What is a Cartesian Diver?
A diver sits in a bottle, and with a small squeeze it begins to dive to the bottom of the bottle. Release the squeeze and it floats back to the top. By adjusting how hard you squeeze you can even control the depth.
A cartesian diver is a toy and science demo that demonstrates Boyles Law. Suitable for most primary school ages as a toy, or a demo for high school students. It is also easy to make at home!
How do Cartesian Divers work?
Pressure and volume are proportional by Boyles Law, PV=k. When you squeeze the bottle, you decrease the containers volume. This in turn causes an opposite increase the pressure within the bottle. This squeezes the air bubble in the diver causing it to become more dense. This denser bubble is unable to support the diver any more so it sinks. When the pressure on the container is released the air expands again to it’s original density and is able to lift diver back to the top of the bottle.
Cartesian Divers
Ingredients
- One empty PET style water bottle, metal or polycarb won’t work.
- One pen lid
- Water
Instructions
- Attach the plasticine or blue tack to the pen cap to block any holes in the top so that it can hold a bubble of air.
- Attach additional plasticine or blue tack to bottom so that it just floats. This can be tested in a bowl of water.
- Place the cap into a bottle full of water and seal.
- Squeeze the bottle to dive.
- Release the bottle to rise.
Practical Application
Submarines use a similar system in order to sink down in the water and rise back up again. Special tanks called ballast tanks can be filled with water, or air. When filled with air, the submarine will float on the surface of the water. When the submarine ‘dives’, large amounts of water are pumped into the tanks. This makes it much heavier. By regulating the water and air ratio in the tanks, the crew of the submarine can make it rise or sink to different depths in a controlled way.