physics question #1076



Marin, a 17 year old male from the Internet asks on November 25, 2002,

Q:

Is it possible to create plasma using two high voltage electrodes , cathode and anode. Would air beetwen them ionize? How high the voltage needs to be to do all this?

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the answer

Donald J. Barry answered on January 8, 2003, A:

What you propose is very easy -- and very dangerous. Air ionizes when an applied voltage exceeds 30 kilovolts (thousand volts) per centimeter -- this is the voltage at which a spark will leap from one electrode to another. The smaller the distance between the electrodes, the smaller the voltage necessary.

Keeping a spark going after it is formed does not require nearly so much voltage -- thus the operation of a so-called "Jacob's ladder" -- a fixture of science museums and Frankenstein films -- in which a spark is formed at the narrow point of two electrodes arranged as a tall V -- then the spark rises up the V to much larger separations than the initial breakdown voltage would support as the plasma buoyantly lifts.

All atoms in a gas move to and fro, bouncing off one another, and we describe their motion by a variety of parameters, including what is called the "mean free path" -- the distance between bounces. To form a spark, a random ion in a gas sample must feel an electric field of such intensity that it is accelerated in the length of a mean-free-path to a speed such that when it hits another atom, it can ionize it. The result -- a chain reaction or cascade of ionizations, leading to an arc in which most atoms are ionized.

Please do not experiment with voltages exceeding about 12 volts (small battery voltages) without the supervision of experts. Even house electricians are not qualified to work on systems exceeding a few hundred volts. Specialized training, equipment, and experience is necessary to deal with the very high voltages necessary to produce substantial arcs. You could all too easily kill yourself or another even though it would seem you are using common-sense precautions.

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