Teaching Kids about Physics(and having fun while doing it)
Compiled & Edited UIUC Physics Van
States of Matter Human Molecules Needed: Several volunteers from the audience WARNINGS: None. What to do: This is done in three parts, one for each state of matter. Begin with the volunteers and participants alternating in a straight line while linking arms, elbow to elbow. As they are doing this, explain that each person is a "molecule". Say that molecules are what makes up everything around us, in other words matter. After they are all linked together, say that they are now like a solid. You can have them try to move, but no one person can move very far. Explain that this is why solids are very hard and rigid. Next you can turn the human molecules into a liquid by heating them up. This is done by having the participants hold hands. Now ask them to move; they will be able to slosh around, but point out that they keep the same basic shape. Lastly, you turn your molecules into a gas. Explain that this is done by adding more heat. Now instruct everyone to let go of each other's hand and move around. Everyone can run freely, but collisions between the human molecules may occur. This adds to the effect and gains the attention of the audience. Variations: During each of the three states you can
ask audience members to guess what state the human molecules are in. Remember, audience
participation is very important throughout the shows, but be careful not to reject any of
the answers given by the audience. Liquid Nitrogen Warnings: Liquid nitrogen is very cold. Avoid prolonged contact with the skin. When it is spilled on a floor or table, liquid nitrogen travels a long distance. Be careful not to spill it when others are close by. Also, some surfaces do not take the cold temperature of liquid nitrogen very well; for example: gym floors and other waxed surfaces. What to do: Pour some of the liquid nitrogen out onto a safe surface. This could be a table which is far from the audience or a lid on top of a chair. Preferably a lid which has raised edges to keep the liquid nitrogen from running off. What to Say: Stress the fact that Nitrogen makes up
most of the air around us. This will dispel any ideas that it is some "magical"
liquid which does very weird stuff. The most important aspect of the liquid nitrogen is
the temperature. The fact that it is so cold is what helps us to use it in so many ways.
For your reference: Liquid Nitrogen boils at -195.82 degrees Celsius. This is
approximately -320 degrees Fahrenheit. For our target age group, Fahrenheit it probably
the most recognized. Liquid Nitrogen and Balloons Needed: Several balloons blown up and tied Warnings: See all warnings for liquid nitrogen. Also some of the balloons may freeze and pop as they are warmed. Warn audience of the loud noise and be careful while holding the cold balloons. What to do: Dip the balloon in the liquid nitrogen until they completely shrink. When you pull it out, you can show the audience that there is some liquid in the bottom of the balloon. Yellow balloons are the best color for seeing the liquid in the balloon. Then, allow the balloon to warm up to room temperature and it will regain its original shape. What to Say: Stress the fact that the air in the balloon never got out; it only changed its state. In dipping the balloon in the liquid nitrogen, we made the air inside the balloon very cold, thus turning it into a liquid. Say that a liquid doesn't take up near as much space as a gas; therefore the balloon shrinks. Then as you take the balloon out of the liquid nitrogen the air inside the balloon warms up and returns to its original state, a gas. Variations: You may refer back to the "human molecules" demonstration so that the audience can get a better picture of what is taking place. Point out that the human molecules took up a lot more space when they were a gas and running all over the room. The Physics: Air, like all matter, is made of
molecules. The air inside of the balloon never escapes; therefore, the number of molecules
inside the balloon remains the same. However, there is a proportionality between the
pressure, volume and temperature of a gas. This proportionality is governed by the Ideal
Gas Law which states that: PV=nRT or pressure, P, times volume, V, equals the number of
molecules, n, times a constant, R, and the temperature, T. Notice that P and V are
opposite T in the equation; therefore, when the temperature is decreased the pressure or
volume must decrease to keep the equation equal. In the case of the balloon, both the
pressure and the volume are decreased. Liquid Nitrogen Cannon Needed: Cannon Warnings: See all the warnings for liquid nitrogen. Avoid shooting the cork over anyone. The cork will leave the cannon before all of the liquid nitrogen turns into a gas; therefore, some liquid nitrogen will spray out of the front of the cannon. Also, be careful not to aim the cannon at anyone or anything that is breakable. The cork comes out at a very high velocity. What to do: Make sure the valve on the cannon is open. Pour liquid nitrogen in the barrel of the cannon and pound the cork into the barrel. Aim the cannon in a safe direction and close the valve. What to Say: Explain that the cannon is much warmer than the liquid nitrogen; therefore, when the liquid nitrogen is poured into the cannon, it warms up and changes into a gas. As seen in the balloons demo, a gas takes up much more space than the same amount of liquid. Since the gas is confined in the cannon, the only way it can take up more space is to push the cork out of the cannon. The Physics: Once again I will refer to the Ideal
Gas Law. Since the liquid nitrogen is very cold, it will boil rapidly when it comes in
contact with the cannon. Liquid nitrogen boils at about 320 degrees below zero Fahrenheit;
therefore, the nitrogen gas inside the cannon will be at this temperature. The cannon,
being much warmer, will warm up the nitrogen gas. As the temperature is increased, either
the volume or the pressure must increase to keep the equation true. As long as the cork
remains in the cannon, the volume of the cannon is fixed; thus, the pressure inside the
cannon will increase. Eventually this pressure becomes great enough to push the cork out
of the cannon. Exploding Bottle Needed: Small plastic bottle with a tight seal Warnings: See warnings for liquid nitrogen. Never try this demo with out an exploding chamber. Make sure that the lid of the exploding chamber is secured tightly. Warn audience of the loud noise. If the bottle fails to explode after several minutes take caution in approaching the chamber and listen for a hissing sound which would indicate that the liquid nitrogen is leaking from the bottle. Check to see if the bottle is bulging before opening the top of the chamber. IF THE BOTTLE IS BULGING DO NOT OPEN THE LID TO THE CHAMBER! What to do: Put the bucket of water into the bottom of the exploding chamber; this will speed up the process. Pour liquid nitrogen into the bottle and close it tightly. Drop the bottle into the exploding chamber; secure the lid and stand back. What to Say: The principle here is the same as in the liquid nitrogen cannon. The liquid nitrogen in the bottle warms up thus turning into a gas. The gas expands the bottle to the point of eruption. The loud bang that is heard it similar to that of a gun going off. The high pressure gasses escaping through a small hole it the bottle create the loud bang. Variations: You may choose to ask the audience what will happen when the liquid nitrogen is poured into the bottle. After getting several answers, explain that this is why scientists perform experiments and approach the demo as a first time experiment. The Physics: The physics of this demo is the same as
the cannon above. The liquid nitrogen in the bottle boils and begins to warm. Since the
volume of the bottle is fixed, the pressure inside of the bottle increases until the
bottle explodes. See Ideal Gas Law above. Racquetball in Liquid Nitrogen Needed: Liquid nitrogen Tongs Warnings: See warnings for liquid nitrogen. When the racquetball shatters, pieces may travel a long distance. Be sure to throw it up in a safe place. Make sure that members of the audience do not pick up broken pieces of the racquetball. Always use glove to handle the racquetball after it has been in the liquid nitrogen. What to do: Show the audience that both racquetballs bounce normally and then place one of them into the liquid nitrogen. When the liquid nitrogen has stopped boiling, remove the racquetball with the tongs and hold it with the glove. Throw both balls up in the air. The one at room temperature will bounce and the other one will shatter into many pieces. What to Say: The racquetball is a solid, but the bonds are loose enough for the ball to flex and bounce. When the ball is made very cold, the bonds are much tighter; therefore, when the ball hits the floor, instead of flexing and absorbing the force, the ball shatters. You may want to refer back the human molecule demo to show how the bonds of a solid are rigid. Variations: This demo can be done with many different substances. For example: flowers, grapes, tygon tubing, ping pong balls (hit with hammer). The Physics: There are many different kinds of bonds
that hold molecules together. The stronger of these bonds are what hold solids together.
However, there are different levels of bonds. Obviously, the bonds inside of steel are
stronger than the ones inside a racquetball. The bonds in a racquetball allow it to flex.
When a racquetball is dropped under the influence of gravity, it gains some kinetic
energy. Under normal conditions, this energy is absorbed by the deformation, or flexing,
of the racquetball when it comes in contact with the floor. The racquetball then returns
to its original shape returning the energy back to kinetic energy. This is what causes it
to bounce. When the racquetball is made very cold, the bonds become much stronger and the
racquetball looses its flexibility. Because the ball can no longer deform to absorb the
energy, the energy of ball act directly on the bonds; thus, breaking them. A Banana as a Hammer Needed: A banana at room temperature Warnings: Both the frozen banana and the liquid nitrogen banana are very cold; therefore, use a glove when holding either of them. When the liquid nitrogen banana shatters, make sure that none of the audience members pick up the pieces. What to do: First try to hammer the nail into the board with the banana at room temperature. After an unsuccessful and very messy attempt, try it again with the banana that was in the liquid nitrogen. This attempt should also fail because the banana will shatter. Finally, use the frozen banana and you should be able to hammer in the nail. What to Say: This is a great demo for the aspiring actor/actress. Feel free to ham it up! Ask the audience it they think that you can hammer in a nail with a banana. You can take a vote and act surprised when few think that it is possible. After the first unsuccessful attempt, remark that the banana was too soft and suggest using a banana that is much colder. After the second attempt fails, explain that the very cold banana was much harder, but that it became very brittle(see racquetball demo). When the third attempt is successful, explain that the frozen banana was cold enough to be hard, but warm enough not to be too brittle. Variations: This demo can be very entertaining when done by two people. Person A may ask Person B to do him/her a favor by hammering in a nail. After Person B agrees, Person A gives Person B the banana to use as a hammer. Although Person B is doubtful, he/she tries it anyway. Person A then uses the explanation above as an excuse to why the banana does not work each time. The Physics: Although a banana seems to have the
characteristics of a solid, it also contains a lot of water. By cooling the banana below
the freezing point of water, we can turn all of the bonds within the banana to solid
bonds. This makes the banana rigid enough to hammer in a nail. Even though the frozen
banana seems very hard, it still "flexes" some to absorb some of the energy.
When the banana is made very cold, the bonds become so tight that the banana no longer
absorbs the energy and it will break upon impact. This is why tall skyscrapers are made to
flex or sway in the wind (The Sears Tower can sway as much as several feet at the top). By
swaying, the building can absorb the energy that is imparted to it by the wind; keeping it
from crumbling. Soap Suds Eruption Needed: Bucket of warm water Warnings: This demo uses a very large quantity of liquid nitrogen; be sure to read all of the warnings for liquid nitrogen. Make sure that the audience is a safe distance from the spot of the eruption. The best thing to do is have a barrier between the surface of the eruption and the audience. As mentioned before, liquid nitrogen travels a long distance when it is poured out on a flat surface. The person pouring out the liquid nitrogen should wear a face shield or some sort of eye protection. What to do: Place the liquid dish soap into the bucket of warm water. Only enough soap to change the color of the water is necessary. More soap will not enhance the effect. Pour the liquid nitrogen quickly and directly into the bucket of water. The faster the liquid nitrogen is poured, the bigger the eruption will be. Use about a 1 to 1 ratio of water to liquid nitrogen. What to Say: When the liquid nitrogen is poured into the warm water it boils very rapidly turning into a gas. The gas expands and carries along with it the soapy water. This creates a huge cloud of bubbles. Variations: This demo can be done by many members of the group. The member who is going to pour the liquid nitrogen will add a little soap to the water and step forward to explain what is going to happen when he/she pours in the liquid nitrogen. As he/she does this, other members of the group pour more soap into the bucket. Since only about 1/8 of the soap in the bottle is needed, the effect can be heightened by filling the other 7/8 of the bottle with water; so it appears as though the entire bottle of soap is being added. Other members of the group can shield the speaker from seeing what is taking place, but be careful not to obstruct the view of the audience. When all the soap has been added and the person who is pouring is ready to go, all the remaining members of the group run as far away as they can. The Physics: Gas molecules of a given substance take
up much more space than the corresponding liquid molecules for the same substance. The
density of a substance is defined by the mass of the substance divided by the volume that
the substance occupies. The density of liquid nitrogen is approximately 1000 kilograms per
meter cubed. However, the density of nitrogen gas at room temperature is approximately 1
kilogram per meter cubed (these are ruff estimates, but order of magnitude is accurate).
Therefore if we pour about one kilogram of liquid nitrogen into a bucket of warm water,
the liquid nitrogen is going to boil and turn rapidly into a gas. One kilogram of liquid
nitrogen will easily fit inside a bucket; however, if we look at the density of nitrogen
gas, we see that it is going to take up much more space. By comparison, the nitrogen gas
will take up 1000 times the amount of space that the liquid nitrogen did. This rapid
expansion causes the water in the bucket to be carried along with the nitrogen gas and the
soap bubbles provide a great visual effect of the size of the gas. Inertia What to say: Explain that inertia is one of Newton's laws. Newton explained it as, "things at rest tend to stay at rest, things in motion tend to stay in motion unless acted upon by and outside force." This is surprisingly a very good and simple explanation. The Physics: Inertia is how much an object will
resist change. By applying a force to an object, we are causing a change. Its inertia, or
resistance to that change, is proportional to its mass. Newton's second law describes
inertia as F=ma or force equals mass times acceleration. The mass of an object remains
fixed; therefore, when we apply a force to something, its acceleration is increased by a
factor of 1/m. The greater the mass of a object, m, is the less the acceleration of that
object will be increased. This principle can be seen in the following demo. Tablecloth and Dishes Needed: Silk(slippery) tablecloth Warnings: Don't perform this demo to close to the audience in case you do break a dish(it happens). If you should happen to break a dish, make sure to clean up the glass very carefully and thoroughly. What to do: Place the tablecloth flat(no wrinkles) on the table and place several plates on top of it. Grab the tablecloth near the edge of the table and pull quickly DOWN. Don't pull the tablecloth straight out or up. What to say: Explain that inertia(explained above) is dependent on mass, i.e. weight. The plates weigh more than the tablecloth; therefore, they have more inertia. Since they have more inertia, it is harder to get them to move. The tablecloth, which is much lighter, will move very easily while the dishes remain where they are. Variations: You can pull the tablecloth slowly the
first time to show that the dishes do move. On the second try pull the tablecloth faster,
but slow enough for the dishes to move again. By this point you are worried that it might
not work. On the third and final try, have the audience count to three and pull out the
tablecloth. Action-Reaction What to say: You can begin this part of the show by talking about Newton and his "law" of action-reaction. "For every action, there is an opposite and equal reaction." Site examples of action reaction. For example: rowing a boat, the space shuttle taking off. Point out that in every case the action and reaction are in opposite directions. The Physics: As stated above, for every action there
is an opposite and equal reaction. "Opposite" just means that the two forces are
in opposite directions and "equal" means that the magnitude of the two forces
are equal. Also, it is important to note that the two forces always act on different
objects. With inertia(see above) we saw that the force on an object is equal to its mass
times its acceleration (F=ma). Therefore, something as heavy as the space shuttle can be
accelerated, or lifted off the ground, by a gas. Even though the gas is much lighter than
the space shuttle; because the gas is accelerated at a very high rate, the space shuttle
will be accelerated at a slower rate in the opposite direction. In other words, the mass
of the shuttle times its acceleration is equal to the mass of the escaping gas times its
acceleration. This principle holds for the following two demos. Balloon Warnings: None. What to do: Blow up the balloon, but do not tie it. Simply let the balloon go and let it fly around the room. Long balloons will travel a long way in one direction if you can get them pretty straight before you let them go. What to say: Explain that the air in the balloon is
under pressure, which means that it wants to get out. The air can only escape in one
direction (out the end ); therefore, the balloon will go in the opposite direction. Fire Extinguisher Cart Needed: Fire extinguisher cart (A small cart capable of carrying a person with at fire extinguisher attached to it) Warnings: Do not point the fire extinguisher at anyone. The fire extinguisher may have dust particles in it and it will also kick up a lot of dust. Replace the safety pin into the fire extinguisher when you are finished with the demo. Make sure that at least two people are prepared to stop the cart. What to do: Point the cart in a safe direction, sit on the cart and squeeze the handle on the fire extinguisher. Only keep t he fire extinguisher open for about half of the trip so that you can stop safely. What to say: This demo uses the same principle as
the balloon and even the space shuttle. The fire extinguisher contains a lot of gas under
pressure. When you squeeze the handle you let out a lot of gas in one direction;
therefore, you and the cart will move in the opposite direction. Pressure and Vacuum Magdeburg Hemispheres Needed: Magdeburg hemispheres Warnings: None. What to do: Show the audience the two hemispheres so that they can see that there is nothing to hold them together. Place them together and pull them back apart. Now place them together and pump out all of the air. Have two members of the audience try to pull them apart. They will be unable to do it. Next, have two demonstrators try. They will also be unsuccessful. Now, have a third demonstrator open the valve while the two demonstrators are still pulling. The hemispheres will come apart and the demonstrators will fall down. What to say: Explain that there is air pressure all around us. We are used to living in a world with air pressure. When the hemispheres were originally placed together, there was air on the inside and on the outside. The air pressure was equally pushing in and out at the same time. But when the air is pumped out of the middle of the sphere, there is no longer any air inside to push out. The air on t he outside continues to push in and keeps the spheres together. When the valve is opened, air gets into the middle of the sphere and once again pushes out on the two hemispheres making it easy to pull them apart. The Physics: This also deals with the Ideal Gas Law
(PV=nRT). The "n" in the equation stands for the number of molecules in the gas.
When the vacuum pump is connected to the hemispheres, most of the gas molecules are
removed. Since the volume of the sphere remains the same, the pressure will decrease to
keep the equation true. Although we never remove all of the air from inside the sphere, we
can approximate the pressure inside the sphere as zero. Since we want only the force
required to pull the hemispheres apart, we need only calculate the pressure on the sphere
as seen from one direction. Looking at the sphere, we see a circle of the same radius of
the sphere. The force holding the hemispheres together can be calculated by: (2
hemispheres)X(apparent area per hemisphere)X(atmospheric pressure). If we have a sphere of
radius 5 cm and the atmospheric pressure is about 1 atm, then the force required to pull
them apart would be (2)X(3.14 X .0025 m^2)X (1.013X10^5 N/m^2)= 1,592 Newtons or about 358
lbs. Vortex Generator Needed: Vortex Generator Warnings: See warnings for liquid nitrogen. What to do: There are several things that one can do with the vortex generator . You can start by having the audience attempt to blow out a lit candle by all blowing at the same time. Hold the candle off to one side so that you do not blow out the candle yourself while talking. After several seconds of the audience trying to blow out the candle, explain why they were unsuccessful (see "what to say"). Then use the vortex generator to blow out the candle. Because you cannot see the air coming out of the vortex generator, use the mylar sheet to see where the vortex of air is hitting. It still may be difficult to blow out the candle, so; you can place a pan of water inside the vortex generator and pour some liquid nitrogen into the pan. The liquid nitrogen will evaporate, but it will still be cold enough to condense some of the water molecules in the air. This will create a water vapor in the air that is visible. When you hit the vortex generator, the air that comes of out of it will carry with it some of the water vapor and make the vortex rings visible. For smaller audiences, you can "shoot" some of the rings into the audience so that they can try to catch them. What to say: Air is a fluid, therefore; it flows like a liquid. You can refer back to the "human molecule" model. If you were to push on one of the persons while they were a gas, he/she would not get that far because he/she would run into another one of the human molecules and be bounced in another direction. That is what actually occurs on the molecular level. When you blow air out of your mouth, the molecules that you push run into other molecules and get spread out into all directions. The vortex generator is different. As air leaves the hole in the vortex generator, some of the air curls around the lip of the hole. This curling effect creates a higher density of air in a ring or donut shape. Because of a higher density, the ring of air weighs more that the air around it and can be tossed across the room in the same way one can toss a baseball. The Physics: The actual physics behind this demo is
very complicated; it is easiest to gain an intuitive picture of what happens. Bernoulli Principle Needed: Electric leaf blower Warnings: None. What to do: This is a very simple demo to perform. Simply turn on the leaf blower and hold in an upright position. Place the ball in the stream of air. It should float in the air one to two feet above the blower. If the ball travels too high it will be difficult to keep it suspended, so you may want to use a heavier ball. However, if the ball is too heavy it will not float at all. What to say: The Bernoulli Principle is a difficult subject to understand; therefore, you may want to use another prop to help in the explanation. A cross section of an airplane wing or a Frisbee may be useful. Show the audience the wing or Frisbee and how it is curved on the top side and flat on the bottom. Explain that, as they each travel through the air, the air that goes over the top must travel faster than the air that travels under them. Bernoulli discovered that the air that travels faster has lower pressure. Lower pressure is just like a vacuum. Just as a vacuum cleaner sucks up dirt, the wing and Frisbee are sucked upward; thus, causing them to fly. The ball is like the curved surface of the Frisbee or the wing. As air travels around the ball, the ball is pulled equally in all directions causing it to float. Variations: As stated in the "what to say" section, there are many things that use the Bernoulli principle including airplanes and Frisbees. Also, a smaller version of the Bernoulli principle can be made at home. All you need is a straw, tape, and a ping pong ball or styrofoam ball. Fold one end of the straw and tape it so that no air can escape. Puncture a hole in the straw about one inch from the taped end. The hole should be large enough so that you can blow through the straw without too much trouble. Now, just blow through the straw while holding the ball above the hole. With a little practice, you should be able to float the ball above the straw until you run out of breath. The Physics: Bernoulli's equation as applied to a
non-viscous, incompressible fluid in steady flow is given by: (P+1/2 pv^2+pgy=c) Where P=
pressure, p= density of fluid, v= velocity of fluid, g= acceleration of gravity and y=
height of fluid. We can see by the equation that the pressure and the velocity of the
fluid are on the same side of the equation; therefore, if the velocity increases, the
pressure must decrease to keep the equation true. Electricity Charges Needed: Two hats that are the same color Warnings: None. What to do: Have two other participants in the show help you with this demo. First make them both the same kind of charge. This is done by placing a hat of the same color on each of their heads. Attempt to hold them together, but they will be able to separate and run to opposite sides of the room. Go to one of the "charges" and replace his/her hat with one of a different color. The two different charges will now run toward one another and "dance" around together. What to say: Say that there are two distinct types
of charges. Most scientists refer to them as positive and negative, but it really does not
matter what you call them, as long as you know that there are two different types. The two
differing colors of hats represent this distinction. As seen from the demo, two charges of
the same type do not like to be next to one another. But, two unlike charges like to get
close to one another. Van de Graaff Generator Needed: Van de Graaff Generator Warnings: Be careful t o ground the large sphere after each time that it is used. This is done by touching it with the small sphere or the ground wire. Although the Van de Graaff generator produces a very low current, it may cause problems with people who have heart problems or a pace maker. Get parent's or guardian's permission before allowing minors to hold the generator while it is running. What to do: First of all, simply turn the generator on. If it is possible, have someone turn off the lights so that the audience can see the sparks more clearly. Also, for large audiences, move the small sphere around in different positions so that everyone can see the sparks. As the charges are jumping from one sphere to the other, hold a fluorescent light bulb next to them. Each time a spark jumps across, the bulb will light up. Next, move the small sphere away from the larger one and place a pan full of styrofoam peanuts on top of t he large sphere. Finally, have someone hold on to the large sphere will insulated from the ground. This can be accomplished by standing on a wooden stool or rubber mat. What to say: The Van de Graaff generator separates charges into the two different types. One type is collected on the large sphere and the other on the smaller sphere. Because different types of charges attract one another. The charges will jump from one sphere to the other. When a pan of styrofoam peanuts is place on the large sphere, each peanut gains the same type of charge. Because like charges repel, like to get away from each other, the peanuts will fly out of the pan trying to get away from one another. When a person takes hold of the large sphere of the Van de Graaff generator they become covered with a lot of the same type of charge, which means that each hair on the individuals head gains the same type of charge. Their hair is light enough to rise, increasing the distance between each hair. Variations: Many different things can be placed on the Van de Graaff generator. When a piece of fur is placed on the large sphere, the individual hairs will stand up. Basically, anything with light strands will work. Also, you can have someone hold onto the large sphere and blow some soap bubbles. The soap bubbles will gain a positive charge and they will be attracted to anything that is grounded, i.e. a person standing nearby. The Physics: Inside the Van de Graaff generator is a
moving belt which collects charge from a grounded source and deposits it on the large
sphere. The large sphere is a hollow conductor and, in principle, all of the charge of a
conductor is transferable to a hollow conductor. This process can reach potential
differences high enough to "breakdown" the air between the two spheres allowing
discharge from one sphere to the other. The breakdown voltage of air is equal to 3 x 10^6
V/m; therefore, a sphere 1 m in radius can reach a potential difference of 3 x 10^6 V. Plasma Ball Needed: Plasma Ball Warnings: The plasma ball is usually harmless, but I would still recommend warning those with any type of heart condition. What to do: Place your hand on different spots of the sphere. It will collect more "sparks" than the rest of the sphere. Also, place the fluorescent bulb next to the sphere. It should light up. By holding the bulb in different places, you can show that the electricity is actually moving from the sphere to you. If you hold the bulb in the middle, only half of it will light. On the other hand, if you hold the bulb at one end, the entire bulb should light up. What to say: The plasma ball is the same concept as the Van de Graaff generator. Instead o f discharging from a large sphere to a smaller one, the plasma ball discharges electricity from a small center ball to a larger surrounding sphere. The sphere is filled with a gas that emits light of different colors as electricity is passed through it. The Physics: The plasma ball uses fluorescence to
produce the different colors of light. Fluorescence occurs as electrons pass through and
collide with the atoms of the gas inside of the sphere. As a result of the collisions,
some of the atoms get raised to a higher energy level. Very quickly, usually about 10^-8
seconds, the excited atoms will return to their normal energy states. This process
involves the emission of a photon. Since the energy levels of a particular substance are
very quantified, the photon emitted will have a particular energy. This particular energy
produces light of a specific wavelength and thus a specific color. Einstein showed that
photons of energy E will have a frequency, f, equal to E/h. Where h is Planck's constant
(6.63 x 10^-34). The wavelength of the light can be found by wavelength = speed of light/
frequency. Light and Sound Laser Needed: Hand held Laser Warnings: Never shine a laser where it could be viewed directly with the eye. What to do: Stand close to a wall or screen and shine both the laser and the flashlight on it. Back away from the wall or screen, keeping the laser light and flashlight pointed at the wall. Next, show that light travels in a straight line by hitting some erasers together or sprinkling baby powder over the laser beam. This needs to be done in a dark room. What to say: Ordinary light spreads out and becomes dimmer as you back away from the wall, but the laser remains in a tight beam no matter how far you get from the wall. Explain that the laser light hits the dust particles of the chalk or powder and bounces off in all directions making it possible for us to see the beam. The Physics: The word "Laser" is an
acronym for Light Amplification by Stimulated Emission of Radiation. Stimulated emission
is produced by electromagnetic radiation incident on a material such as ruby, helium-neon,
argon and krypton. To create a laser you must first achieve population inversion which is
the act of gaining a majority of atoms in an excited state. The material is then place in
an optical cavity which selects only certain natural frequencies and emits light in a
single direction. The light emitted from a laser is said to be coherent which is to say
that all of its photons have the same frequency, phase and direction. The Dancing Spot Needed: Keyboard Warnings: See warnings for laser. What to do: Shine the laser of the mirror on the speaker to a place on a wall or screen where it can easily be viewed by the audience. This may take some time therefore it should be done ahead of time. Play several different notes, some low and some high, on the keyboard. Then play some different styles of music. Most keyboards will have different styles of music already programmed into them. What to say: Explain that sound is created by movement. You can have the members of the audience put their hand on their throats as they hum. They will be able to feel their throat moving. Or ask if they have ever felt a speaker while it is playing music. Then explain that we have a speaker with a mirror attached to it. Then we shine a laser on the mirror. Show them the laser spot on the wall. Explain that the type of sound we hear depends on the frequency of the sound, or how fast it moves. Low notes move slow, but far, therefore; the spot gets very big. High notes move very fast, but do not get that far, so; the spot stays pretty small.
(Please note that this page describes a dated version of our show and is here for archival and reference purposes only. Please use caution when performing any of these demonstrations and be aware that not all of the information provided here is necessarily complete. Any demonstrations with warnings listed should be performed by children only under adult supervision. For our current list of demos, with more thorough descriptions, click on the "Demos" link at the bottom of this page.)
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