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Why do light bulbs break? (burn out)

  1. Mar 9, 2016 #1
    1. The problem statement, all variables and given/known data

    When a light bulb brakes it is usually when the circuit is complete, like when there is no cut in a thread or anything. give a reason for this that has something to do with physics. the only thing I can come up with is that the current gets too high and the effect gets too high so the thread in the bulb get very hot so it burns
    2. Relevant equations


    3. The attempt at a solution
     
  2. jcsd
  3. Mar 9, 2016 #2

    berkeman

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    Maybe read about light bulbs on wikipedia -- that should give you some better clues about why light bulbs eventually burn out. :smile:
     
  4. Mar 9, 2016 #3
    now that I reread the question again I am not sure if it is why light bulbs break or why light bulbs break only when the current is on. Anyway, I found this:

    Tungsten gains resistance as it heats. Resistance is the amount of ‘push back' a material has against an electric current. The only thing that heats tungsten in a light bulb is electric current flowing through it. Imagine if a rubber hose gained strength only after water flowed through it. After some use, it would be able to handle a heavy stream of water. At first, though, it would bulge and strain like a water balloon before regaining its shape. Unless the rubber is in good condition, it will snap. In the same way, a tungsten filament is overloaded with current in the first few seconds after being turned on. The heat causes it to expand, and the filament experiences thermal stress, the strain of the material trying to expand due to sudden changes in temperature. Unless it is in good condition, it snaps.
     
  5. Mar 9, 2016 #4

    berkeman

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    They don't always burn out right at turn-on, though. Please read the wikipedia article to see if that helps fill out the picture for you. :smile:
     
  6. Mar 9, 2016 #5
    you didn't link an article and I don't know which one you are talking about
     
  7. Mar 9, 2016 #6

    berkeman

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    You don't know what wikipedia is?
     
  8. Mar 9, 2016 #7
  9. Mar 9, 2016 #8
    I asked this question in an engineering class many decades ago. For the most part, the answer is as noted above, thermal stress. If the bulb is being vibrated, it could also be mechanical stress. More rare is a surge in power causing the weakest part of the filament to fry. It's also possible that the bulb wasn't properly evacuated or has a small leak which would cause oxidation of the filament, but that's very rare these days. Note that the filament goes from ambient temperature of about 25 degC to 2500 degC in a fraction of a second. The thermal stress on the filament is significant. After hundreds or thousands of cycles, it's gonna break.
     
  10. Mar 9, 2016 #9

    berkeman

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    The first part of the article talks about what had to be done to extend the lifetime of bulbs to make them practical. And there is a section near the end about lifetime.

    Note that the wikipedia article does not seem to mention the On/Off cycles as part of the lifetime equations. They probably should, but there certainly are mechanisms to cause the lifetimes of bulbs that are left on all the time to be finite....
     
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