eli5: How does water siphoning work?


I see people put one end of a tube in some water and then suck the other end, and then the water seems to flow out of the tube on its own? Shouldn’t there be an external force pulling the water out?

In: Physics

The weight of the water does the pulling.

As long as there’s a steady flow of water through the tube gravity and the water itself does all the work.

Think of it like this, a four foot long hose is used and marked with four different colors, each one foot long.

If the red section of the tube is full of water and then that water moves down to the green section, what’s left in the red section? It can only be filled by more of the water from the container.

Each small amount of water through the hose pulls an equal amount of water behind it. That’s because the space has to filled with something, and if no air is available, water is the only available option.

Without getting into a bunch of Physics terminology there are two ways water has potential energy: pressure (high pressure seeks lower pressure) and gravitational (this one is obvious).

A siphon can only work with an elevated source of liquid and an arching pipe. Basically as water is pushed into the descending portion of the pipe (via gravity) it creates low pressure in the tip of the arch.

This creates an endless loop where the high pressure water in the source sees the low pressure in the arch and *really* needs to get up there but just falls down the pipe pushing low pressure back into the top.

There is an external force pulling the water out, gravity. The weight of the little bit of water in your tube that is below the waterline of your reservoir will pull the next bit of water up and over the bend of tube on its way out, which pulls the next bit of water and so on, creating flow.