MadSci Network: Chemistry
Query:

Re: Does an electrical arc spark need air to happen?

Date: Fri Jul 25 18:08:37 2003
Posted By: Aaron J. Redd, Post-doc/Fellow, Plasma Physics and Controlled Nuclear Fusion, University of Washington
Area of science: Chemistry
ID: 1050069563.Ch
Message:

The short answers: yes, a spark can occur in a vacuum; and, no, the spark
isn't really a "fire", but such a spark can start a fire.  So it is still a
safety hazard.

The spark or electrical arc is not a fire, in the sense that it is actually
a superheated gas, also known as a plasma.  The plasma of the arc does not
burn, in the sense that it is sustained by the electrical current traveling
through the arc -- just as a wire will heat up when electrical current
travels through it, the plasma stays hot because of the current in the arc.
 In theory, two metal surfaces with a vacuum gap between them can safely
hold off an arbitrarily high voltage between them.  In the real world,
though, there are two important ways for the spark to happen:
(1) No vacuum is perfect, so there will be some small amount of gas
present.  As shown by Paschen in the early 20th century, there is a maximum
voltage that can be sustained between two (metal) electrodes, determined by
the density of the trace gases in the area and (speaking loosely) the
distance between the metal plates.  If the voltage is above the so-called
Paschen voltage, then there will be an arc between the metal surfaces. 
Also, the gas doesn't need to be air: helium, neon, hydrogen, carbon
dioxide -- any of these will show the same Paschen breakdown.
(2) Real surfaces (such as metal surfaces) aren't perfect either, and if
the electric field at the surface is too high, then the material will
sputter and/or vaporize, creating some gas which can then become an arc plasma.

Once the arc is occurring, more material can be sputtered off from the
surface, adding more gas to the plasma arc and pitting the metal surface. 
  This is part of the reason why electrical fires are so hard to fight: 
the spark that causes the fire won't stop until the electrical current
stops, so whoever is fighting the fire first has to shut off the current,
and then try to put out the fire.

As for this pitting on the electrode surfaces, it is quite noticable --
electrodes tend to look very "beat up" after being exposed to arcs.

If your light switches (or any other switches!) generate visible sparks
when they are switched on or off, then they need to be replaced
immediately.  Like I said above, the spark isn't a fire itself, but it can
start a fire.




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