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Negative pressure in a cylinder.

  1. Apr 26, 2010 #1
    hey everybody, I need to figure out a few things about calculating the pressure inside a vacuum

    Basicly I have a tube and one side of the tube is capped and air tight and the other side is open to the air. In the tube there is an air tight piston like stopper that is pulled through the tube towards open end of the tube, creating a vacuum on the opposite side of the piston.

    I need to figure out how much force is needed to pull the piston various distances in the tube.
    I have a 1 inch inner diameter tube right now but I would like to be able to test different sized tubes.
    hope that wasn't to confusing, hope this picture helps.

  2. jcsd
  3. Apr 26, 2010 #2


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    A negative pressure would be a negative differential pressure or a gage pressure with respect to an ambient atmosphere.
  4. Apr 26, 2010 #3
    I get that but i still need to know how to figure out what the pressure in the tube is.
    I thought I should say that the piston is starts a few inches away from the closed end of the tube and the pressure is equal to the outside pressure.

    suppose the piston is 3 inches away from the sealed end of the tube would the pressure decrease by half every 3 inches the piston is pulled towards the front
    Last edited: Apr 26, 2010
  5. Apr 26, 2010 #4
    Write what pressures you have on either side of the piston.

    Hint 1: Look at what you wrote on the extreme right of your drawing.
    Hint 2: There must be a pressure on the left hand side for volume to matter.

    Working out the force required is a very simple matter from first principles of pressure and force.
  6. Apr 26, 2010 #5
    i understand that negative pressure is really just the difference in pressures and correct me if i am wrong,but to figure out how much force I need to pull the piston i use
    the difference in pressure (Pa) multiplied by the area of the piston (m2)

    lets say they are both at 101325 pa to begin with, the area of the piston is 0.0125 m2 and there is 50 mm of tube between the piston and the tight cap. I want to know what happens when the piston is pulled towards the open end decreasing the pressure. i want to know how to figure out what the pressure inside the tube is when i pull the piston.
  7. Apr 27, 2010 #6
    You've answered your original question at the end of the 1st paragraph.

    To find the pressure behind the cylinder you need something realating pressure and volume of a gas in a closed space. (ie boyles law)
  8. Apr 27, 2010 #7
    hahah yeah I did some research and ran some equations and figured it out.

    so would the equation p1 v1=p2v2 be used to solve this?
    Last edited: Apr 27, 2010
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