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Creating a vacuum manually?

  1. May 12, 2008 #1
    I have a 6" diameter , solid bottom, transparent , upright tube, top end open.
    In the bottom of the tube is 3" of water, and on top of that is 3" of air
    at atmospheric pressure, ~15 psi.

    Now i insert an , air tight , plunger down thru open top , til three inches above the water.
    I close the valve in the plunger, and begin to pull the air tight plunger toward the top of tube. Creating a vacuum between the water and the plunger.

    When the column of air reaches 6", the atmospheric pressure should now be 7.5 psi. , and
    so forth.

    How much psi did it take to raise the plunger from 3" of air to 6".
    And from 6" to 12".?

  2. jcsd
  3. May 12, 2008 #2


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    The pressure inside the tube follows the gas law; pressure * volume = constant ( if you ignore any temperature change) so you can work out the pressure inside easily.
    The pressure pushing down on the piston is always 15psi (from the atmosphere above you)

    So you can work out how much force there is on the piston from simply knowing the difference in pressure and the area. Pressure is just force / area, hence pounds/square inch.
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