1. Sep 21, 2008

### drsmith99

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. Sep 21, 2008

### 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.

3. Sep 21, 2008

### drsmith99

Thanks!
Any ideas about which generates more negative pressure or suction?

4. Sep 22, 2008

### x3qt0r

if you understood what the guy in second post replied you would have asked that :|

5. Sep 22, 2008

### lagwagon555

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!

6. Sep 22, 2008

### 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.

7. Sep 22, 2008

### stewartcs

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

8. Sep 22, 2008

### 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.