H and He Balloons
15" balloons filled with either H or He are popped with an 8" "torch" lighter. This is used as an exercise in the scientific method. In science, we make educated guesses based on observations, experiments are performed to test those guesses, the guesses are modified if necessary, and the process continues. Students are shown the balloons, and after noting that whatever is in the balloons must be lighter than air, the students are asked what might be in them. If mentioned, hot air can be discounted since there is not an obvious heat source. Heat must be continuously be added to a hot air balloon to keep it hot. Once choices are narrowed down to H and He, the students are asked to come up with an experiment to tell the difference. This results in the "flame test", since H is very flammable, while He is not.
Pinto Bean Buoyancy
A ping pong ball is buried in a jar of pinto beans and a brass ball of the same size is placed on top of the pinto beans. The ping pong ball is less dense than the pinto beans, while the brass ball is more dense. The brass ball will not sink, and the ping pong ball will not float, unless the pinto beans can move out of the way. Some "thermal energy" is added by gently shaking the jar. This is equivalent to increasing the temperature in an atomic system. The brass ball will then rapidly sink, and the ping pong ball will rise.
Air has Weight
We do not notice it much, but the air around us takes up space and it has weight. Two 20" balloons are taped to the ends of a long dowel that is suspended by a string from it's middle, like a balance. When one balloon is popped, the other will fall, illustrating that air has weight. Actually this shows that the air inside the balloons is heavier, or more dense, then the air around them. This is because of the elastic tension in the latex. If plastic baggies were used instead, so that the pressure is the same inside as outside, neither balloon would fall if the other is popped.
Some gasses are heavier than air. CO2 (carbon dioxide) is a good example. We use it in fire extinguisher because it sinks to the base of a fire and pushes the lighter oxygen away. This will put out a fire, since fire needs oxygen to burn. Our "candle ladder" consists of 128 2' candles evenly spaced on a flat 3'x5' platform with two 2"x2" boards along the edges of the long sides to act as channels for the CO2 when the candle ladder is tipped at 30 . With the candles lit, room temperature CO2 (from apx. 1 lb of powdered dry ice allowed to warm and turn to gas) is poured from a 20 gallon barrel down the ladder, and the invisible gas puts the candles out is a clear, downward moving, wave. This illustrates that even warm CO2 is heavier than air.
An 18"x18" piece of 1/8" neoprene with a small hole in the center to fit a small weight hanger handle serves as a suction cup, with out the cup. When placed on a smooth surface with all the air pressed out from between them, the magic lifter can transmit a considerable lifting force. To be precise, the effort in pulling up on the handle creates a small region of low pressure between the lifter and the surface. The unbalanced pressure on the bottom of the table, box, desk, etc. is what actually provides the lift.
A cheap wooden ruler balanced on the edge of a table cannot normally be snapped with a fast blow from a hammer. This is because of the very low "inertia" of the ruler. However, a large, light piece of paper pressed flat over the half of the ruler on the table will allow one to break the ruler. The inertia of the ruler is increased by essentially adding a large amount of air directly over the paper. This air must be moved in order to move the ruler.
A vacuum pump is used to pump out most of the air from between two metal hemispheres. Normally, the force of the air on the inside pushing out will equal that of the air outside pushing in. This force is due to the "air pressure", or the weight of the atmosphere above us. With most of the air gone from the inside, not much opposes the force of the outside air pushing in. The weight of the atmosphere above us will hold the two hemispheres together. Two of the biggest and strongest students each pulling on one hemisphere will not be able to pull them apart. Using two students is simply good theatrics. One student could exert the same force as two on each hemisphere if the other hemisphere is tied to the wall.
Vacuum Chamber Tricks
A vacuum pump hooked up to a big glass bell jar is used to demonstrate what happens to things if the air is pumped out from around them. The weight of the atmosphere can be essentially removed. First, the jar is filled with several small balloons, not inflated all the way. As the air around them is removed, not much opposes the outward push of the air on the inside of the balloons, so they inflate ant pop. Next, a "marshmallow" man is placed in the jar. Marshmallows are mostly air inside, so like the balloons, they will inflate as the air is pumped out from around them. However, once the air is let back in, the marshmallows shrink down to much smaller than they started, because a lot of air was also sucked out of them. Finally, a can with a small hole in the top, filled with shaving cream colored with food coloring, is placed in the jar. Like the marshmallows, shaving cream is mostly air, so that it will expand dramatically as the air is pumped out. Streams of shaving cream "worms" will shoot out of the can. When the air is let back in, this all turns to a slimy mess.
Most of the air around us is nitrogen. Most materials can be either solid, liquid, or gas. If you warm up a solid enough it will melt into a liquid. If you heat that liquid, it will boil and turn into gas. Similarly, a gas can be cooled to form a liquid, and a liquid can be cooled to form a solid. A bucket of liquid nitrogen (often called liquid air) at -321 F is used to demonstrate what happens to things when they get very cold. Balloons will shrink down as the air inside cools, then turns into liquid. a Liquid takes up much less volume than a gas. The balloons will inflate back to normal after they are removed and allowed to warm. Flowers will become very brittle, and shatter if dropped. A piece of lead will ring like a chime once it is cooled. Superballs will act like marbles, and a racquet ball will shatter and implode, because the air inside is liquid and almost a vacuum. Rubber bands, when frozen while stretched, will squirm and contort as they warm due to the long chain molecule nature of rubber. Liquid nitrogen can be handled by an experienced demonstrator without danger due to the Leidenfrost effect. His or her hands are so hot, compared to the liquid nitrogen, that the nitrogen first touched is flashed immediately to vapor, and the rest of the liquid will be insulated from the hand by that vapor layer. Liquid nitrogen splashed on a smooth surface will bead up into balls and roll around for a long while like mercury, because it will float on an insulating vapor film. Water will do the same thing if dropped on a very hot skillet, while a colder skillet will evaporate the water away almost immediately.
Gas takes up much more volume than liquid, and for most materials, liquid takes up much more volume than solid (water is an exception to this). To demonstrate this, a small amount of liquid nitrogen is poured into a metal tube that is closed at one end. A giant 36" balloon is put over the open end of the pipe and duct taped down. As the nitrogen boils and turns into gas, the balloon will inflate. As the cold gas warms up, it also expands. Only 10 oz or so of liquid nitrogen are needed to inflate the giant balloon to bursting.
Pop Can Crush
Another way to demonstrate volume changes during phase changes is to boil a small amount of water in an aluminum pop can. Once the can is filled with steam and all the air is pushed out, the can is quickly placed upside down in a shallow bowl of cool water. The steam inside the can quickly condenses to liquid, taking up a much smaller volume, and drastically lowering the pressure inside. The can cannot suck water up into it fast enough, and the pressure of the outside atmosphere crushes the can.
50 Gallon Drum Crush
This demo is conceptually identical to the Pop Can Crush demo, except that a 50 gallon steel drum is used. A giant propane torch is used to boil one gallon of water inside the drum while the drum sits inside a 1.2 meter (4 foot) diameter circular livestock watering trough. The drum is then sealed and doused with ice, and water from super soakers. The collapse of the drum is loud and dramatic as the steam turns back into liquid and atmospheric pressure crushes the drum.
Ball and Ring
Most solids, like gasses, expand when heated and contract when cooled. To demonstrate this, a metal ball, specially constructed so that it is just a little too small to fit through a similar metal loop, is cooled in liquid nitrogen and the hoop is heated on a propane stove. The ball will then easily fit through the loop, with lots of room to spare.
Different materials contract or expand with temperature at different rats. A strip made of two strips of different metals joind together will then bend one way, then the other, as it is heated and cooled. Bi-metal strips wound up as springs are often used as thermometers because of this. They can be found inside most themostats.
Instead of topping the metal tube discussed in the N Balloon description above with a balloon, the open end of the tube is stopped up by a cork pounded in with a hammer. The pressure builds up, and pops the cork out at a high speed. Corks can easily be launched 60 yards or more.
A 12 oz plastic pop bottle is filled halfway with liquid nitrogen and sealed. It is quickly placed in a bowl of water made from a cut up two liter pop bottle that is inside a specially constructed blast chamber. Our blast chamber is a 3'x3' wooden box open on two opposing sides, with three sides pierced by many 1/2" holes covered in mesh and shielded by baffles. The open end facing the audience is covered by a 1/4" lexan shield clamped in place. A second 1/4" lexan shield is clamped parallel to, and 6" in front of the first lexan shield. The pop bottle will start to inflate like a balloon, and explosively rupture within about 30 seconds.
Pulsating Fountain of Joy
Once liquid nitrogen is no longer required, the remainder (no more than 12 oz) is dumped into a large bucket half filled with one part Joy dish washing liquid and four parts water. As the liquid nitrogen boils, large volumes of soap bubbles spew out of the bucket in a fountain that lasts 20 seconds or more. This is a good distractor after filling the pop bottle in the LN Bomb demo described above, while you are waiting for the blast.
A vacuum cleaner set to blow instead of suck is used to levitate ping pong and styrofoam balls vertically, and at angles as great as 40 . This looks impossible, but the balls are actually attracted to the air stream. Wherever fluid or air is moving fast, the pressure there is lower than where it is moving slower.
We use this effect to make airplanes fly. Wings are shaped to force much of the air over the curved top of the wing. The air that moves over the top moves farther than the air that just passes underneath, so the air moves faster over the top of the wing, The pressure is lower where the air is moving faster, so there is a pressure difference between the top and the bottom of the wing. This provides the lift that keeps the plane in the air. The engines are there just to make sure that the plane is going fast enough to get enough of a pressure difference between the tops and bottom of the wings. This effect also explains why doors sometimes open when there is a wind outside, and why houses sometimes explode when a tornado goes by. High speed winds can dramatically reduce the pressure outside, while the pressure inside is comparatively very large because the air is still.
Air blown between two suspended light bulbs will pull them together and produce an audible "clink", and air blown through a straw under a piece of paper supported by books on two sides will pull the paper down. When a funnel is attached to the hose of the vacuum cleaner (still blowing) like a horn, a ping pong ball pushed up into the narrow part of the funnel will get stuck there by the low pressure created by the faster air in the narrow part, even when upside down.
The audience is challenged to blow out a single candle, held out about 6' from the closest student. After a bit of huffing and puffing from the crowd, a demonstrator blows it out from 30' away with a small vortex cannon. The cannon is a 5 gallon bucket with a 5" circular hole cut in the bottom and a drum-like diaphragm made from an old inner tube stretched over the top. Once the drum is struck, a gust of rapidly moving air is forced out the hole. This lowers the pressure at the hole due to the Bernouilli effect, and air around the hole is sucked towards the low pressure. The air forced out of the hole and the air sucked in from the sides combine to form a doughnut-shaped vortex, just like a big smoke ring. Such vortices can be very stable and can travel long distances. Where the air is spinning around, the pressure is low. This sucks in more air, which makes it all move a bit faster. This lowers the pressure even more, which sucks in even more air, which makes it all move faster, etc... Hurricanes and tornados are as nasty as they are because of this same effect. After this introduction, the audience is blasted by a giant 4'x4'x3' cannon made from a big cardboard box with a 24" circular hole and a "diaphragm" of plastic sheeting pulled in tight in the middle by elastic tubing attached to the four opposite corners. The vortex ring is made visible with atomized mineral oil from a fog machine. These rings can be visible for over 100 yards in still air.
A cylindrical wire cage is spun on a rotating platform to create a tornado vortex. Wherever air or any fluid is forced to circulate, the pressure drops due to the Bernoulli effect. The low pressure sucks more air into the vortex, which makes it spin faster, which lowers the pressure even more, etc.., as described with the Vortex Cannon. Instead of making the vortex visible with smoke, flame is used. A small tray with an alcohol soaked rag is fixed to the center of the rotating platform inside of the wire cylinder. When lit, the flame normally rises 10 to 15 cm (4 to 6 inches), but when the flame is caught up in the vortex, it rises to 1.2 meters (4 feet) or more in a beautiful corkscrew-like pillar.
Leaf Blower Tricks
Vacuum cleaners and Shop Vacs are all well and good for levitating ping pong balls, but nothing beats a leaf blower for putting out a large and fast volume of air, ideal for Bernoulli effect demonstrations. Large balls, screwdrivers, 2 liter pop bottles filled with a bit of water, small furry animals, and long streams of toilet paper can be impressively levitated at large angles with these wonderful devices. We save this one for last, for audiences are often difficult to control after they are blanketed by falling toilet paper streams from a dozen rolls or so.