| MadSci Network: Physics |
Straws and Water.
Start with a glass of water, put a straw in it, and take a close look at it. Here is a diagram:
air air air
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v v v
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| | straw
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glass | | | |
| water | | |
|-_ _ _ _ _ _-|-|-_ _ _ _ _ _ _-|
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Air has weight, so gravity tries to pull it down to the ground. The
air that is already down on the ground is in the way of the rest of
the air, so the rest of the air stays above our heads. The important
thing is that gravity pulls air down equally everywhere you look.
That means that the of weight of air on the water outside the straw is
the same as the weight of air on the water inside the straw. The
level of the water in the glass is almost the same everywhere, because
the weight of the air is the same everywhere.
The exception is inside the straw, and at the edges of the glass. The water level is slightly higher there. This is something that water can do by itself; the air has nothing to do with it. There is a small amount of attraction between the water and the glass, and between the water and the straw. That attraction lets the water climb up the side of the glass (and the straw) a little bit. Here is another diagram, so we can see it more closely:
| | | straw
glass | | |
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|\ /|\_/|\
| \_ _ _ water level _ _/ | | \_ _ _ _
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The thing to think about is the amount of water that is lifted near
the side of the glass. Water not only attracts the glass, it also
attracts itself. Only gravity keeps it from climbing all the way up
the side of the glass. But notice how the water inside the straw has
climbed higher than the water level inside the glass. This will only
happen if the straw is narrow, and the narrower the straw, the higher
the water can climb inside it. The closer together the walls around
the water, the easier the uppermost water can attract the straw. And
there is less total weight of water to be lifted, as the uppermost
water attracts the water underneath it. The very narrowest straws are
usually called "capillaries"; capillaries inside of trees let water
climb hundreds of feet upwards. This climbing process is called
"capillary action".
The attraction that water has, as just described, is mostly noticed only at the surface of water. The usual name for it is "surface tension". Surface tension causes small drops of water to become completely round, like a tiny ball. This is because the ball-shape, otherwise known as the "sphere", can hold the largest amount of water with the smallest amount of surface. Remember that we are talking about attraction at the moment. Attraction causes things to move closer together, and they take up less total space as a result. Since the surface of a sphere is smaller than the surface of any other shape, surface tension always causes water to form round droplets.
We often do not see a droplet of water after it touches something; it 'splats' into a flat film. This is just another consequence of the fact that water is attracted to other things. Whenever a drop of water flows into a flat film, the process is called "wetting". Anything that can truly get wet is something that water attracts. Other things, like wax paper, repel water, so if a drop is placed onto wax paper, the drop remains drop-shaped, thanks to surface tension.
Now back to the straw in the glass of water. If you put your finger over the top of the straw, and pull the straw part way out of the glass, you will see something like this:
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/ fingertip
\_______
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|\_/| water in straw
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glass | | |
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|\ /| |\
| \_ _ _ water level _ _/ | | \_ _ _ _
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Remeber that first sketch showing air pressure everywhere? When you
lift the straw with your finger on it, you prevent air from getting
in. The air outside the straw, pushing down on the water in the
glass, weighs enough to hold the water up inside the straw. The
water will stay there as long as no air can get into the top of the
straw, where its weight can push the water back down to the level in
the glass.
Now, what about when you pull the straw all the way out of the glass?
_______
/ fingertip
\_______
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|\_/| water in straw
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\_/
weight of water forms a droplet at bottom of straw
Remember that a droplet has surface tension, and attracts the sides of
the straw. The only way that air can get into the straw is by
BREAKING the droplet's surface tension. If the straw is narrow, air
is not strong enough! Sometimes, for a medium-diameter straw, if you
tilt the straw, the weight of the droplet will pull itself away from
part of the straw. This will help the air break the surface tension,
so it can get in. The water will drain out, drop by drop.
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