Under Pressure

The Primaria kids are learning about the ocean, starting from the deep
sea and moving up, so I decided to focus on pressure Friday.  I
started with a very simple giant syringe with the end capped, mounted
in a wooden block for stability.  I filled it with water and had each
of them press down as hard as they could.  Many of them already knew
(having encountered it when they studied the Marianas Trench) that
pressure at any point in the ocean is just the result of the weight of
the water above that point pressing down.  The syringe reminded them
of this point and made it vivid; they were playing the role of the
upper layers of water pressing down, and we imagined how much pressure
a creature would feel if it were in the syringe when they pressed on
it as hard as they could.  (Warning to future self: supervise more
closely because the syringe is easily broken!)

As a bit of an aside from the main focus of water pressure, we then
filled the syringe with air and repeated a round of pressing.  What's
different is that the air shrinks in response to the pressure!  The
plunger actually goes down when you press on it, and you can see that
that is not due to escaping air because the plunger springs right back
when you let go (assuming you have a good syringe where air really
does not leak around the plunger).  It's an interesting feeling to
feel the air pushing back like that; it really feels like squeezing an
invisible spring.  Water is very different: it pushes back without
changing its volume.  Physicists would say that air is compressible
and water is incompressible.

Next, we did the "three-hole can" experiment.  This is just a vertical
glass tube with three stoppered holes at different heights.  Fill with
water, and ask the kids to predict what will happen.  After they say
water will squirt out the holes, ask them if it will squirt out
equally fast (or far) from each hole.  You will probably get a variety
of opinions, at which point you can talk about the importance of doing
experiments to settle issues in science.  If there is a unanimous
prediction, I ask if we should still do the experiment, and we
conclude yes, because sometimes everybody is wrong.  This worked out
well Friday because one group had a unanimous prediction which was
correct, and then later had a unanimous prediction which was
incorrect, so they experienced both sides of it. 

In any case, if you unstopper all three, it's clear that water squirts
out fastest from the lowest hole.  This is because it is under more
pressure, having more weight of water above it.  (Technical note: if
you judge the pressure by how FAR the water travels before hitting the
ground, the lowest hole is at a bit of a disadvantage because its
water has less time before it hits the ground.  But that's a minor
factor for most setups.)

Next, I had set up a big tub of water with a stoppered hole at the
same height as one of the holes in the tube.  I restoppered the tube
when the water levels in the two containers matched, so the heights of
the water, as well as the heights of the hole, match.  Now the kids
have to decide, having removed the variable of height, which will
squirt out faster when unstoppered: the skinny tube or the massive
reservoir of water.  They turn out to be equal; pressure is determined
only by the height of the water above you, not by the volume.  A
practical application of this is that the strength required of a dam
is determined by the depth of the water it holds back, not by the
volume of the lake.

Next, we looked at how you can hold water in a straw by putting your
finger over the top.  This is related to pressure because if the water
started to fall out of the straw without air getting in, the air in
the straw would have to occupy more volume and thus be at lower
pressure.  The higher pressure of the outside air then provides an
upward push on the water to prevent it from falling.  (I simplified
this a bit for the kids; the summary for them was that the water could
not fall out without a way for air to get in to fill the space it
left.)  Then came the cool part; I have a similar setup in which air
is prevented from escaping from an inflated balloon, and it is amazing
to see how the balloon stays inflated even when you let go of the
neck!

Here's how it works. It start with a piece of glass which is shaped
more or less like an inflated balloon.  The exact shape doesn't
matter, but it needs a good amount of space inside for the balloon to
inflate, and it also needs a neck on which to mount the balloon's
neck.  Stuff most of the limp balloon inside, and mount the balloon's
neck on the glass container's neck.  Then inflate the balloon, which
surreptitiously pushes air out of the glass container through a hole
in its back side.  Then insert a stopper into that hole, and there's
no way the balloon can deflate; in order to do so, you would need to
remove the stopper so air can take up the space in the glass
container.  So take your mouth off the balloon's neck and watch the
kids gape...its neck is held wide open by the glass neck, but the
balloon does not deflate!

You can then ask them to predict what will happen when you remove the
cork, and there are further variations such as pouring water in the
balloon before removing the stopper (which creates a nice squirt of
water when you do remove the stopper). 

I found that the whole thing went pretty quickly, and there would have
been time to also do the vortex bottle, which I will write about next
time.  Some groups had extra time to blow up balloons and release
them, and do other hands-on experiments with the equipment, but that's
always a good thing.

I also (re)discovered something about managing the kids.  I had them
all sit around a big round table, and that eliminated a lot of the
annoyances which had wasted time in previous activities: kids
jockeying for better position, kids getting distracted by the
playground equipment (we were outdoors for obvious reasons), etc.  In
the future I should set things up so that they are seated if at all
possible.

A simple, fun extension of this activity you might want to do at home is to
make a giant version of the three-hole can.  Stand an 8-foot PVC pipe
straight up, make the bottom end watertight, drill a bunch of holes in it,
and put a hose in the top.  This will make a good thought-provoking
sprinkler for the summer!