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Electrical Conductivity - Non-ohmic Conductors

electrons ionized gas ionic

Non-ohmic conduction is marked by nonlinear graphs of current vs. voltage. It occurs in semiconductor junctions, electrolytic solutions, some ionic solids not in solution, ionized gases, and vacuum tubes. Respective examples include semiconductor p-n diodes, battery acid or alkaline solutions, alkali halide crystals, the ionized mercury vapor in a fluorescent lamp, and cathode ray tubes.

Ionic conductivities are much lower than electronic, because the masses and diameters of ions make them much less mobile. While ions can drift slowly in a gas or liquid, their motions through the interstices of a solid lattice are much more restricted. Yet, with their thermal kinetic energy, ions will diffuse through a lattice, and in the presence of an electric field, will wander toward the appropriate electrode. In most instances, both ionic and electronic conduction will occur, depending on impurities. Thus, for studies of ionic conductivity, the material must be a very pure solid.

In gases, the gas atoms must be ionized by an electric field sufficient to supply the ionization energy of the gas in the tube. For stable currents, the ratio of field to gas pressure, E/P, is a major parameter. Electrons falling back into bound states produce the characteristic spectrum of the gas, qualitatively associated with color, e.g., red for neon, yellow-orange for sodium vapor, or blue-white for mercury vapor.

The basic definition of a plasma in physics includes all material conductors, ohmic and non-ohmic. A plasma is a medium in which approximately equal numbers of opposite charges are present, so that the medium is neutral or nearly so. In a metal the negative electrons are separated from an equal number of positive ion cores. In a semiconductor there may be holes and electrons (intrinsic), holes and ionized acceptors (p-type), or electrons and ionized donors (n-type). In an electrolytic solution and in an ionic solid there are positive and negative ions. An ionized gas contains electrons and positive ions. A small distinction among these may be made as to whether the medium has one or two mobile carriers.

In contemporary usage, the term plasma usually refers to extremely hot gases such as those used in the Tokamak for nuclear fusion experiments. High-energy plasmas are discussed in the article on fusion as a means of generating electric power.

The remaining non-ohmic conduction category is the vacuum tube, in which a beam of electrons is emitted from either a heated cathode (thermionic) or a suitably illuminated cathode (photoelectric), and moves through evacuated space to an anode. The beam in its passage is subjected to electrostatic or magnetic fields for control. The evacuated space cannot be classed either as a material with a definable conductivity or as a plasma, since only electrons are present. However, there are relations of current and voltage to be analyzed. These graphs are generally nonlinear or linear over a limited range. But vacuum tubes are not called ohmic even in their linear ranges because there is no material undergoing the lattice behavior previously described as the basis for ohmic resistance.

Electrical conduction in the human body and other animal organisms is primarily ionic, since body fluids contain vital electrolytes subject to electrochemical action in organs. Further information is available in other articles, particularly those on the heart, the brain, and neurons.

Resources

Books

Halliday, David, Robert Resnick, and Kenneth Krane. Physics. 4th ed. New York: John Wiley and Sons, 1992.

Serway, Raymond A. Physics for Scientists and Engineers. 3rd ed. Philadelphia: W. B. Saunders Co.


Frieda A. Stahl

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Lattice

—The structure of atoms in a solid. In a conducting material, ion cores make up the lattice.

Potential difference

—In a conductor carrying an electric current, it is the difference of potential energy per unit charge.

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