This Week's Experiment - #330 Water in a Glass
This week's experiment has its roots in a phone conversation I had with my
friend Mik Jacobs (The Fire Guy, http://members.aol.com/findmik/). He is on a
whirlwind tour, including Hong Kong, California and South Carolina. We were
discussing different ways to explain simple science experiments. Later as I
was doing the dishes, I followed up some of the ideas and wound up with this
one. You will need:
a drinking glass
a sink or bathtub
water
a paper plate or piece of cardboard
We will begin with a common, simple experiment. Fill the glass to the top
with water. Place the paper plate firmly over the top of the glass. Holding
it over the sink (in case it doesn't work), hold the plate firmly in place
and turn the glass upside down. Then release the plate. It should stay in
place, and the water should stay in the glass.
Why? It is tempting to think that the plate is holding the water in, but it
is not. The plate is keeping the air out. The air pressure outside the
glass is holding the water in. How can we show that? Well, first, we can let
some air in to take the place of the water. Keeping the glass over the sink,
pull down on one side of the plate, just enough to let a little air get in.
What happens? The water gushes out. But wait. Doesn't that also fit
with the theory that the plate was holding the water in?
To check that, we need to get rid of the plate. Fill the sink most of the
way with water. Holding the glass sideways, place it in the water. Once it
is totally filled with water, turn it upside down, keeping it under the
surface. Lift it slowly upwards. Though most of the glass is above the
surface,
the water stays inside the glass. As long as the mouth of the glass is under
water, no air can get into the glass, and the water can't come out. The
external air pressure holds the water in, even without the plate. As soon as
the edge of the glass is lifted above the surface, air rushes in, and the water
rushes out.
How can something as light as air hold up that much water? The air around
you is pressing on everything with quite a bit of pressure. At sea level, it
presses with 14.7 pounds on each square inch of surface. If you draw a grid
of one inch squares on a piece of paper, you can place the mouth of the glass
on the paper and see about how many square inches it covers. Multiply that
my 14.7 pounds and you can see that you have quite a bit of pressure pushing
upwards. When the glass is full of air at the same pressure, you have the same
amount of push downwards as upwards. They balance each other. With the
glass full of water, when the water starts to move downwards, there is nothing
to take its place. This produces an area of extremely low pressure. The
higher pressure on the outside is then plenty strong enough to keep the plate in
place.
Have a marvelous week.
From Robert Krampf's Science Education Company
PO Box 60982
Jacksonville, FL 32236-0982
904-388-6381
krampf@aol.com
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