MadSci Network: Physics

Re: what physics laws enable a siphon to function?

Date: Wed Apr 1 20:00:16 1998
Posted By: Robert L. Judge, Faculty, Chemistry/Science Department, Holy Cross High School
Area of science: Physics
ID: 887582185.Ph

When you use a siphon, one end is in the larger portion of the water (if 
that's what you're siphoning), the other end is below the "tank" or 
reservoir. What makes things fall toward the earth? That's right, gravity.  
Because the terminal end is below the "tank" from which the liquid is being 
transfered, the water flows downhill.  

But then how does the water or other liquid get uphill?  

The air pressure differential between the reservoir and the receiver tank 
causes the water to go uphill, once it gets up, it has to fall. It is gravity 
which causes things to fall toward the center of the earth. Once the water is 
started moving downward, gravity continues the process.

But why does the water in the tank continue to "follow" the water in the tube? 
There is another process invloved but it has to do with chemistry.  This would 
be cohesion. 
Molecules of a substance "stick" togeher with other molecules that are the 
same.  There are tiny forces between the molecules that cause this.  In the 
case of water, these forces are called "hydrogen bonding". As a result of these 
forces, the molecules "stick" together and one follows the other as they are 
pulled along, out the tube. 

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