Many people have seen or experienced a Van de Graaff generator even if they don’t know what it is called. People familiar with this device will probably remember a small metallic sphere “full” of static electricity that makes the hair of the one touching it stand on end. However, as one may suspect, it was not invented for the sole purpose of making hair stand up. There
was a reason for its invention and a science that made its existence possible.
The science of the Van de Graaff involves static electricity. Everyone has experienced it at some (probably many) points in their lives. Anyone that has shuffled their feet across carpet and zapped someone else has had a peek into this “shocking world.” Commercials trying to peddle their products have also warned us of the “evils” of static cling, another result of static electricity. On dry winter days it is quite easy to build up a charge in our bodies that can then be passed to someone else or a to conductor, say a doorknob. The same effect is
attainable by rubbing certain materials together. For instance, rubbing glass with silk or amber with wool creates static electricity. These materials will then attract or repel small bits of paper or plastic. We have all seen these sorts of effects produced by static electricity, but few are familiar with the mechanics of how it is created and why it behaves as it does.
To gain a reasonable understanding of how static electricity operates and is generated requires a knowledge of the atom itself. All matter is made of atoms which are also made up of other charged particles. At the center of all atoms, known as the nucleus, is a bundle of one or more protons and zero or more neutrons. A “shell” of electrons surrounds the nucleus. Electrons have a negative charge and protons have a positive charge (neutrons have no charge.) Consequently, the charges associated with protons and electrons are equal in magnitude but opposite in direction. So, in a neutral atom, the number of protons and electrons is equal; this is most commonly the case. When an atom is negatively charged, there are more electrons than protons and positively charged when there are more protons than electrons.
When an atom becomes charged it can only do so by gaining or losing electrons. The protons are bundled tightly together in the nucleus and remain there, at least in non-nuclear reactions. However, not all atoms hold their electrons with the same vigor. Some atoms have a very tight grip on their electrons while others hold on very loosely. The strength in which the atom holds its electrons in place determines its ranking in the triboelectric series. A material that is apt to give up electrons to another material it comes in contact with is more positive on the triboelectric series whereas a material more likely to take electrons is more negative. Many household items in the triboelectric series are as follows, the most positive at the top and most negative at the bottom:
- Human hands Very positive
- Rabbit fur
- Human hair
- Steel Neutral
- Hard rubber
- Saran wrap
- Scotch tape
- Teflon Very negative
The relative position of two materials in relation to one another on the triboelectric scale will indicate the manner in which they will react with one another. Glass that is rubbed by silk causes a transfer of charge because glass is more likely to give up electrons than is silk. Therefore, the glass transfers some of its electrons to the silk making the glass positively charged and the silk negative. The farther the separation in the table, the greater the effect between the two materials.
When non-conducting materials come in contact with one another a chemical bond known as adhesion occurs between them. Depending on the triboelectric properties of the substances involved, one material may capture some of the electrons from the other. When they are removed from one another, the “capturer” leaves with more negative charge and the “captured” leaves with less (making it positive.) This charge imbalance is what creates static electricity. Now, the substance with fewer electrons (the captured) is attracted to substances that are negatively charged, and even some things that are neutral by attracting the electrons in the neutral material leaving it “lopsided” as to charge as seen below:
The capturer is attracted to a neutral material because the positive charge attracts the electrons in the neutral material toward it. The neutral substance still retains the same number of protons and electrons.
The capturer and captured are now attracted to each other because they each have an opposite but equal charge imbalance. If allowed to touch, they would both become neutral again.
The negatively charged “capturer” does the same thing only by repelling the electrons in the neutral material. The charge transfer that occurs in such a situation creates what we call “static electricity.” Of course the word static does not mean lack of motion as it does in other instances. A “flow” is necessary for the transfer of electrons to occur. For example, when you touch your friend and shock them, The charge “flows” from your body to theirs.
This being the case it may seem inconsistent that sparks aren’t flying around things all the time. For instance, why is it that electricity doesn’t jump between a piece of paper and desk as the paper is removed? The reason is that the amount of charge is dependent on the materials involved and the amount of surface area that is contacted between them. Many materials viewed more closely, say with a microscope, appear rather jagged and bumpy. This reduces the surface area that may be contacted between objects.
If materials were actually “flat” they would be able to transfer much greater charges because there would be a great deal more contact between them. This would certainly increase the voltage by allowing more adhesion to occur.
Humidity is also of great importance in electrostatics because of the dipole nature of the water molecule. Water is polar with a positive and a negative side making it a great conductor. So, when static electricity is produced in the presence of much moisture it will quickly dissipate through the water molecules on the surface of the conducting material and into the air. By the same token the voltage on a surface can build to extraordinary levels when there is very little moisture. Imagine walking across the floor on a dry winter day. The charge that builds up can have enough potential to jump from your hand to the doorknob. The same thing happens in the dryer. Clothes are tumbled around together in a dry environment allowing a great deal of contact and therefore adhesion between clothes. When they are removed from the dryer they “stick” to other things that are of a different charge like other clothes or you. This is easily resolved with a mist of water, allowing the charge to “leak” away through the polar water molecules.
Dirt in the air is also of respectable importance when dealing with static electricity. This is because the air will break down more easily when dirt is present with it (in an electric field.) This is because dirt allows the air to become more easily ionized or stripped of its electrons. This condition causes the air to conduct very well because it makes the air molecules more positive than before and therefore more attracted to electrons. Most impurities in the air have the same effect and are less than convenient for dealing with electrostatics because some of it inevitably ends up on your conducting surface.
Now, with a proper understanding of the way static electricity is produced and behaves, it is easy to see that these generators are used to produce charge that can be used for experimental purposes. It was invented by a physicist named Robert Jemison Van de Graaff in 1931. It can produce up to 20 million volts supplying the high energy required for particle acceleration. By colliding particles together, scientists are able to analyze the subatomic particles created by such collisions. This is what became the foundation of nuclear physics.
In the generator the current remains the same, only the voltage changes. This is why they are often referred to as constant current electrostatic devices. If you approach a Van de Graaff output with a grounded object, the current remains constant but the voltage decreases the closer the object comes to it. Batteries are just the opposite. A 9-volt battery is referred to as such because it always has a constant voltage of 9V, only the current changes. In a car for instance, the voltage on your battery will read around 12.75 volts regardless of how many radios, wipers, or headlights are turned on.
There are actually two types of Van de Graaffs. One uses a high voltage power supply for charging while the other uses belts and rollers to accomplish the same purpose. The belts and rollers model will be the focus of this discussion. This type of generator is made of the following parts:
- 2 rollers
- 2 brush assembly
- Output terminal (typically a metal or aluminum sphere)
When the motor is turned on the lower roller begins turning the belt. Because the belt is made of rubber and the lower roller is covered with silicon tape, the lower roller begins to build a negative charge and the belt a positive one. Note the triboelectric series of rubber and silicon. Silicon is more negative than rubber and therefore captures electrons from the belt as it passes by. Also note that the charge on the roller is more concentrated as well because it has less surface area than the belt. This greater concentration creates a greater electric field around the roller and brush assembly than is created by the belt at the same location. This does two things:
1) It repels electrons near the tips of the lower brush assembly. Metals are good conductors because their electrons are so easily moveable. At this point the brush assembly has positive tips because the electrons have moved toward the connection at the motor housing.
2) It begins to strip nearby air molecules of their electrons. When electrons are stripped from an atom it is said to be plasma, matter’s fourth state. So there are positively charged atoms of air existing between the roller and the brush. The electrons are repelled by the roller, but attracted to the brush tips while the positive atoms are attracted to the (negatively charged) roller.
The positively charged air molecules move toward the negatively charged roller, however the belt is in the way. The belt then gets covered in positive charge that is then carried by the belt away from the roller. When there is air between the lower roller and brush assembly, the generator will continue to charge. The charge will continue to grow indefinitely until it is hindered at some point by air impurities such as dirt or humidity (which always exists to some extent.)
Now at this point, the positively charged belt is continuing to roll toward the upper roller and brush assembly. The upper roller (nylon) repels 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. The electrons in the brush are attracted to the belt and therefore move to the tips of the brush. The air then breaks down as before, then the positive air moves toward the tips of the brush. At the same time the free electrons in the air move toward the belt. Since a metal container will take the charge of an object that touches it, the charge shows up on the surface of the conductor, here it is the sphere. This is how it is able to hold such a large voltage, by being delivered a continuous positive charge.
The Van de Graaff has opened the door to a better understanding of the subatomic world. But perhaps more importantly, it has proved quite the crowd pleaser for possibly millions of children and adults alike with the inkling for their hair to stand straight up. What more could you ask for from science?
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Chester, Michael. Particles. New York: Macmillan Publishing Company, 1978
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Mann, Martin. Revolution in Electricity. New York: The Viking Press Incorporated. 1962.
Spruch, Grace Marmor; Spruch, Larry. The Ubiquitous Atom. New York: Charles Scribner’s Sons, 1974.