### Re: can you measure air pressure in a closed container?

Date: Mon Jan 13 16:39:45 2003
Posted By: Aurelio Ramos, Grad student, Computer Engineering
Area of science: Physics
ID: 1039566322.Ph
Message:
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Hi Marissa,

It's good to see your interest in physics, your experiment is showing
very good thinking, you have correctly assumed that if the bottle is
tightly closed and the amount of oxygen changes, the water must
experience the force and move accordingly. All this things, so far, are
correct. But there is more.

There are at least two problems with the way you are measuring pressure.

The first problem is that you are assuming the loss of oxygen as the only
factor, when the production of carbon dioxide and monoxide are also
there.

As you probably know, combustion is a chemical reaction where a fuel is
broken down by oxigen, and there are byproducts, mostly carbon dioxide
and carbon monoxide. So, without doing the math, I don't know if the
pressure is going to go up because of the production of those two gases,
or go down because the candle uses up the available oxigen.

Certainly, the net effect is the result of the individual pressures of
each gas combined. Perhaps your can settle this matter once you figure
make it work. Your experiment will only measure the NET pressure effect.

The second, and most important problem, is that water is not an easily
compressible fluid. Compressible means squeezable, compliant. Because
water is not compressible when a force pushes on a water volume, the
volume changes very little (practically nothing) for a given force. You
would need to measure very tiny changes (almost invisible) or have
incredibly large forces acting on the water. From your experiment
results, I suppose the pressure change is not that enormous to change
water volume.

So, even though, in fact when the air pressure changes, the water
experiences a force either pushing or pulling on it, the force has little
effect on the volume of the water (is just like pushing on a concrete
wall and trying to measure the displacement, you are doing things right
(at least in principle), but it will move very little to see the
difference) It will move a tiny bit though. If you push with something
stronger like a big monster truck...well.. better not try that!

Air, for instance is compressible, you can squeeze an air balloon, or if
you take an empty soda bottle (its really full of air) and put the cap
back on it (tight), you can still squeeze it. Try that with water
instead! its so hard it barely moves! If its half full of water, its
somewhat easier to push on.

Perhaps if a small amount of air could be placed inside your water...
that might just work! The trick is to put ALL of the air separate from
the candle's oxigen supply, and completely under the water, so that the
pressure can still be SEEN as a change in water level. This air cavity

A bladder full of air, like this, is a cavity that can change its volume,
while the pressure inside it changes very little.

Here is something you can try to add to your experiment to increase how
far the water moves for a given change in pressure. In your water jar,
submerge under the water a small balloon. Inflate the balloon with some
air, just enough so its not limp, but not so much that it does not fit
completely under the water. Also, tie something very heavy (perhaps a
small metal object like a metal bolt or fishing weight) to the balloon to
prevent it from floating. Remember that air is very light and it will try
to float back up. You may even have to GLUE the balloon's "tie rope" to
the bottom of the jar with very good waterproof glue from the hardware
store. Avoid white glue or the super glue. Also, wait for it to dry, the
floating force (buoyant force) will pull too hard on it)

Now, here is what happens: your water is still not compressible, but the
air inside the balloon submerged under the water is compressible, as you
can see from the soda bottle experiment. Water that has an air balloon
inside is compressible because you are really squeezing the balloon, the
water is just there to help push.

So, when your candle finishes the oxigen (and finishes producing its
carbon byproducts) the change in pressure will be more visible because
the balloon is being squeezed (or pulled) by that change.

With this "modified" version of your experiment, the role of water is:
1. To make the floating candle "float"
2. To transmit the force of the pressure change to the balloon. The study
of how water can be used to "transmit" forces is hydraulics. Your jar is
a hydraulic system.

Yet, another variation on your experiment can be made by drilling a hole
on the lid for the jar, then glue with silicon based glue a straw that
goes down into the water in vertical position. You still want to keep the
lid airtight, and the straw better be a good quality hard straw, not the
cheap soft ones. Try the Wendys or McDonalds one, not the ones from
Burger King :) If you find a straw that lets you see the water inside it,
all the better.

When pressure pushes on the water, it will try to push the water "out" of
the bottle" via the straw... Only, it will not push that hard to drive
the water out, just enough to move a bit up or down the straw. This is
just like the ballon idea, but instead of a small air volume in the
water, your straw makes sure the pressure in the bottle remains at
atmospheric pressure. In essence, the whole atmosphere has become the
balloon! With the candle, as oxigen gets used up (and other gases
produced) the volume will change just like with the balloon idea.

Without a candle, the straw jar trick makes a neat barometer. Leave it
closed very airtight for some days, and see what happens as the weather
changes.

Fortunately, its barometric abilities will not interfere with your candle
experiment because weather does not change that fast. Were the
atmospheric pressure to change in a few seconds, just when your candle
burns, that could interfere, but that's nearly impossible. You can do the
*same* experiment 5 or 10 times and compare the results just to make sure.

You could even make the balloon jar and the straw jar and see side by
side. I bet the jar with the straw shows the most difference in water
level. Try to find out why...

Good luck and keep up the good work!

Aurelio R. Ramos

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