Demos: 2B-06 Cartesian Divers

A variety of Cartesian divers is available to illustrate Pascal’s principle, as well as the compressibility of air and relative incompressibility of water. The basic demo consists of a tall cylinder filled with water and capped by a soft rubber diaphragm. The cylinder contains an inverted test tube, filled with just enough air to cause it to float at the top of the cylinder. When the diaphragm is pushed down with a finger, the test tube sinks. When the diaphragm is released, the test tube rises.

Other divers include a “Tony the Tiger” toy inside a capped soft plastic soda bottle, a package of fast food sauce in a sealed plastic bag in a soda bottle, and a small inverted test tube in a whiskey bottle.

Directions: The demo technician will have set up the apparatus ahead of time, so no preparation is needed. Push firmly on the rubber diaphragm until the test tube sinks.

Tony the Tiger floats at the top of the capped soft soda bottle. Press on the sides and Tony will sink. Release and he will rise to the top. The same goes with the bottle containing the package of sauce.

The whiskey bottle is “rigid” so that it appears that it shouldn’t work. However, by pressing firmly on the flat sides, the test tube can be made to sink. (NOTE: Because the sides of the bottle flex so little, the test tube must be just on the verge of sinking. This can be arranged by pushing the rubber stopper farther in or pulling it slightly out to adjust the system.) You can push in the rubber stopper to just make the tube sink to the bottom. Then press on the “ends” (the narrow part of the bottle) and the tube will float to the top.

Suggestions for Presentation: Tell the class you are going to perform a little magic. Without touching the floating test tube, you will simply press your finger on the top of the cylinder and make the test tube sink. With a little skill you can make it hover. To show the students what is happening, you might need to use the projection camera. When you push down on the diaphragm, the water is incompressible and by Pascal’s principle, the change in pressure is transmitted to every part of the water. The air inside the test tube is compressible so the volume of air is reduced. This means that the volume of water displaced is reduced, thus decreasing the buoyant force. Close inspection shows clearly the water level changing in the test tube.

The sauce package contains air and material having approximately the same density as the water. Squeezing the bottle compresses the air in the package, causing the package to sink.

Applications: The intake and discharge of water in a submarine allows it to rise or fall in the water. Although this isn’t related to Pascal’s principle, the concept of more or less air in a rigid container is applicable.

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