Cartesian Divers
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PURPOSE
Prepare Cartesian divers and study how pressure affects
the density of the divers. |
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Selected by the
SciLinks program, a service of the National Science Teachers Association.
Copyright 1999-2002. |
PERFORMING THE
EXPERIMENT (work over a waterproof tray at all times)
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Prepare one or more divers by steps 2 and 3. (Note: Take water into
the dropper by squeezing the plastic or rubber bulb, placing the tip of
the dropper well under the surface of water in the cup, and releasing your
squeeze to let water into the dropper. You let water out of the dropper
by gradually squeezing the bulb.)
- Draw water from the cup into the dropper until the dropper is about half
full.
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Place the dropper back into the cup full of water to determine whether
it sinks or floats. You must experiment carefully with how much water
to use. If the dropper sinks to the bottom, let some water out of
the dropper. If the dropper floats, but well above the surface of
the water, draw more water into the dropper. If properly adjusted,
the top of the dropper floats either at or barely above the
surface of the water. An improperly adjusted dropper won't work in step 5.
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Place your diver carefully into the soda bottle, and screw the cap on tightly.
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Command the dropper to "dive," squeezing the soda bottle with both hands
and considerable force as you say the words. Command the dropper
to "rise" to the surface as you release your squeeze. Did it work?
What changed inside the diver? Why?
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If the diver did not dive, squeeze the open soda bottle just enough to
lift your diver to where your adult partner can remove it with tweezers.
Repeat step 3, then steps 4-5 again.
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Prepare a couple of divers so that on command one of them will dive, then
another, and then another in succession. What is the secret in causing
this to happen?
QUESTIONS TO THINK
ABOUT
1. How does the diver's behavior apply to your trying to float motionless when you are swimming?
EQUIPMENT NEEDED
Clear, soft plastic,
2-liter soda bottle with tight fitting cap, filled with water almost to
its narrow neck; plastic droppers with nuts screwed on them to add mass
and/or medicine droppers with rubber bulbs and glass tips; a cup at least
twice as deep as the droppers are long, filled 3/4 full with water; tweezers;
waterproof tray in which to work.
EXPLANATION
The Cartesian diver
is named after the French philosopher, Rene Descartes (1596-1650), and
is a very old experiment. The volume of a gas decreases as the pressure
on the gas increases. As you squeeze the bottle, the pressure is
transferred from your hand to the water and from the water to the air trapped
inside the diver. As the volume of air in the diver gets smaller,
more water enters the diver, making it heavier and less buoyant, and the
diver sinks to the bottom. As the pressure is released, the air inside
the diver expands and increases the buoyancy so that the diver rises.
REFERENCE
Modified from Andy
Sae, Chemical Magic from the Grocery Store, Eastern New Mexico University,
1991, pages 24-25.
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