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Welcome to the Greenwood.Net Curiosity Corner

Let's Siphon

Jan 15, 2019

Curiosity Corner
Dr. Jerry D. Wilson,
Emeritus Professor of Physics
Lander University

Question: I used garden hose to siphon water from the ditch on one side of the road to the other, and got to wondering how siphons work. I found one theory that involves gravity acting on one side more than the other. Another involves atmospheric pressure. I wondered if those have been tested at the space station. They have zero gravity there, so they could test both. (Submitted by Jim Miller via Cyberspace.)

Reply: A siphon (sometimes spelled syphon) is a handy thing for transferring liquids. Like Jim, I used a garden hose to siphon water, but I was emptying a hot tub. A siphon allows you drain a liquid from an higher level to a lower level (as shown in Figure 1). To get the siphoning actions started, you need to fill the tube with the liquid and place the upper end below the liquid level. Once the siphon begins to work, it will continue to drain the liquid from the upper container until the liquid level reaches the bottom of the tube, or the end of the tube is lifted about the liquid level to stop the siphon.

The first time you use a siphon, it impresses you … scientific principles at work. Even though siphons have been used for centuries, there is still debate on exactly which forces make a siphon work. As Jim pointed out, there are two theories, atmospheric and gravity. However, these won’t be tested in the space station. The vacuum in space is outside the station and would require a spacewalk. The astronauts have regular atmospheric pressure inside the station. Also, there’s gravity there. That’s what keeps the space station in orbit. (Analogous to the string that keeps the whirling ball in orbit; and astronauts appear to float or be weightless because they are “falling” toward the Earth with the space station.) The terms “zero gravity” and “weightlessness” are misnomers. Better terms are “microgravity” and “apparent weightlessness.”

Back to the siphon. First, let’s consider atmospheric pressure. When the liquid in the tube begins to empty (fall) into the other container, a decrease in pressure is caused at the highest point in the tube (the hump portion). The atmospheric pressure on the surface of the container liquid then forces the liquid up into the tube toward the area of lower pressure, causing the siphoning action. (Atmospheric pressure allows you to drink with a straw by pushing the liquid upward when you aspirate and reduce the pressure at the top of the straw.)

Sounds good, but this theory requires the presence of air. So just to make sure, scientists tested a siphon in a vacuum chamber … and it still worked! It seems there must be some other force also at work. Another theory was developed in which gravity is the key. When a liquid goes up and over the hump, the force of gravity continues to pull the liquid down through the tube. This theory relies on liquid cohesion, which means a continuous chain of cohesive bonds must exist in the liquid. When you pull the liquid chain through the tube and over the hump, gravity takes over and pulls the liquid chain to the lower level.

So take your pick. It may be the case that atmospheric pressure and gravity (with liquid cohesion) may act together to make siphons work the way they do. That’s what makes science so interesting … figuring out how things actually work. And there’s still work to be done!

C.P.S. (Curious Postscript): “I’m sorry; if you were right, I’d agree with you.” –Robin Williams

Curious about something? Send your questions to Dr. Jerry D. Wilson, College of Science and Mathematics, Lander University, Greenwood, SC 29649, or email jerry@curiosity-corner.net. Selected questions will appear in the Curiosity Corner. For Curiosity Corner background, go to www.curiosity-corner.net.

Caption for Figure 1: Liquid is continually siphoned from the upper container through a tube to a lower elevation until the liquid level reaches the bottom of the tube, or the end of the tube is lifted above the liquid level to stop the siphon.

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