What is the Cartesian Diver?
Well, if you do not know or have never seen a Cartesian Diver, you can make your own by following the directions on this page here:
Now that you either have your own Cartesian Diver, or can recall your observations, let's figure out why the Cartesian Diver works.
What happens as you squeeze the sides of the bottle slowly, then release the sides of the bottle?
What you should notice is that as you apply a force on the bottle by squeezing it is that the diver descends and then it floats back up to the top once you stop squeezing the bottle.
What happens to the dropper? More importantly, what happens to the air and water levels inside the dropper?
Well if you take a close look at the water level. When you apply the squeezing pressure and the diver is at the bottom of the bottle, you should notice that the water level is higher. Then, when you release the pressure, the water level decreases and goes back to its original water level.
Why does this happen?
Observations & Explanations:
Well,
when the walls of the bottle are squeezed, the pressure
increases inside the bottle. This added pressure
decreases the volume
of the gas in the bottle, including the gas in the medicine dropper.
This is a direct result that gases are easily compressed which can be explained
by the kinetic-molecular theory. Since the gas, the air in the medicine
dropper is easier to compress than water, the gas is compressed and more
water is allowed to go into the medicine dropper.
So why does the cartesian diver sink and float?
When the bottle is not being squeezed, the medicine dropper floats because there is not enough mass in the volume of the medicine dropper to make it float. In other words, the total density of the medicine dropper is less than that of water. In fact, it is mostly air and this air is a lot less massive than the body of water that surrounds it.
When the bottle is squeezed, the pressure increases inside the bottle. This increase in pressure causes the gas to compress and more water is able to fit inside the diver.
This is known as Boyle's Law which states that if the amount and the temperature of of a gas remains constant, the pressure exerted by the gas varies inversely as the volume. Therefore, since the pressure of the gas was increased, the volume of the gas decreased!!
Since there is no more water in the constant unit of volume, the mass is much greater at this point and causes the overall density of the diver to increase which makes it sink because at this point, the density of the diver is greater than that of the diver. More mass in the same amount of volume results in a greater density, which in turn makes the diver sink.
When the pressure is decreased by stopping the squeezing on the bottle, the volume of the gas returns to its original volume and therefore causes the water to leave and the density to decrease which in turn causes the diver to float again!