In order to understand the Van de Graaff, one must have a firm grasp on the physical concepts that make it possible. The Van de Graaff uses a principle called static electricity. Most of us have either heard of this or felt its effects in the form of a shock from a pestering sibling. Whether your sibling was pestering or not is irrelevant as I am speaking from my own experience ?. You can feel and see the effect on dry winter days. These days of low humidity allow for quite a charge to build from shuffling feet, and you can feel and see the shock created when you touch metal or another person.
You may also be familiar from experiments in science classes. You can attract small bits of plastic or paper by rubbing a piece of silk onto a tube of glass. To fully understand this, we must go to the atomic level. All materials are made out of atoms. These atoms are made of charged particles, with positive protons in the center, or nucleus, and outer shells of negatively charged electrons. Usually these atoms are neutral in charge, meaning that the amount of negative charges, the electrons, equals the amount of positive particles, the protons. If an electron is stripped from a neutral atom, there is more protons than electrons, and the particle is said to be positive. Likewise, if an additional electron is added to a neutral atoms, it is said to be negative.
Not all atoms are the same. Some hold more tightly to their electrons than others. This is quantified by the triboelectric scale or series. If a material readily gives up electrons, it is more positive on the triboelectric series. Also, if it is more apt to steal electrons, it is more negative. Here is a list of common house-hold materials and how they measure on the triboelectric series:
• Human hands (usually too moist, though) (Very positive)
• Rabbit Fur
• Human hair
• Steel (Neutral)
• Hard rubber
• Nickel, Copper
• Brass, Silver
• Gold, Platinum
• Styrene (Styrofoam)
• Saran Wrap
• Polyethylene (like Scotch Tape)
• Vinyl (PVC)
• Teflon (Very negative )
(List taken from Nature's Electricity, by Charles K. Adams.)
In order to tell how two substances will act when in contact with each other depends on their relative position on this scale. If two materials are several positions away from each other on the triboelectric series, there will be a charge separation. The greater the position displacement, the greater the separation. When two non-conducting materials come in contact, a chemical bond forms, and depending on the triboelectric properties of the materials, a charge imbalance occurs due to one material capturing an electron from another material. This imbalance is where the “static electricity”. Many scientists say that static is a deceptive term because static implies no motion, but in reality it is necessary for these imbalances to flow. The sparks you feel and see come from the flow of this charge.
There are several important factors in dealing with static electricity. One is the surface area of the materials involved. On the microscopic level, most materials, even seemingly flat materials like paper, are very jagged. The more surface area that is contacted, the greater the charge development. Another important factor is humidity. If it is too humid, the charge imbalance won’t last long enough to do anything with, and is therefore undesirable. Humidity is the amount of water vapor in the air. If humidity is high, water droplets will form on the surface of the material and will provide a low-resistance path for charges to leave the surface and recombine to neutralize their positive or negative charges. Obviously, in order to build up tens of thousands of volts, we wouldn’t want any charge to have a leak to neutralize through.
Back to a dry winter day, depending on the type of soles you have, and the material of the floor that you walk on, you can build up enough charge to have a spark jump to a doorknob, thus leaving you neutral again. Another everyday example of static electricity is “static cling” in a laundry dryer. In a dryer, the conditions are a very dry, very warm environment where clothes can repeatedly and continually contact each other and the metal dryer surfaces. This can build a charge in the clothes that will attract to oppositely charged particles, like your body or other clothes for instance. One method of correcting this is to lightly mist your clothes. Again this provides humidity that will allow the charge to leak and neutralize.
It is important to note that electrostatics are not caused
by friction as most people initially think. 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, which is the
chemical bond that we talked about earlier. Rubbing materials together can increase
the electrostatic charge because more surface area is being contacted, but friction
itself has nothing to do with the charge.