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Q & A: Plasma balls’ lifetimes

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Most recent answer: 10/22/2007
Q:
Will a Palsma Ball last forever or will it run out like a light bulb?
- Rob Talks (age 14)
UK
A:
A light bulb "runs out" because atoms from the tungsten filament gradually evaporate (actually, "sublimate" may be a better word) from the filament when it is hot. The filament becomes progressively thinner and weaker (and sometimes you can see a dark splotch of the atoms accumulating on the inside of the glass). The filament usually oscillates with the power frequency a bit, it may get shaken around if the bulb is moved, and it undergoes thermal expansion and contraction when it heats up and cools down when switched on or off. This is eventually too much for the poor filament and it will break.

Not so with a plasma ball! There is no filament, merely a spherical electrode in the middle. There may be special coatings on the electrode which may evaporate away with time, I don't know exactly. Small neon bulbs, the kind used in night-lights, have two electrodes with a coating that does in fact degrade with time (either the coating degrades or deposits from impurities in the gas collect on the electrodes). Sometimes you'll find an old one of these neon bulbs which only lights up when you shine light on it. That's because the "work function" (how much energy it takes to free an electron from the electrode) has gone up as it got old, and shining light on it adds energy the electrons can use to help escape from the electrode. It could well be that plasma balls don't need any special coating on the electrode.

They still aren't expected to last forever, though. Plasma balls require a source of high voltage which alternates from positive to negative and back rapidly. Power supplies don't live forever, and I suspect that even those which come in the bases of plasma balls will eventually stop working.

Tom

(published on 10/22/2007)

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