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Why are Sink Pipes Curved?

Author: Edgar Zhu

Editors: Shamsia Ahmed and Simran Gohel

Artist: Tiffany Gao

In our daily lives, it is the norm to find water pipes in kitchens and bathrooms twisted and entangled with one another. How many times have you popped out with curiosity on the reason behind this, and how many times did you abandon the question, considering it as another trivial, futile idea? Or, have you dug into the question and found the scientific theory behind this phenomenon in depth with further exploration?

The fundamental theory of the phenomenon is called the Principle of Communicating Vessels. If a U-shaped glass tube contains the same liquid, we can envisage a small liquid plate, AB, at the bottom center of the join. If we suppose the liquid is still, and the pressure of the liquid in the tube on the left side of tablet AB must be equal to that in the tube on the right side of tablet AB because the tube contains the same liquid so it has identical density everywhere in the tube. According to liquid pressure expression, p=ρgh, the pressure due to the left and right water columns will be the same only if they are the same height. Therefore, in the case that the liquid does not flow, the liquid level in each container of the communicator should be flat.

The characteristic of the communicating vessels is that the liquid level in each container is flat only when the container contains the same kind of liquid. If the container is tilted, then the liquid in each container will begin to flow from the high end of the column to the low end of the column until the level in each container is normal, stopping the flow. If two glass tubes are connected to a rubber tube and the container is filled with the same liquid, one tube is fixed and the other tube is raised, lowered, or tilted. It can be seen that the liquid level in the two tubes is always level when it is still.

In this case, water pipes in kitchens and bathrooms are U-shaped and there will always be some water stored at the bottom of the “U”. The surface of the water on two sides both exceeds the lowest point of the upper cover of the pipe. Therefore, the water body functions as an isolating set, preventing the gases in the downcomer from back flowing to our houses. The gases are not only smelly but also dangerous in some cases. This is the reason why the pipes look strange. Next time when you are taking a shower, try not to just wash your body, but shower your brain knowledge.



Spellman, Frank R.; Whiting, Nancy E. (2005). Environmental engineer's mathematics handbook. CRC Press. ISBN 978-1-56670-681-0

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