Tech Note 400 Detail

Van de Graaff Generator does not produce sparks.

Affected Products:

SF-9722 Van de Graaff Generator
SE-8691 Van de Graaff Generator, High Voltage

Problem/Symptom:
Van de Graaff Generator does not produce sparks.

PASCO Solution:

Theory

When two dissimilar materials in contact are separated, there will often be a preferential transfer of electrons from one material to the other, leaving the two materials with equal and opposite charges. If one or more of the surfaces is a good insulator (such as latex), the electric charge may get trapped in an electrical potential well, preventing the charge from rapidly recombining when the surfaces are separated. This is known as the triboelectric effect. With a Van de Graaff generator, triboelectric charging occurs when the inside of a belt separates from the lower felt-covered (SE-8691) or acrylic (SF-9722) pulley and starts up toward the dome. This charge is "trapped" on the belt and stays on the inside of the belt for anywhere from seconds to years (depending on the depth of the local electrostatic potential well). When that triboelectric charge is sufficient, the presence of the charge on the inside of the belt ionizes the air around the sharp points of both of the corona brushes. Electrons will then start to conduct from the grounded base to the outer surface of the belt, where they are mechanically transported by the belt to the upper corona brush. There the electrons jump conduct across the second path of ionized air to the upper sphere in an attempt to maximize their separation because of the mutual repulsion. They then continue to accumulate on the outside surface of the sphere until the sphere is discharged.

Troubleshooting

  1. Condensation On The Rollers

    In condensing conditions, the pulley or belt will absorb moisture over time and may need to be dried with a hair drier. Remove the base of the cabinet and lay the machine on its side. Remove the belt from the lower pulley and, in the case of the SE-8691, being careful not to melt the plastic, blow on the felt for fifteen to twenty minutes or, in the case of the SF-9722, on both the acrylic and teflon rollers and the belt for a couple of minutes.
  2. Condensation On The Belt

    The belt may have a small amount of moisture on its surface or absorbed into the light coating of talcum powder. It can often be revived by putting a small amount of fresh talcum powder in a plastic bag along with the belt. Shake it up for a minute or so to get it completely coated. Remove the belt and wipe it off with a clean dry paper towel.
  3. Contamination On the Belt and Rollers

    The SF-9722 is especially susceptible to surface contamination because the exposed belt and rollers electrostically attract microscopic airborne contaminants such as dust and volatile organics. We suggest wrapping the supports in saran wrap to prevent accumulation of dust and volatile organics on the belt and rollers. If this is not done, you will need to frequently wipe down the belt and rollers with a soft clean cloth wetted with isopropanol.
  4. Oxidation

    If the belt is more than 1 year old or has not been stored with a protective cover, the surface of the belt is probably oxidized. We suggest either scraping off the inner surface of the belt or replacing the belt with one that has been stored in a sealed package for less than 2 years.
  5. Misaligned Brushes

    Both the upper and lower brushes should be positioned with points or ends of the wires pointing directly at the belt with an air gap of 4-5 mm. Putting them closer will not improve performance, but does increase the chance of snagging the belt or scratching a pulley.
  6. Worn Pulleys

    After several years of use, it is possible that the felt on the SE-8691 has worn away from the lower pulley or that the coating on the upper pulley is embedded with grime and rubber particles. If this is the case, the pulleys should be replaced.

Creation Date: 05/1/2003
Last Modified: 05/21/2004
Mod Summary: