Atmospheric electrostatics. wahlin l. chapter 3


CHAPTER 3
CHARGING MECHANISMS
3.1 SUMMARY
There are several basic charging processes proposed which in one way
Or another might contribute to atmospheric electric phenomena.
Some of the more important are contact electrification, electrochemical
Charging, influence charging, diffusion charging and mechanisms
Involving freezing and splinting of ice particles. Most of the above
Charging mechanisms were devised to explain the charging of thunder
Clouds. Some theories, such as those involving influence charging for
Example, will not operate in the earlier stages of cloud growth, and
Others, which involve freezing and ice, cannot be considered
Dominating since warm tropical thunderstorms exist which do not
Contain ice. There are numerous other theories which will not be
Mentioned here and there are even theories which argue that a
Combination of all charging mechanisms might be at play at one and
The same time. The situation is very challenging. The latest theory
To be proposed is that of the electrochemical process put forward by the
Author. The electrochemical charging mechanism has the advantage
Of being able to explain both thunderstorm charging and fairweather
36 ATMOSPHERIC ELECTROSTATICS
Electricity. So far it is the only charging mechanism that can be readily
Demonstrated by working laboratory models. Contact electrification
Involves mechanical contact between solids where electrons from a
Lower work function material spill over to a higher work function
Material. Contact potentials are of the order of a few tenths of a volt
And might occur in the atmosphere when solid precipitation particles of
Different temperatures collide or when solid precipitation particles
Bounce off material surfaces either on the ground or in the atmosphere.
Any process in which charge is captured or transferred by ions is by
Definition an electrochemical process. Charge transfer by ions can be
Referred to as oxidation-reduction reactions. An ion which gains an
Electron is reduced and an ion that loses an electron is oxidized.
Electrochemical potentials are encountered in everyday life and can be
Found in batteries and dry cells for example, and has been known to
Chemists and included in their text books for centuries. One problem,
However, is that the language and

conventions used by chemists are not
Exactly tailor-made for physicists who therefore, in the author’s
Opinion, seem to shun the electrochemical effect and often confuse it
With contact electrification. An attempt will be made later to explain
The difference between contact potentials and electrochemical
Potentials as seen by a non-chemist.
Influence charging deals with charges that appear on material
Surfaces which are exposed to an electric field. For example, dust
Resting on the earth’s surface will be negatively charged during normal
Fairweather conditions since the electric field lines from the positive
Space charge in the atmosphere above must terminate on the
Negatively charged earth’s surface (the surface in this example being
Covered with dust). During strong winds the negative charged dust
Particles can become airborne and form highly electrified dust clouds.
Dust and sandstorms are most often found to be negatively charged.
The same explanation can be applied to waterfall electricity where the
Negative charge, induced by the fairweather field on a surface of water,
Is being carried over the edge of a waterfall. As the electrified water
CHARGING MECHANISMS 37
Falls over the edge it breaks up into small drops and forms a mist of



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Atmospheric electrostatics. wahlin l. chapter 3