MadSci Network: Chemistry |
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.
Try the links in the MadSci Library for more information on Chemistry.