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Special thanks to John M. Zavisa for contributing this article!
a close encounter with a Van de Graaff generator! |
Understanding Static Electricity
To understand the Van de Graaff generator and how it works, you need to understand static electricity.
Almost all of us are familiar with static electricity because we can see and feel it in the winter. On dry
winter days, static electricity can build up in our bodies and cause a spark to jump from our bodies to
pieces of metal or other people's bodies. We can see, feel and hear the sound of the spark when it jumps.
In science class you may have also done some experiments with static electricity. For example, if you rub a glass rod with a silk cloth or if you rub a piece of amber with wool, the glass and amber will develop a static charge that can attract small bits of paper or plastic. [fun fact: the word "electron" in English comes from the Greek word for amber!]
To understand what is happening when your body or a glass rod develops a static charge, you need to think about the atoms that make up everything we can see. All matter is made up of atoms, which are themselves made up of charged particles. Atoms have a nucleus consisting of neutrons and protons. They also have a surrounding "shell" which is made up electrons. Typically matter is neutrally charged, meaning that the number of electrons and protons are the same. If an atom has more electrons than protons, it is negatively charged. Likewise, if it has more protons than electrons, it is positively charged. Some atoms hold on to their electrons more tightly than others do. How strongly matter holds on to its electrons determines its place in the Triboelectric Series. If a material is more apt to give up electrons when in contact with another material, it is more positive on the Triboelectric Series. If a material is more apt to "capture" electrons when in contact with another material, it is more negative on the Triboelectric Series.
The following table shows you the Triboelectric Series for many materials you find around the house. Positive items in the series are at the top, and negative items are at the bottom:
The relative position of two substances in the Triboelectric series tells you how they will act when brought into contact. Glass rubbed by silk causes a charge separation because they are several positions apart in the table. The same applies for amber and wool. The farther the separation in the table, the greater the effect.
When two non-conducting materials come into contact with each other, a chemical bond, known as adhesion, is formed between the two materials. Depending on the triboelectric properties of the materials, one material may "capture" some of the electrons from the other material. If the two materials are now separated from each other, a charge imbalance will occur. The material that captured the electron is now negatively charged and the material that lost an electron is now positively charged. This charge imbalance is where "static electricity" comes from. The term "static" electricity is deceptive, because it implies "no motion", when in reality it is very common and necessary for charge imbalances to flow. The spark you feel when you touch a doorknob is an example of such flow.
You may wonder why you don't see sparks every time you lift a piece of paper from your desk. The amount of charge is dependent on the materials involved and the amount of surface area that is connecting them. Many surfaces, when viewed with a magnifying device, appear rough or jagged. If these surfaces were flattened to allow for more surface contact to occur, the charge (voltage) would most definitely increase. Another important factor in electrostatics is humidity. If it is very humid, the charge imbalance will not remain for a useful amount of time. Remember that humidity is the measure of moisture in the air. If the humidity is high, the moisture coats the surface of the material providing a low-resistance path for electron flow. This path allows the charges to "recombine" and thus neutralize the charge imbalance. Likewise, if it is very dry, a charge can build up to extraordinary levels, up to tens of thousands of volts!
Think about the shock you get on a dry winter day. Depending on the type of sole your shoes have and the material of the floor you walk on, you can build up enough voltage to cause the charge to jump to the doorknob, thus leaving you neutral. You may remember the old "Static Cling" commercial. Clothes in the dryer build up an electrostatic charge. The dryer provides a low moisture environment that rotates, allowing the clothes to continually contact and separate from each other. The charge can easily be high enough to cause the material to attract and "stick" to oppositely charged surfaces (your body or other clothes in this case). One method you could use to remove the "static" would be to lightly mist the clothes with some water. Here again, the water allows the charge to leak away, thus leaving the material neutral.
It should be noted that when dirt is in the air, the air will break down much more easily in an electric field. This means that the dirt allows the air to become ionized more easily. Ionized air is actually air that has been stripped of its electrons. When this occurs, it is said to be plasma, which is a pretty good conductor. Generally speaking, adding impurities to air improves its conductivity. You should now realize that having impurities in the air has the same effect as having moisture in the air. Neither condition is at all desirable for electrostatics. The presence of these impurities in the air, usually means that they are also on the materials you are using. The air conditions are a good gauge for your material conditions, the materials will generally break down like air, only much sooner.
[Note: Do not make the mistake of thinking that electrostatic charges are caused by friction. Many assume this to be true. Rubbing a balloon on your head or dragging your feet on the carpet will build up a charge. Electrostatics and friction are related in that they both are products of adhesion as discussed above. Rubbing materials together can increase the electrostatic charge because more surface area is being contacted, but friction itself has nothing to do with the electrostatic charge.]
Understanding the Van de Graaff Generator
Now that you understand something about electrostatics and static electricity, it is easy to understand
the purpose of the Van de Graaff generator. A Van de Graaff generator is a device designed to create static electricity and
make it available for experimentation.
The American physicist Robert Jemison Van de Graaff invented the Van de Graaff Generator in 1931. The device that bears his name has ability to produce extremely high voltages - as high as 20 million volts. Van de Graaff invented the Van de Graaff Generator to supply the high energy needed for early particle accelerators. These accelerators were known as atom smashers because they accelerated sub-atomic particles to very high speeds and then "smashed" them into the target atoms. The resulting collisions create other subatomic particles and high-energy radiation such as X-rays. The ability to create these high-energy collisions is the foundation of particle and nuclear physics.
Van de Graaff Generators are described as "constant current" electrostatic devices. When you put a load on a Van de Graaff generator, the current (amperage) remains the same. It's the voltage that varies with the load. In the case of the Van de Graaff generator, as you approach the output terminal (sphere) with a grounded object, the voltage will decrease, but as stated above, the current will remain the same. Conversely, batteries are known as "constant voltage" devices because when you put a load on them, the voltage remains the same. A good example is your car battery. A fully charged car battery will produce about 12.75 volts. If you turn on your headlights and then check your battery voltage, you will see that it remains relatively unchanged (providing your battery is healthy). At the same time, the current will vary with the load. For example, your headlights may require 10 amps, but your windshield wipers may only require 4 amps. Regardless of which one you turn on, the voltage will remain the same.
There are two types of Van de Graaff generators, one that uses a high voltage power supply for charging and one that uses belts and rollers for charging. Here we will discuss the belts and rollers type.
Van de Graaff generators are made up of a motor, two rollers, a belt, two brush assemblies, and an output terminal (usually a metal or aluminum sphere), as shown in the figure:
When the motor is turned on, the lower roller (charger) begins turning the belt. Since the belt is made of rubber and the lower roller is covered in silicon tape, the lower roller begins to build a negative charge and the belt builds a positive charge. By now, you should understand why this charge imbalance occurs, but you may want to look at the Triboelectric Series again. Silicon is more negative than rubber; therefore, the lower roller is capturing electrons from the belt as it passes over the roller. It is important to realize that the charge on the roller is much more concentrated than the charge on the belt. Because of this concentration of charge, the roller's electric field is much stronger than the belt's at the location of the roller and lower brush assembly. The strong negative charge from the roller now begins to do two things:
You should now understand that as long as there is air between the lower roller and brush assembly, the Van de Graaff generator will continue to charge the belt. Theoretically, the Van de Graaff generator can continue to charge forever. Unfortunately, dirt and other impurities in the surroundings will limit the actual charge that develops on the sphere.
Let's return to the belt. The belt, as we left it, is positively charged and rolling toward the upper roller and upper brush assembly. Since I used nylon for my upper roller, it wants to repel the charge on the belt. The upper brush assembly is connected to the inside of the sphere and hangs near the upper roller and belt location. The electrons in the brush move to the tips of the wires, because they are attracted to the positively charged belt. Once the air breaks down as before, the positive atomic nuclei of air are attracted to the brush. At the same time, the free electrons in the air move to the belt. When a charged object touches the inside of a metal container, the container will take all of the charge, leaving the object neutral. The excess charge then shows up on the outside surface of the container. Here our container is the sphere. It is through this effect that the Van de Graaff generator is able to achieve its huge voltages (over one million volts). For the Van de Graaff generator, the belt is the charged object delivering a continuous positive charge to the sphere.
One last note before going on to actual construction tips. Normally, a neutral material is used for the upper roller; thus the belt becomes neutral after the sphere sucks its excess charge away. Because I used a nylon upper roller (which is positive on the Triboelectric Series), I cause the belt to actually deliver more positive charge and actually become negative. This is a technique used for doubling your current. The belt is positive on one side as it approaches the upper roller and negative on the other side as it approaches the lower roller.
Building Your Own Van de Graaff Generator
If you are mechanically adept it is easy to build your own Van de Graaff generator from scratch (if not, you may wish to buy a kit or a finished generator - see the third link in the links section below for some ideas). The following is a list of the parts and materials I used to build my Van de Graaff generator.
Belt: The surgical tubing that I use performs flawlessly. It's extremely durable, easy to keep clean (wipe with rubbing alcohol), and easy to work with. Purchase the tubing at a good hardware store or a medical supply store. You will then need to cut the tubing to make a plat strip. Do this with scissors or by inflating the tubing. To form a belt, form the strip into a circle and overlap the ends slightly. Cut a 45 degree angle thru the overlaped portion (cut all the way thru). Now butt the two ends together and bond with rubber glue. The 45 degree seam will help the belt to travel over the rollers when the seam reaches them. Remember that the belt must not be conductive. Avoid using any material that is black. It probably contains carbon, which is conductive at the high voltages a Van de Graaff generator develops. The belt width should be as close to the roller width as possible. You want to ensure that the brush is "coating" the belt and not losing charge to the roller.
Brushes: The brushes must be a conductive material like metal. I've found that the smaller and the sharper the brush tips, the better the performance. Try placing the brushes at various distances to the rollers. Do not allow the brushes to contact the belt. This will cause debris to build up and will ruin your belt. Unbraiding the fine wires in grounding strap wire works well and is recommended.
Motor: The motor is arguably the least critical aspect of the Van de Graaff generator. Obviously, you want one with enough horse power to drive the belt. Try looking at local motor repair shops. I even used a circular saw motor at one point. For the motor speed, I would not use anything less than 1000 rpm. The speed determines how fast a Van de Graaff generator charges up (do not confuse this with how much charge is built up).
Sphere: Any hollow metallic sphere will work fine. The two salad bowls that I use had some leakage where the rims came together. I remedied this by sealing the seam with epoxy and covering it with electrical tape.
General Construction
Mount the lower roller to the motor shaft. Mount the lower brush assembly to the motor housing. Enclose the lower unit. Do not use wood: Wood is easy to work with, but it does absorb moisture from the air, which can adversely effect the Van de Graaff generator. Make the case from plastic - Plexiglas from a hardware store works well. Remember to leave access to set the belt to your roller and to leave an opening at the top to route the belt to the top roller.
For the column assembly, I used a 6-inch diameter, 32-inch length piece of PVC tube. I mounted one end of the tube to the top of the housing and drilled holes in the other end of the tube. Now you can mount your upper roller to the top of the tube via a bolt or rod through the drilled holes. Depending on how your upper roller mounts to the tube, you probably want to put the belt on the roller before you mount it. After the top roller is mounted, you can then seat the other end of the belt to the lower roller and close your housing.
Finally you are ready to mount the sphere and upper brush assembly. To do this, I cut a hole in the bottom of one of the salad bowls. I then used conductive "metal bond" to secure the braided grounding strap to the inside of the bowls. Next, the bowls were mounted to a 6-inch to 4-inch PVC reducer. The 4-inch end of the reducer was inserted into the hole in the bowl and then coated with silicon caulk. The brush end of the grounding strap was then routed to the inside of the reducer and mounted (you may have to play with this a bit in order to get the best separation distance from the upper roller assembly). At last, all you have to do now is put the reducer over the top of the PVC. Make sure that the brush is facing the belt and on the same side as the lower brush. Now you have your very own Van de Graaff generator!
It's a good idea to ground a piece of wire to the motor housing, because you can then touch the other end of the wire to the sphere when you turn it off. This will keep you from getting a nasty little pop when you touch the switch. Also, you may want to discharge the sphere without turning it off. Keep in mind though, if you do not hold the end of the wire during operation, you will get a little pop when you pick it up.
There are millions of interesting experiments you can perform with your new Van de Graaff generator, but I will concentrate on the "hair raising" one. Have the lucky participant stand on top of an insulated surface (a Rubbermaid container top works well). It is critical for the person to be insulated from ground. If the charge can not build up on the person, his/her hair will not stand up. Now, have the person put a hand on the sphere. Turn on the Van de Graaff generator and watch it go! When the Van de Graaff generator starts charging, it transfers the charge to the person who is touching it. Since their hair follicles are getting charged to the same potential, they try to repel each other. This is why the hair actually stands up. It would not make a difference if the polarity of the Van de Graaff generator was reversed. As long as the person is insulated, the charge will build up; assuming of course, that the hair is clean and dry.
My Van de Graaff generator will create sparks about 10 - 12 inches in length. I like to charge myself on it and point at the aluminum blinds on the window. The charge (electronic wind) will then cause the blinds to actually move. I can do this from about 8 feet away with ease. Soap bubbles are also interesting to play with around the Van de Graaff generator. They initially are attracted to the Van de Graaff generator and float towards it, then they become charged by the Van de Graaff generator and float away due to repulsion. There are multitudes of fun things you can do with your Van de Graaff generator. Use your imagination.
Help: If your Van de Graaff generator does not seem to be charging properly, make sure that it is clean. Avoid oils or debris. You can also use a hair dryer on it to remove any moisture. I go through this ritual every time I want to use my Van de Graaff generator. You will be amazed at the difference it can make. You may want to turn off all of the lights and run the Van de Graaff generator in the dark. You will see bluish-purple sparks shooting out where ever you have leakage. Try to eliminate the leakage with tape, epoxy or silicon. It may even take combinations of the three, but it will be worth your while to do so.
Hopefully, you now have enough understanding to build your own Van de Graaff generator. I do not think you will be disappointed. ENJOY !!
Special thanks to John M. Zavisa for contributing this article. If you have questions you may contact him at Johnz99@hotmail.com.
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