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Question about syringes

  1. Sep 21, 2008 #1
    Hi, I have a simple question about syringes. I'm trying to find our which is able to generate more pressure with a given force, a syringe with small diameter, or one with a large diameter. An explanation with a simple formula would be much appreciated. Also which is able to generate more suction? Thank you.
  2. jcsd
  3. Sep 21, 2008 #2


    Staff: Mentor

    Hi drsmith99, welcome to PF

    A small one:
    P = f/A
    where P is pressure, f is force, and A is area.

    So for a constant force the pressure is largest if applied over a small area.
  4. Sep 21, 2008 #3
    Any ideas about which generates more negative pressure or suction?
  5. Sep 22, 2008 #4
    if you understood what the guy in second post replied you would have asked that :|
  6. Sep 22, 2008 #5
    Try to see for yourself, assign variables to the equation, taking into account that you will use a vector for force (so, when sucking in, a 'negative force' is applied, making force a negative number). Keep the negative force the same, change the areas, and see what gives you a more negative pressure. Hope that helps!
  7. Sep 22, 2008 #6


    Staff: Mentor

    For negative pressure just use the formula above with a negative force.

    However, suction is different from negative pressure. Suction is the movement of fluid from a region of high pressure to a region of low pressure. So, are you interested in just applying a negative pressure or in moving fluid? If moving fluid then the syringe with the biggest volume will move more fluid, even if it does so more slowly for a given force.
  8. Sep 22, 2008 #7


    User Avatar
    Science Advisor

    Just to clarify, there is no such thing as negative absolute pressure. A gauge pressure only appears to be negative since it is measured relative to atmospheric pressure.

  9. Sep 22, 2008 #8


    Staff: Mentor

    stewartcs brings out a good point. Because there is no such thing as negative absolute pressure the maximum negative gauge pressure you can even theoretically get is -1 atm. For a syringe with a given area that also implies a maximum force you can apply without just pulling the stopper out.
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