Allen, George Austin Community Academy

Objective The purpose of these demonstrations is to aid the student in learning basic principles of air pressure. Apparatus Needed Balloon Glass Jar Water Soap Pencil Balloon In Jar Inflate the balloon so that it is slightly larger than the mouth of the jar. Try to force the balloon into the jar; it is difficult, if not impossible. Then slide the pencil down beside the balloon and the balloon may be pushed into the jar. The balloon seals the opening of the jar so that air cannot escape around it, and as it is pushed, it tends to compress the air in the jar slightly. It cannot be easily pushed against the air pressure. Apparatus Needed Plastic Glass Hot Water Smooth Surface Gliding Glass Rinse the glass with hot water. Leave a little water in it, and invert the glass onto the smooth surface. The glass will "skitter" around as if on ice, with almost no friction. As the water is poured out of the glass it is replaced by room air. Heat stored in the glass and water, heats the air somewhat; it expands and the pressure lifts the glass a tiny distance from the surface of the table. The glass floats on a film of water and a cushion of air. This is the same principle used by the surface-effect vehicles or "hovercraft." Apparatus Needed Erratic Ball Table Tennis Ball Thread Scotch Tape Soda Straw With Flex Attach the ball to the end of the string with a tiny piece of tape. Suspend the ball by the string. Blow upward against the ball through the straw. Blow harder, then slowly, and reverse. Blow gently up against the ball, just off center, and the ball will try to "outflank" the air jet by going around it to where it can hang vertically. The ball's motion will be quite erratic. Blow harder and the ball will go into the center of the air stream and tend to remain there. Moving air exerts less pressure than still air, so the ball tends to remain so that the pull of the moving air around it is nearly equal on all sides. Apparatus Needed Crushed Jug Gallon Size Plastic Jug with screw on lid Boiling Water Put boiling water into the jug and shake it with the lid closed, but loose. When steam and water stops coming out, screw the lid on tight. The jug will begin to collapse. The action can be speeded up by using cold water on the jug. As the steam in the air condenses, the pressure in the jug diminishes. Atmospheric pressure crushes it. Apparatus Needed The Hovercraft An Old Long Playing Record A Wooden Spool A Candle A Large Balloon Smooth Surface Fix one end of the wooden spool so that the balloon can be slipped over it. Attach the other end of the spool to the center of the record with candle wax or glue. The holes in the spool and record should match. Inflate the balloon, slip its mouth over the spool and place the record on a smooth surface. Release the balloon and the record will glide over the smooth surface with little friction. When the record rests on the surface it tends to remain there because of the friction created when the surfaces move against each other. The air stream from the balloon puts a thin layer of air between the surfaces, eliminating most of the friction.
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