It’s our fourth week back at school and we have to scratch our heads to come up with something interesting. So this week we’ll demonstate the tremendous power of air pressure. The air around us is pressurised to 14 pounds per square inch or about 1000 millibars for the more metrically minded. This pressure acts in all directions and we don’t feel it only because we are so used to it. Find an old gallon can (or even a 5 litre steel container) with a decent screw-on top. I used an old Shell petrol can which I cleaned out thoroughly. Be extremely careful about using a can that contained any chemicals. Take off the top. Put about 2 cms of water in the end of the can and heat it up until it boils. When plenty steam is coming from the top, take it off the heat and cover the can with the screw-on lid sealed with a bit of a plastic bag. As the steam condenses back into water, no air can get into the can to equalise the external pressure. The air outside squeezes the can until it is practically flat! This demonstration should only be done by a teacher or an adult.
Our Water Clock.
We made an alarm clock using the materials shown above. The plastic cup is full of water and is attached to the plank of the see-saw with plenty of tape. The cup has a small hole to let the water out. The see-saw is balanced on an empty 500ml bottle which is also taped on. At the far end of the see-saw is a foil ash-tray attached to the plank using a darawing pin. The ash-tray has been bent out of shape to allow the marbles fall out easily.
As the water drains from the cup, that end gets lighter. The other end gets heavier and eventually falls leaving the marbles fall into the biscuit tin.
If you have an idea for an alarm clock, we'd like to hear from you.