[Prev][Next][Index][Thread]
Re: Primary Q - A Brain Teaser
Tesla List wrote:
>
> Original Poster: ElectronX-at-aol-dot-com
>
> What conditions cause the losses in the spark gap to be V*I rather thn
I^2*R?
> This is new to me.
>
> Matt Behrend
Matt,
The only condition is that the arc plasma channel be unrestricted. As
the current level increases, the diameter of the plasma channel
increases accordingly, increasing its conductivity. The
electrode-to-electrode voltage drop stabilizes to a relatively constant
value which is governed by the material(s) used for the electrodes and
the gas(es) between them. The terminal arc-voltage drop is the sum of
three things: the drop between the cathode and the main plasma channel
(the "cathode drop"), the drop across the main arc channel itself (the
positive column), and the drop between the channel and the anode
electrode (called the "anode drop").
In an arc discharge between copper electrodes (as in a typical static
gap), the anode drop is in the range of 2-6 volts, and the cathode drop
is in the range of 8-9 volts. Most of the overall voltage drop is across
the positive arc column, and this voltage drop tends to decrease with
increasing current (negative V-I characteristic), and increase with
increase arc-length. The result is a relatively constant voltage drop
between arc electrodes, resulting in energy dissipation with a V*I
characteristic. The overall voltage drop is a function of arc length,
and for typical short gaps will be in the 50-150 volt range for copper
electrodes.
BTW, if the arc diameter is constrained in some fashion (by cooler
surrounding walls, for example), the overall arc voltage versus current
will develop a positive resistance characteristic, since the positive
column can no longer expand in unrestricted fashion with increasing
current.
Hope this helps!
-- Bert --