How does siphonic action work

The siphon is a liquid reservoir with an inverted U-tube. The liquid initially fills the tube. There are several methods to achieve this initial state.

The purpose of the siphon is to drain liquid from the reservoir by liquid flow that passes over a higher level than the liquid surface in the reservoir. The net result is that the liquid passes from the reservoir to a lower level and does so continually and without external energy input until the reservoir level falls below the ouput end of the U-tube.

The practical difference between a siphon and a leaky bucket is the fact that the liquid passes to a higher level before draining out to a lower level.

So we have several things to explain. Water rising in the input tube. The liquid barometer is an example of water rising above the level of a reservoir. A tube closed at one end is filled with the liquid, then the closed end is raised up while the lower end is immersed in the liquid reservoir. The liquid in the tube stays there, until the tube height is greater than a certain height that depends on the density of the liquid and the pressure of the air outside the tube.

Then a near-vacuum forms above the water in the tube, and the water level does not rise further. Air pressure is sufficient to sustain a liquid height of about 34 feet for water, only about 32 inches for liquid mercury. Above that height, a near-vacuum forms above the liquid in the closed tube. Actually, the pressure there is the vapor pressure of the liquid.

The vapor pressure of mercury is 0. This tells us that if the height of the U-tube of a siphon were too great, the same thing would happen, and a near-vacuum would be created near the top of the U-tube. This would interrupt the continuity of liquid in the tube, and siphon action would stop. This is exactly what happens. Ancient Roman engineers who built siphons into their water aqueducts were quite aware of this limitation of siphons. In this case we must be clear about the reason that the siphon would fail if the U-tube were too high.

It is simply because air pressure on the input side is insufficient to raise the input liquid column as high as the top of the tube. If it doesn't get to the top, it won't flow over to fall down the output tube.

So air pressure is important to siphons by putting a limitation on how high they can lift water, and without lifting the water to the top of the U-tube, no siphon flow can occur.

What sustains the liquid flow? But this isn't the whole story. Is it air pressure that sustains the water flow in a siphon? It is not. If siphon flow is analyzed, with a nearly incompressible liquid like water, the work done at each end of the siphon against air pressure is NET zero, for equal volumes of air are displaced at each end. The two ends water surface in bucket and at the output end of the tube are both at the same atmospheric pressure, p , so pdV is the same size for equal volumes displaced, but the signs are opposite.

So air pressure does not drive the siphon. Siphoning in a vacuum. If there were no air pressure, could a siphon work? Yes, at least for some liquids, but something else would be required to cause the liquid to form a continuous path through the elevated U-tube.

Very cohesive liquids can do this, the molecules attracting each other so strongly that they can maintain a chain-like continuity up and across a U-tube, yet still maintain liquid properties.

Hence, the liquid starts to flow from a higher elevation tank to a lower elevation tank continuously. Hence, the pressure at point 2 is When a siphon is used like U-tube or U-pipe, instead of an inverted U tube, it is called an inverted syphon. So, we have learned syphon along with various working principles.

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We will capture all types of Mechanical Engineering articles in the simplest way with various diagrams, practical examples, etc. Fluid Mechanics 10 Questions No. What is the unit of kinematic viscosity? Which property is responsible In this article, we will learn all about hand pump along with definition, parts, how it works, advantages, disadvantages, application, etc.

Let's explore! What is Hand Pump? Liquid should start traveling from the source container to the new one. Be sure to keep an eye on your source liquid and make sure the hose stays fully submerged, otherwise you'll end up with bubbles. When you need to stop, lift the new container and hose higher than your source container. Then, remove the hose and let the excess fluid in the hose drain back into the source.

Or, if you're draining something large like a fish tank or Jacuzzi, lift the hose out of the source. The Straight Dope homepage. Filed under:. By Cecil Adams Jan 5, Reddit Pocket Flipboard Email.

Dear Cecil: I've asked every physics professor I've ever known but have never found a satisfactory answer to the question: How does a siphon work? Why is it called a restroom, anyway? How did public libraries get started? Thanks for signing up! Check your inbox for a welcome email. Email required.