MadSci Network: Physics

Re: why water not spurts from top?

Date: Thu Feb 8 10:31:08 2007
Posted By: Valdis Kletnieks, Staff, Computing Center, Virginia Tech Computing Center
Area of science: Physics
ID: 1168097575.Ph

Consider a glass of water as a container that has a very large hole in
the top.  Why doesn't the water jump up out of the glass?  Well, there's
two ways to look at it:

1) Remember there's gravity pulling downward on the water. And gravity is a
*vector* field - that means it has a *direction*.  Gravity pulls *down* on
it, so the molecules are exerting force *down*.  Consider the molecules at
the surface - gravity is pulling them down, so they exert a force downward
on the layer of molecules under them. And that second layer exerts force of
its own, *and* propagates the force from above, and so on. The end result
is that at the surface, there's zero up/down pressure.  32 feet down,
there's enough water above you to equal the weight of atmosphere above you,
so you have about 28 pounds/square inch of pressure total, and so on - at
320 feet, you have 11 atmosphere's worth of things pressing down.

2) The water *is* exerting a small upward force caused by (mostly) the fact
it isn't very compressible, which just happens to perfectly balanced by the
*downward* pressure of many miles of atmosphere above it.  To demonstrate
what happens when there's an imbalance, use a straw to either blow air into
a concentrated spot on the surface and watch the added pressure cause a
small dip in the water surface.  Or suck on the straw, and watch the
imbalance cause the water to rise up the straw.  This force is quite strong
- about 14 pounds per square inch, which over the entire surface of the
glass is quite a bit pushing down on the water, keeping it in place.  In
fact, if you fill a glass *almost* full of water, then place a small
*stiff* piece of cardboard or plastic across the top, and then invert the
glass while holding the cover in place, you can hold the glass *upside
down* and the air pressure will keep the cover in place.  Just two notes: 
(a) practice over a sink and (b) a *totally* full glass won't work as well.

And yes, some of the molecules *do* exert enough force upwards to leave the
surface.  The usual term for that is "evaporation".

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