Discovering Syringe Pressure & Suction: Small vs. Large Diameter Explained

  • Context: High School 
  • Thread starter Thread starter drsmith99
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Discussion Overview

The discussion revolves around the pressure and suction generated by syringes of different diameters. Participants explore the relationship between force, area, and pressure, as well as the concepts of negative pressure and suction in the context of syringe operation.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant inquires about which syringe diameter generates more pressure with a given force, suggesting a formula for pressure as P = f/A.
  • Another participant explains that for a constant force, pressure is greater when applied over a smaller area.
  • A follow-up question asks about which syringe generates more negative pressure or suction.
  • One participant suggests using a vector for force and changing areas to analyze negative pressure, indicating that a negative force could yield more negative pressure.
  • Another participant distinguishes between negative pressure and suction, stating that suction involves fluid movement from high to low pressure, and that a syringe with a larger volume can move more fluid.
  • One participant clarifies that negative absolute pressure does not exist, and that gauge pressure can appear negative when measured relative to atmospheric pressure.
  • Another participant adds that the maximum negative gauge pressure is limited to -1 atm, which relates to the maximum force that can be applied without displacing the syringe stopper.

Areas of Agreement / Disagreement

Participants express differing views on the definitions and implications of negative pressure and suction, indicating that the discussion remains unresolved regarding the relationship between syringe diameter and suction capabilities.

Contextual Notes

Participants reference the need for clarity on the definitions of negative pressure and suction, as well as the limitations of applying force in the context of syringe operation. There are unresolved assumptions regarding the effects of varying diameters and the implications for fluid movement.

drsmith99
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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.
 
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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.
 
Thanks!
Any ideas about which generates more negative pressure or suction?
 
if you understood what the guy in second post replied you would have asked that :|
 
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!
 
drsmith99 said:
Thanks!
Any ideas about which generates more negative pressure or suction?
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.
 
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.

CS
 
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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