Hi Rebecca,
It sounds as if you are planning some interesting demonstrations.
A vacuum cleaner's suction probably isn't strong enough to pull enough
air out of the tube to lower the air resistance enough to make the
point. Many teaching supply companies sell inexpensive vacuum pumps
which are appropriate for the purpose. There might even be
hand-operated ones for low cost.
The name "back pump" is sometimes used to refer to a mechanical
piston pump or other kind of pump used for creating a fairly good
vacuum but not state of the art. Vacuum pumps for research can evacuate
a chamber to very very very low pressures using turbines, or by
literally freezing the gas molecules out, or by absorbing them in
activated charcoal, or by ionizing the molecules and collecting them
electrostatically. These high-tech pumps cannot pump large quantities
of air, but they can get most of the remaining molecules out once the
bulk of them are taken away by a mechanical pump. The reason the
mechanical pump is called a "back pump" is because it pumps on the
exhaust (the back end) of the fancy pump so that the fancy pump does
not have air flowing the wrong way through it -- these fancy, high-tech
pumps often do not work well or at all if their exhaust is at high
pressure. For the classroom, you most likely do not need a fancy pump,
but can use the "back pump" all by itself.
None of the pumps I've seen for aquarium use would be appropriate,
but maybe there are some that are better than others. The ones I am
familiar with circulate air through a bubbler in the tank and use a
thin rubber diaphragm moved by a little magnet. These do not handle
large pressure differernces (as you can easily tell by putting your
finger over the input or output of one -- it just doesn't pull or push
that hard.) Maybe people have other kinds of pumps for aquariums.
I'd go for the low-cost solution from a teaching supply company.
You may need some additional materials, such as a fairly stiff tube to
connect your pump to the hole in the stopper, and I'd recommend getting
some vacuum grease to prevent the inevitable leaks. You also will
probably want a valve or two so you can remove the pump and keep the
vacuum in the tube if you need to.
One very low-cost pump is a water suction pump, which has no moving
parts except some flowing water. It's hooked up to a faucet, and a
third opening (other than the water inlet and outlet) draws a vacuum
that can get close to the vapor pressure of water.
You should know that there will be some air friction left even with
a good vacuum. The part of the friction that goes as the sqaure of the
velocity will be mostly remioved, but another part that is just
proportional to velocity doesn't drop off until the vaucuum is much,
much better.
Tom J. (w Mike)
(published on 10/22/2007)
