## Cartesian Diver

The cartesian diver is a hollow cylinder clear tube sealed at one end. The diver is place in the 2 liter pop bottle with a small amount of air in it. The open end of the diver is on the bottom. The 2 liter pop bottle is sealed without any air remaining at the top.

With this demo we tested to see if the students comprehended the ideas of buoyancy and incompressibility of a fluid. We showed them the cartesian diver in the 2 liter pop bottle and asked to explain why the diver sank when the bottle was squeezed.

The water is incompressible where as the air is not. As the bottle is squeezed, the pressure inside increases and therefore the volume inside must decrease. (PV=nRT) The only place where this can occur is in the air inside the cartesian diver. The air compresses and more water enters into the cartesian diver causing the diver to sink.

Students from grades 5-7 were answering correctly. (Without formulas) We found this very rewarding since this was one of the demonstrations used at the Physics Olympics this year!!!!!!