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Wearing Out

  1. Sep 5, 2006 #1
    I understand why mechanical systems that are not capable of self repair wear out, but for years now I've been totally puzzled as to why things which do not have wearing parts wear out, such as - phosphores, rechargable batteries, capacitors, fluorecent lights, electrodes, vacuum tubes, electroluminescent plastics, LEDs and so on. I understand why incandescent bulbs burn out though - because the tungsten filament subliminates under the intense heat until a place on the filament gets thin enough that the filament snaps in two, breaking the electrical circuit and putting the light out.
    So, why do they?
     
  2. jcsd
  3. Sep 6, 2006 #2

    Gokul43201

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    Different reasons. For instance, fluorescents can die bacause of a bad ballast or starter or even a broken filament. Electrolytic capacitors fail when electrical contact between the electrode and electrolyte is broken. This can happen if the electrode forms an oxide layer or if the electrolyte degrades (especially with high temperatures) or if any one of a handfull of other unlikely events occurs.

    In short, and being poetic, things eventually fail because everything in the world will react with everything else in the world, given enough time.
     
  4. Sep 6, 2006 #3

    DaveC426913

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    Because there are no moving parts on a macroscopic scale does not mean there isn't a storm of activity on the atomic and subatomic scale. Even small amounts of electrical current will cause corrosion of parts.
     
  5. Sep 6, 2006 #4
    Ah, thanks. :)

    That doesn't explain what kills phosphores though. Also not why the bulb part of http://en.wikipedia.org/wiki/Sulfur_lamp goes out. There's nothing inside the inert glass container but sulfur and argon, what is reacting inside to ruin it after 60,000 hours?
     
  6. Sep 6, 2006 #5

    Gokul43201

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    Ain't no such thing as a truly isolated system!

    Specific outgassing rate of air through glass is at least 10^{-9} mbar/cm2-sec.
    Surface area of bulb = 100cm2 say
    60,000 hrs = about 200 million sec

    If your bulb started out at absolute vacuum, it would end up with about 20mbar worth of air in it at the end of its rated life, assuming all joints and seals are perfect and have no leaks. That's a 2% contamination.
     
  7. Sep 7, 2006 #6
    btw if you buy a pc today, and use it non-stop everyday, how many years does it take normally for the chips, transistors (and all p-n junctions) to get "corroded" by the fluctuating microcurrent? because these streams of electrons eventually destroy the band structure of the material won't it?

    (neglect the hard drive, because it is mechanical)
     
  8. Sep 30, 2006 #7
    One of the biggest problems with long-term use of semiconductor devices is electromigration. When you have relatively high currents passing through small areas, over time, the electrons act as a type of "wind," physically moving the metallic contacts of the devices and creating shorts and opens.
     
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