Communicating vessels

Leon has a container filled with water. The surface of the water is flat — the water is at rest. The water in the container exerts pressure in all directions. The further away from the surface, the more pressure there is. Leon connects the container to another, empty, container using a straw attached at either end to the bottom of each container.

This setup is an example of communicating vessels. Let’s see what happens now. Initially, the pressure is higher in the first container filled with water, and lower in the second, empty container. The pressure in the first container pushes the water through the straw into the second container. As the water flows from one container to the other, the water level changes, and so does the pressure in the containers.

Where the water level drops, the pressure decreases. Where the water level rises, the pressure increases. Water keeps flowing from one container to the other, until the pressure in both containers is the same. Because the pressure in liquids depends on the distance from the surface, the pressure in both containers evens out when the water levels are the same. It is important that the containers, or vessels, are connected in a way that the liquid can flow freely between them, and that air pressure above the surface is the same.

If these conditions are fulfilled, the liquid will always flow from one container to the other until it settles at the same level in each container. That is, until pressure in each container is the same. This will happen regardless of the shape or size of the vessels, or the type of liquid. This is called the principle of communicating vessels. So this is why I always get all wet when I water Grandma Ellen’s garden?

My watering can is a communicating vessel, and the pressure pushes water out! The spout and the main container of the watering can are connected, so you're right Leon, they can be seen as communicating vessels. When Leon fills up the main container through the opening on top, the level of water inside rises. As the level of water rises, the pressure in the main container increases, and pushes the water up the spout. Unfortunately, the opening of the spout in Leon’s watering can is lower than the opening in the main container.

So, the level at which the water should settle is slightly above the spout’s opening. Pressure pushes out the water through the spout’s opening, until the liquid can finally settle at the same level in both the spout and the main container. Next time, to avoid getting wet, Leon should remember how communicating vessels work. He can try not to overfill the watering can, or he could get another one, where the spout is longer and higher than the main vessel! Communicating vessels are used in many situations!

One of the most common uses is in water towers. A water tower is a large elevated water tank. The water tank is connected by pipes to buildings nearby. The pipes and water tower form a communicating vessel. The pressure at any given point inside the pipes is the same as pressure at the same level inside the water tower.

So when water is pumped into the tower, water fills the pipes too. When a tap in one of the nearby buildings is turned on, the pressure causes water to flow through the pipe and out through the tap. Communicating vessels are an extremely useful part of life. ...when used right!