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Q & A: Syringe Physics

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Most recent answer: 04/03/2008
Q:
How does a syringe works?
- maneesh (age 6)
cranbury,nj,usa
A:
Maneesh -

Syringes work because of a thing called pressure. A syringe is basically a long plastic tube with a plastic plunger inside that sticks out from one end of the tube and can slide back and forth through it. The thin metal needle goes on the other end. It basically looks like this:

A syringe
(http://www.life-assist.com/fh/safe.html)

The plastic on the end of the plunger makes an airtight seal with the plastic tube, so when the plunger is pulled back, the inside of the tube is /totally/ empty. [not quite. see below/mw] We like to say that there's a vacuum inside. This doesn't mean that you could use it to clean your carpets... instead, it means that it's an empty space that doesn't even have air in it.

Well, there's liquid (like blood or medicine) on the outside of the needle, so it gets pulled into the empty space by the vacuum. This is because the liquid pushes up against the hole at the end of the needle, but since there's no air or anything there to push back on it, it rushes in.

When you press the plunger back down, it just pushes the liquid back out through the hole in the end of the needle.

-Tamara

(published on 10/22/2007)

Follow-Up #1: syringe corrections

Q:
this is not reliable at all. the question about the syringe is not correct.
- alex (age 11)
bethesda, maryland, united states
A:
Alex- thanks for the tip. I see only one correction that's really needed. When you pull back on a syringe you increase the volume V that the air is distributed in. That causes (approximately) the pressure to go down as 1/V. Of course this pressure isn't zero, but it is less than atmospheric pressure. Even if there were no air at all in the syringe, starting to reduce the pressure would cause net evaporation from the liquid, which also prevents the vacuum from being perfect. The pressure can get down to about the vapor pressure of the fluid, the pressure at which the rates of molecules entering the vapor and entering the fluid just balance.

Mike W.

(published on 04/03/2008)

Follow-up on this answer.