|Why are we only able to push so far on the plunger
of a syringe when we put our finger over the nozzle?|
|Question Date: 2017-10-23|
We are squeezing the air in the syringe together
when we push on the plunger, but we can only
squeeze the air together so much, and then the
plunger won't go any farther, because the air
molecules are squeezed as close together as we can
squeeze them by pushing on the plunger.
When you push the plunger of a syringe without
your finger plugging the hole, the force that you
are applying doesn't have an "opposing
force", or a force pushing back. This
allows whatever is in the syringe to come out of
the hole because of the pressure applied to the
fluid inside. When you cover the exit, the fluid
that pushes on your finger is blocked and an
opposing force is applied to the fluid.
This makes it much harder to push the fluid back
since the two forces cancel out and the net
movement is 0.
It is because there is air inside of the
syringe, and you are not letting it escape. Air
exerts a pressure on anything it touches, and the
denser the air, the greater the pressure. By
compressing the air inside of the syringe, you are
making it denser, to the point where you are
no longer strong enough to keep pushing the
plunger in. The force of the air on the plunger
equals the force of your finger pressing on it.
When you put your finger over the nozzle of a
syringe, you are making a closed volume of
air. When you push on the plunger, you are
decreasing the volume of air inside the syringe
by increasing the pressure. More pressure
makes the closed volume of air even smaller. In
order to press all the way down, you need to
decrease the volume to zero. This can only be done
if you can apply infinite pressure to the plunger.
Unfortunately, it does not matter how much you
work out your thumb because you won't be able to
keep covering the nozzle when the pressure becomes
extreme. Your finger will hurt! So, what
happens if we put a perfect seal on the nozzle and
use a very strong machine to push on the plunger?
Will we get to infinite pressure?
Unfortunately, the answer is "No!" The syringe
will explode before you can get to infinite
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