QUESTION #674

Vacuum is a dielectric. Where does the impedance to charge flow come from?

Answered by: Ted Pavlic, B.S., Electrical Engineer, The Ohio State University

In order to answer this question, first let's consider the two main mechanisms of charge transport in materials. In a metal, the valence electrons exist in a delocalized state-- that is, an electron has a high probability of being found anywhere inside the metal. This gives a high conductivity because the electrons are essentially free to move when acted on by an electric field. Next, in a semiconductor or conducting polymer, the electrons are not in delocalized states. They are restricted to orbit given atoms, and they can only move from one to the other by quantum tunneling. This takes advantage of the fact that, even though you may start with an electron around one atom, there is some probability that at some time later, the electron may be found orbiting another atom. Electrons still have some probability of moving under the influence of an electric field, but it is much less than that of the electrons in a metal. So, semiconductors and conducting polymers have much lower conductivity than metals. Now, ask yourself-- what is the mechanism of charge transport through vacuum? Assuming that your electrodes are separated by some reasonable distance, no electrons will be delocalized between the two electrodes, so they won't be able to flow directly from one to the other. The only possible mechanism, therefore is by tunneling. If you have electrodes separated by distances much larger than, say, the diameter of a typical atom, the probability for one electron to tunnel from one electrode to the other is incredibly small, unless you have a very large applied electric field. And, saying you have a small probability of tunneling is the same as saying you have a small current for some applied electric field, which is the same as saying you have a high resistance to charge flow.