MadSci Network: Physics

Re: Why does water rise higher in a straw with a wider diameter?

Date: Wed Feb 7 11:40:34 2001
Posted By: Michael Richmond, Faculty, Physics, Rochester Institute of Technology
Area of science: Physics
ID: 981134440.Ph

Indeed, the diameter of the straw should not have a (big) effect on the 
height to which water can be drawn upwards.  Water will rise slightly
in small straws due to capillary action, even if no suction is applied.
I do not see how Pascal's Principle (pressure is transmitted equally
to all portions of a container holding an incompressible fluid) would
explain any difference between narrow and wide straws.

I cannot explain the outcome of your student's experiment.  I speculate
that there might be differences in the materials from which the straws
are made, or that he might be able to fit his lips and tongue more firmly
against a wide straw than a narrow one.  

As far as I can tell, "physics" does _not_ explain the result.

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