Is that a cap or a tap?
The science behind this activity is roughly the same as the one for the previous activity (index card sticking to the glass and not falling off).
You will need a plastic bottle with a cap (like a soda bottle or plastic water bottle) for this activity.
1. Use a nail to poke a hole on the side of the bottle, say an inch or so from the bottom.
2. Fill the bottle with water and close the bottle tightly.
3. Water will not flow out off the hole.
4. Open the cap a little and water flows out. The cap works like a tap -- open or close it to let the water flow or stop.
Explanation:
When the cap is screwed on, it is air tight. Initially the air inside
is at the same pressure as outside. But, when a small amount of water
flows out, the empty space inside the
bottle expands and as a result the air pressure inside the bottle decreases. The
air
pressure outside is higher now and stops the water from flowing out.
But when you loosen the cap, it is no longer air tight and air from the
outside comes in. Once
again the air pressure inside and outside the bottle are the same. They cancel
each other out and the weight of the water now causes the water to flow
out. Screw the cap tightly again, the air pressure inside becomes low again
and ...
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