being raised (fig C)


    Attached Files:

  2. jcsd
  3. Sep 10, 2012 #2

    Simon Bridge

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    Please show us how you have been thinking about it so we can figure out what is behind you getting stuck.

    Note: in B and C, the box, itself, is made out of material less dense than water yet can hold it's shape with a vacuum inside?

    You can see what happens in a lesser situation, where you have a gas at the top of the column ... how does the column balance? What happens if you reduce the pressure of the gas?

    For the picture in B: what happens if the volume of the box is bigger than the volume of water in the dish?
    For the picture in C: what happens next?
  4. Sep 10, 2012 #3
    Judging from the experiment shown in the illustration, I'll assume it would take place at normal atmospheric pressures. Say, on Earth, in your conventional classroom?

    When the box containing the vacuum is ruptured, the pressure gravity exerts on the air surrounding the setup would continue pushing the water inward, filling in the empty space, as shown in figure-B of your illustration.
  5. Sep 10, 2012 #4


    Staff: Mentor

    It is B. The height of the water column is determined by he external air pressure, which will push the liquid in the dish up into the box.
  6. Sep 11, 2012 #5
    so B is the correct option. the vacuum box, once ruptured would simply suck inside it a quantity of the water equivalent to its internal volume. no ""air gap"" will be created at the top of the column.

    thank for all the replies. i have understood the physics behind this setup.
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