Lightning - Dramatic Electrostatics When

Lightning - Dramatic Electrostatics

When people are asked to name a force of nature, far more will refer to the force of gravity than to the electrostatic force. True, the force of gravity works over much larger distances than does the electrostatic force, but the fact that matter exists at all is directly due to the electrostatic force itself. Without it, electrons would not orbit the nucleus of an atom and chemical compounds would be impossible. Water exists solely due to the electrostatic attraction between hydrogen and oxygen atoms, similarly for every other chemical compound in the universe. Without the electrostatic force, the universe would exist (if at all) as only an undifferentiated swarm of subatomic particles whose interactions would be entirely random.

Nevertheless, all it takes is a stumble on rocky ground to convince one of the power of gravity, whereas electrostatic phenomena are usually far less dramatic - a shock gotten from touching a doorknob in winter, for instance.

Electrostatic phenomena have been the subject of human curiosity and comment for ages. The ever-observant ancient Greeks knew that under certain circumstances, pieces of amber could influence small pieces of fabric or dust. But it was Franklin's 18th century efforts to understand electricity that should be considered the birth of real scientific inquiry into the field of electrostatics, because it was Franklin who first seems to have identified the positive/negative nature of electricity and who invented the lightning rod, after rightly understanding what it was about rooftops and church steeples that lightning seemed to favor (Jefimenko, 1973).

Although lightning can even strike on a sunny day (Wikipedia, 2005), most terrestrial lightning is generated through two interactive processes: triboelectricity and electrostatic induction.

The triboelectric effect is the tendency of objects of dissimilar materials to accumulate electric charges when put in contact with each other and depends on the relative tendency of these materials to either accumulate or give up electrons (though even identical materials will tend to develop small differences in charge if rubbed together). Materials' relative tendency to do this has been plotted on what is called the Triboelectric Series (Noon, 1992). Polyvinyl chloride (PVC) tends to accumulate electrons and so is toward one end of the series, while glass tends to give up electrons and is positioned toward the other end. If glass and PVC are brought into contact with each other, the PVC will become negatively charged and the glass positively charged. This is the basis of many electrostatic generators, such as the Van de Graaff generator.

In a thunder cloud, the two dissimilar materials are ice and liquid water. Most thunderclouds' bottoms are at about 5km above Earth's surface, where it is usually sufficiently cold to freeze any liquid water. Powerful winds circulate up and down the interior of a thunderstorm, bearing up small drops of freezing water which collide with descending hail stones. Even though these two objects are made from water, the fact that one is large and solidly frozen while one is small and only partially frozen means that they occupy different positions on the tribolectric scale. The hail stones tend to become negatively charged and the smaller particles positively so. The negative charge accumulates at the base of the thunderstorm, and the positive at its top. In essence, thunderstorms act like immense Van de Graaff generators.

The induction effect of static electricity is what allows capacitors to work. Simply put, a charge of any given value on one object will induce an opposite charge on the adjacent surface of any nearby object. Imagine two metal spheres, separated from each other by a small distance. If we charge sphere a negatively, sphere B. will develop a positive charge on the hemisphere adjacent to sphere a. In fact, the charges on both spheres will be concentrated on the spheres' adjacent hemispheres. Negative charge will be concentrated on a's side facing B, and positive charge concentrated on B's side facing a. The degree to which sphere a can be negatively charged without a spark leaping from a to B (equalizing the charges on both spheres) is a function of the distance between the two spheres, the surface areas of the two spheres, and the dielectric constant (the tendency of a material to resist the flow of electricity through it) of the material intervening between them. In our example the material is air and it is the baseline against which all other substances' dielectric constants are scaled, so the dielectric constant of air is equal to 1 as in Table 1 (Ford, 1991). The general formula expressing this function is C = 0.08854(a/S) in which C. is capacitance in picofarads, a is the area of one side of one plate in square centimeters and S. is the distance between plates in centimeters. The rule of thumb from this equation is that capacitance increases as the area of the plates increase, but decreases as the distance between plates increases. With air as the dielectric and the bottom of a thunder cloud representing one plate and the surface of the Earth as another, each many square kilometers in area, the practical capacitance thus formed is staggering. A thunderstorm's base may possess a charge of around 300 million volts and can be hundreds of square kilometers in area.