# Cartesian divers

Cartesian divers

Cowens, Jim

This classic activity will fascinate students at any grade level.

The French philosopher Rene Descartes (best known for the statement, “I think, therefore I am”) was the inspiration for the name of this incredible device, though no one is sure why. The Cartesian diver functions with the balance of weight and buoyancy forces.

Weight is the force which keeps us on Earth and stops us from floating off into space. Buoyancy is when a body is immersed in a fluid and is buoyed up by a force equal to the weight of the displaced fluid.

Rise and fall

Two examples of things that use buoyancy are fish and submarines. Fish use muscles to squeeze or relax a small sac (with an air bubble inside) in their bodies. By squeezing the sac smaller, the fish sinks. When the fish retaxes its muscles, the sac increases in size, displaces more water, and the fish will rise.

A submarine uses a similar principle to control buoyancy. By pumping water in and out of buoyancy air tanks in the submarine, the ship can be made to rise and sink. In these air tanks, the ratio of air to water is varied to achieve the buoyancy force desired. Compressed air is forced into the tanks to displace water and thus achieve greater buoyancy, which makes the submarine rise. The sub can expel air from its tanks and collect enough water to reduce buoyancy and get the submarine to dive. Submarines can “sit” at a desired depth by adjusting their effective buoyancy in much the same way as the Cartesian diver can be made to rest at a given depth.

How the Cartesian diver works in a large plastic bottle

The Cartesian diver descends to the bottom of the bottle when the bottle is squeezed. Water is forced into the diver – the only place it can go – the tiny space of air in the diver is compressed and the density of the diver increases, making it sink. When the bottle is no longer squeezed, the bubble inside the diver becomes large again and buoyancy pushes the diver to the top.

Ways to make divers

1. Condiment Packet – Place a condiment packet such as those given in fast food restaurants for ketchup, soy sauce or taco sauce into a 1-2 liter plastic bottle. (These packets usually contain a small amount of air, which is important for the Cartesian diver.) Fill the bottle with water. Screw on the top of the bottle securely. If the condiment packet floats, your diver will work. If the packet sinks, replace it with a different condiment packet.

Next, squeeze the bottle. If you squeeze it hard enough, the packet should sink. Once you select the appropriate packet, it will float when you aren’t squeezing the bottle and sink when you are. (Your geographic location and air pressure makes a difference when choosing a condiment packet. I had a difficult time trying to get this diver to work properly here in the Pacific northwest.)

Note: Buoyancy of this diver is affected by temperature. If your diver is left in the cold the packet will not float any more. Why might this happen? Will the packet float when the water is warmed? Try it!

2. Ink Cap – Use an ink pen cap that does not have a breathing hole in its tip. Straighten a paper clip, cut the wire in half, and bend it into a “U.” Place a small nut on the wire and tape it to the pen cap. Drop it gently into a bowl of water so that air is trapped in the cap. Adjust the weight of the nut on the cap until the cap floats upright with just its tip above the water. After you have the right amount of weight hanging on the paper clip wire, fill the bottle with water and carefully drop the pen cap into the bottle. Cap the bottle tightly and your Cartesian diver is finished.

3. Eye Dropper Cap – Take the rubber bulb fi-om an eye dropper. Straighten a paper clip, cut the wire in half, and bend it into a “U.” Place a small steel nut on the wire and tape the wire to the bulb.

4. Drinking Straw – Take half of a drinking straw and bend it in half Straighten a paper clip, cut the length of wire in half, and bend it into a “U.” Thread a small steel nut on the wire and secure the wire inside the straw. As with the medicine dropper and ink cap, get the straw to barely float on the surface of the water.

5. Pipette – Head for your nearest high school chemistry classroom and ask the teacher for several plastic pipettes (ak.a. dropper applicators). Cut all but I cm of the stem off. Then screw a steel nut onto the stem.

6. Match Heads – Break the heads off three or four wooden matches. The air bubbles trapped in the splintered wood make the match heads act as Cartesian divers.

Extensions

Once you get the Cartesian diver to work properly, try some of these fun activities:

1. Sunken Treasure – Open the water– filled plastic bottle, pull out the diver and hang an L-shaped hook onto it. Then, drop a small non-floating object inside the bottle. Secure the cap tightly. Squeeze the bottle until the diver sinks to the bottom. Try to catch the sunken object with the hook. Then, release the bottle and try to lift the sunken object to the top.

2. High Water, Low Water – Put different amounts of water into the large plastic bottle. As you squeeze the bottle, which amount of water requires more squeezing pressure to make the diver descend?

3. “Steady As She Goes” Strength Test — Squeeze the plastic bottle with enough pressure to keep the diver hovering at midpoint. Time yourself and see how long you can keep the diver at midpoint.

4. Holdin Pattern — Most large plastic bottles are about 30 centimeters tall. Starting from the bottom of the bottle, mark every centimeter from 0– 25 up the outside of the bottle. Next, cut an index card into 25 2 cm x 2 cm squares. Number the cards from 1-25. Place the squares in a hat, mix them and draw out three. Then, squeeze the bottle and move the diver to each of the three positions and hold for three seconds. (Example: if you draw 3, 13 and 6, the diver descends to the third mark on the bottle for three seconds, ascends to the 13th mark for three seconds, then descends to the sixth mark for three seconds.)

5. Hot and Cold – Fill one bottle with ice water and another with hot water. Insert a diver into the bottle of ice water, secure the cap and squeeze. Do the same with the bottle of hot water. Does temperature affect the diver’s descent when the bottle is squeezed with the same amount of pressure?

John Cowens teaches science at Fleming Middle School, Grants Pass, OR, and is a Teaching Editor of Teaching K-8. E-mail: snewoc@yahoo.com

Copyright Early Years, Inc. Feb 2002