MadSci Network: Physics

Re: Will a siphon work in a vaccuum?

Date: Sat Jan 27 19:29:16 2001
Posted By: Renafaye Norby, Faculty, Science Education, Black Hills State Univ.
Area of science: Physics
ID: 979074465.Ph

Siphons work on the principle of using air pressure, so in a vacuum, a 
siphon will not work. The questioner is muddying the waters by stating the 
tall narrow cylinder has less air pressure than the large wider container. 
Pressure is defined as force per unit area, so a wider area has the same 
amount of pressure as a smaller one, by definition.
If water is in two containers, one of which is "lower" than the other, then 
a tube running from the higher container to the lower container will allow 
water to flow as long as the water level of one is higher than the water 
level of the other, and the tube is below the water level. This is gravity 
in action, and is a different case from a siphon. Since I haven't looked at 
the diagram I may be missing part of the details, But, simply put, a siphon 
will not work in a vacuum, since there is no air pressure. For further 
ideas about water pressure and vacuums, read the chapter in Richard 
Feynman's book, "Surely you're joking, Mr. Feynman," where he describes his 
experiment with a lawn sprinkler, a large carboy, and a vacuum pump. 

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