# Suction force and pressure difference

1. Feb 3, 2014

### mellotango

I am doing a study to see whether suction force can act as a mechanism to compress air. Lets say I have a normal syringe, which consist of the small cylindrical tip at the top of syringes where you attach the needle. Then there is the larger middle barrel for the liquid, medication, etc. Lastly there is the suction plunger at the bottom. If I apply a constant suction force by pulling the plunger constantly for x amount of time till it reaches the end. During this time, would the air pressure at the tip column be more than the air pressure in the barrel? I assume this since the tip tube circumference and volume is much smaller, hence the pressure of air sucked in must be a lot greater in order to fill up the bigger space in the barrel. I do understand this pressure difference equalizes and cancels out once the suction motion by the plunger is halted. But during the time of the sucking motion, is the column of air in the tip subject to a huge pressure increase? And is there a way to calculate this?

2. Feb 4, 2014

### CWatters

Consider this.. Lets say you were to put a cap on the needle end of the syringe, then remove the plunger and connect a vacuum pump in it's place. Now run the pump until there is a vacuum inside the syringe. The pressure difference between the inside and outside of the cap will be limited by atmospheric pressure.

Removing the cap cannot increase the pressure difference.

Using the plunger instead of a pump to reduce the pressure inside can't make the pressure difference any greater than the atmospheric pressure outside.