Cartesian Diver
| Polish version is here |
Simple and Fascinating
The Cartesian Diver is a very simple physics toy. Anyone can build it from items commonly found around the house. The diver itself is simply a tube open on one end, for example, a small glass tube (I used a small test tube). It’s immersed in water, upside down. As a result, there’s a bubble of trapped air inside the tube, causing the toy to float in the water instead of sinking. Instead of a test tube, you could also use a small bottle originally containing cake flavoring. I’m referring to those elongated, test-tube-shaped bottles. They often have a flat bottom, but that doesn’t really matter for this experiment. The diver should float in the water as shown below:
Next, place the diver in a sizable plastic bottle (e.g., 1.5 L). It should be filled with water up to the neck. Of course, you need to experiment a bit to find the right amount of air trapped in the tube so that the entire setup floats in the water. Then you screw the cap onto the bottle. Everything is ready. And now—the magic happens! A light squeeze on the bottle makes the diver sink to the bottom; releasing the pressure makes it resurface. With the right pressure, you can keep the diver at any chosen depth, make it do underwater “jumps,” and perform other moves. Below, you can see how it looks on the model I built:
How can we explain this behavior?
In the Cartesian Diver, we use a well-known fact that water (compared to air) is practically incompressible. When we squeeze the bottle, the pressure inside it increases. The principle here states that if an external pressure is applied to a fluid (liquid or gas) in a closed container, then (neglecting hydrostatic pressure) the pressure inside the container is uniform and equals the external pressure. The external pressure here is our squeeze on the bottle. At every point inside the bottle, the pressure must be the same, so the bubble of air inside the diver is compressed or expands accordingly. Meanwhile, according to Archimedes’ principle, a body immersed in a fluid experiences two forces: gravity and the oppositely directed buoyant force. If the buoyant force is larger than the gravitational force (situation A in the figure below) acting on the body, it floats. In case B, the opposite is true—gravity wins, and the diver sinks to the bottom. By squeezing the bottle, we reduce the bubble’s volume and thus the buoyant force. When it drops below the weight of the diver… Yes, that’s right—total submersion!
Have fun experimenting!
Further readings:
- Błażejewski R., 100 prostych doświadczeń z wodą i powietrzem, Wydawnictwa Naukowo-Techniczne, Warszawa, 1991, pp. 71-72
- Glücksman S., Technika demonstracji z fizyki, Państwowe Wydawnictwo Naukowe, Warszawa, 1956, pp. 74-75
Marek Ples