05 Aug DIY: Communicating Vessels
This DIY communicating vessels experiment teaches students about fluid mechanics – that’s the science of how liquids such as water behave under different conditions.
What you need:
- Bendable straws (paper straws with bendable tips will work)
- Two cups (make sure they are clear so you can observe changes in water levels)
- Water
- Scissors
- Optional: food dye
What to do:
1. Fill up one of the cups with water. You can add a few drops of food dye so you can more easily observe changes in water levels!
2. Attach two straws together at the tip. You may need to cut one of the straws so that it can more easily slide into the other straw.
3. Fill the attached straws with water. You can fill the straws up under the tap. Once the straws are completely filled with water pinching the middle of the attached straws will stop the water from coming out as you transfer them.
4. Place one end of each straw into the two cups. For the experiment to work the straws must be filled with water, so make sure the water has stayed in the straws as they are transferred.
5. Watch as water fills the empty cup until the water levels are equal!
6. Bonus experiment: try placing something under one of the cups so it is elevated. The water levels will change again so that they are at the same height!
What is happening?
In this experiment, water moves between two containers until the height of the water is the same in both. This happens because nature always tries to find the lowest energy state in a system.
In this case, water at a higher level has more potential energy because of gravity and atmospheric pressure. Even though water has to flow upwards against gravity at first, the overall system will have lower energy when the water levels are at the same height in both containers. A lower energy state is reached because water at a lower height has lower potential energy, and because the two jars are connected by a pipe that is also full of water, it acts as one vessel.
For this to work, the straws need to be completely filled with water to begin with. This is because gases are compressible, while liquids are not. So, when the straws are full of water, the force exerted by atmospheric pressure forces it to move until it reaches a stable state where atmospheric pressure is equal at all points in the system, which is to say when the water level is the same. This is the same as the state with the lowest energy. However, when air gets into the system, it will rise to the highest point – in our case, where the straws connect – because it is less dense than water, and the effect of the force exerted by the atmospheric pressure is simply to compress the air in the straws, rather than to move it anywhere.
Watch the Youtube Video here: https://www.youtube.com/watch?v=7gm4_xtlmBg