Sunday, July 17, 2011

Charles, Boyle, Divers, and Balloons

If you scuba dive, you know these gentlemen; even paramedics are taught their laws when learning about diving-related emergencies.

They are introduced in chapter 3 of Liquids and Gases: Principles of Fluid Mechanics.  Solids and liquids, in general, are non-compressible; gases, however, are and that changes pressure and density.  Robert Boyle invented the air pump, something with which any child that plays with a ball or rides a bike is familiar.  His law states that the volume of a gas is inversely proportional to its volume, if held at a constant temperature.

Taking what we learned from Archimedes, Pascal, and Boyle, we created something named after Rene Descartes--a Cartesian Diver. The book shows the same principle using an eye dropper, but this example in Gizmos and Gadgets is much more fun, if it is a bit more work.

1 small foil container
1 bendable straw
1 small paperclip
A small amount of modeling clay
2L bottle with a cap filled with water

I used a small foil container to cut out my 3 inch "diver." Next I cut off the bendable portion of the straw, leaving about an inch on either side.

I used the paperclip to attach the straw to the diver by first clipping one side of the straw and then clipping that onto the diver.  I then gave the diver some clay boots to weigh it down a little but not have it sink altogether.  You can see in the picture that when I put it into the water it bobbed at the top; I just pushed it in a little and put the cap on.

Archimedes taught us that objects float by displacing the fluid they are in, so that the object is less dense than the fluid.  Pascal taught us that water exerts pressure in all directions and is basically non-compressible. Boyle taught us that gases are compressed, and that compressing them increase the pressure and the density.  Our diver has a straw tank of air and is surrounded by water.  What happens when you squeeze the bottle?

The chapter wraps up discussing the effect of temperature on gases. Jacques-Alexandre-Cesar Charles, while ballooning, noticed that hot air expands, but the law named after him was first stated by two other great scientists, John Dalton and Joseph-Louis Gay-Lussac. Charles law states that for a constant volume of gas, the pressure of the gas is directly proportional to the temperature. While I did this with the group, I also did it awhile back with my boys and the videos came out better:

ExploreLearning has  two Gizmos to reinforce these concepts: Temperature and Particle Motion and Boyle's Law and Charles' Law

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