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I don't really understand the mechanism of electrical breakdown, as occurs in a spark gap.

I know that once a certain voltage threshold is passed (around 30kV/cm in air) a gap in a circuit can become electrically conductive, drastically reducing the resistance of a gap and producing a spark while the current flows over the gap.

Usually it's explained like this: The strong electric field across the gap ionizes the air, which now becomes conductive. But could somebody explain it to me in more detail? I mean, the air ionizes, but what does it mean? Are the molecules somehow arranging themselves so that different charges "point" different ways? Or are the "electrodes" of the gap somehow attracting different kinds of particles from the air? Or are the air molecules losing electrons? And how does any of this make the electrons from the high voltage side cross the gap to the lower voltage side? Thank you!

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here is a simplified explanation: Imagine two electrodes near each other, separated by air. a very large electric field is imposed between them. then a random ultraviolet photon enters the gap between the electrodes and just happens to knock an electron off an oxygen or nitrogen molecule there. the electric field immediately urges the loose electron and the ionized molecule towards the positive electrode (for the electron) and the negative electrode (for the ion). when the ion strikes the negative electrode, it dislodges a waiting army of electrons which then fly off towards the positive electrode. occasionally these electrons will strike other O2 or N2 molecules hard enough along the way to ionize them and the field then separates the ions and electrons and this "avalanche" process continues.

Meanwhile at the positive electrode, incoming electrons smack into the metal surface, completing the circuit and if the gap is narrow enough, the field is strong enough, and the voltage supply creating the field is stout enough, all the air in the gap becomes ionized and its DC resistance falls to a low level. the charge transport current then rises accordingly and ohmic losses in the ionized air then heat the air to a high temperature and the resulting plasma emits light. Superimposed on this blackbody spectrum are emission lines corresponding to the excited states of the N2 and O2 molecules.

the process is sufficiently energetic to decompose some of the O2 to ozone, which you can smell in the air while the spark is conducting.

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  • $\begingroup$ So basically it works by the charged electrodes attracting electrons/positive ions that are already in air? Is it possible that in certain situations these particles do not exist in the air are therefore no sparks occur? Or is it possible for the air to "run out" of charged particles this way? $\endgroup$
    – S. Rotos
    May 2, 2018 at 15:58
  • $\begingroup$ for accurate and repeatable measurements of electrical breakdown in gases, it is customary to furnish artificial UV illumination of the electrode surfaces while the experiments are being run, to ensure the presence of ions in the gap. If you don't do this, there will be a lot of scatter and uncertainty in the data. Lots has been written about this- try googling "electrical breakdown of gases". the standard works are by Penning and and Paschen. I can get you a more complete bibliography if you wish. $\endgroup$
    – niels nielsen
    May 2, 2018 at 16:28

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