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Offline billy55

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drinking straws
« on: 15/01/2008 00:28:09 »
How come when you cover a straw with your thumb, the water stays inside of it?


 

Offline Karen W.

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drinking straws
« Reply #1 on: 15/01/2008 01:27:06 »
I am guessing and am no expert so here is my guess someone fix it if I fall on my face! First of all you put the straw in the water forcing water into the end of the straw. Then you put your thumb over it thus entrapping air between your thumb and the water.. I believe this causes a suction and allows the water to remain in the straw until you release the suction by letting the air out.
 

Offline lightarrow

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drinking straws
« Reply #2 on: 15/01/2008 11:38:30 »
I am guessing and am no expert so here is my guess someone fix it if I fall on my face! First of all you put the straw in the water forcing water into the end of the straw. Then you put your thumb over it thus entrapping air between your thumb and the water.. I believe this causes a suction and allows the water to remain in the straw until you release the suction by letting the air out.
Hmmm...not exactly, but you went near. You have the effect even without air trapped. The fact is that you have air pressure on the liquid down the straw and no air pressure up so the liquid is held up and cannot fall down. Of course this is true until the liquid's weight becomes higher than the air force and this happens, for water, when the liquid column is 10.33 metres.
 

lyner

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drinking straws
« Reply #3 on: 15/01/2008 11:39:45 »
Karen!!!! There is no such thing as suck! (It is one of the Ace Schoolboy howlers of all time.)
Fluids do not hold together (ignoring the tiny forces of surface tension). They will only move when PUSHED.
The drink stays in the straw because there is atmospheric pressure pushing  down on the surface of the drink and, once the liquid level drops a tiny bit, the pressure under your finger is low and balances the pressure in the column from the bottom. This is what keeps the liquid column up in the straw.
If you have a long enough straw - nearly 10m - the atmospheric pressure is not enough to keep the column any higher and you will get a space at the top of the liquid. (Ignore the problem that it will boil under such reduced pressure).
Mercury is about 14X more dense so a mercury barometer uses a column of about 760mm of mercury. The actual height of the column depends upon the  Atmospheric Pressure so you can measure AP accurately with a few quid's worth of mercury in a tube and a ruler.
This is yet another thing which Gallileo found out.
 

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drinking straws
« Reply #3 on: 15/01/2008 11:39:45 »

 

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