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.
Candle Ladder
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.
Magic Lifter
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.
Ruler Snap
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.
Madelung Hemispheres
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.
Liquid Nitrogen
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.
N Balloon
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.
Bi-Metal Strips
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.
Cork Cannon
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.
LN Bomb
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.
Bernoulli Effect
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.
Vortex Cannon
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.
Flame Tornado
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.