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Re: [TCML] Re: Spark models, revisited



On 10/21/12 10:45 AM, Udo Lenz wrote:
That poses another question. A thin conductor does not have much
capacitance. The model calculations for a real arc indicate about
10 to 20 pF/m.
That is much more than a 0.2mm wire would have. Possibly a thin
conductor like that would be a perfect breakout point for sideways
arcs or create corona around the main arc, adding capacitance.


Antonio Carlos M. de Queiroz wrote:

Thin conductors have really this range of capacitance. The inductance
of a thin wire is approximately 1 uH/m. Adding distributed
capacitance, a transmission line is formed. With L being the
inductance per meter and C the >capacitance per meter, the speed of a
signal
travelling through the line is 1/sqrt(LC). As this speed must be
smaller than the speed of
light c, c^2<1/(LC), C<1/(Lc^2) = 1/(1e-6 x 9e16) =11.1 pF/m.

If I apply the equations for a wire of 0.2 mm diameter 1 m
above a conducting plane from
http://en.wikipedia.org/wiki/Capacitance
I get 5.6 pF/m. For 11.1 pF/m I would need wire diameter of 2.7 cm.

The arc model discussed here doesn't work well with 5.6 pF/m. At a low
capacitance like that the arcs become impossibly long if I adjust the
series resistances, so that the arc consumes 20 kW at 70kV peak
voltage.


For the reasons Antonio gave, the Wikipedia equation must be invalid in that scenario.

Which Wikipedia equation are you using, the one at the bottom of the pate?


There is another reason I'm thinking about the pulsating electron
cloud. It could be modelled by adding a resistor in series with every
cap. A circuit like that could take care of the problem of the phase shift
between voltage and current draw of the arc.

I'd be delighted to hear about other estimates of arc capacitances.
Also I'd be thankful for any pointers to literature about RF arcs.


Bazelyan and Raizer, "Spark Discharge", CRC Press..


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