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Light bulb

  1. Jun 11, 2008 #1
    1. The problem statement, all variables and given/known data

    We did an experiment in class abvout resistance and temperature. The teacher told us to find out why light bulbs usually blow when you turn on the light. It must be related to the experiment we did which was seeing how resistance changed with temperature

    2. Relevant equations
    I'm not sure if there are any, If there are please tell me


    3. The attempt at a solution
    I have no idea why, and i wouldnt know where to look on the internet

    thanks

    _Muddy_
     
  2. jcsd
  3. Jun 11, 2008 #2

    mgb_phys

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    The light bulb fails because the current through the filament is too large.
    The current depends on the resistance
    How does the resistance of metal change with temperature ?
     
  4. Jun 11, 2008 #3
    The higher the temperature the higher the resistance.

    So then, the light bulb fails because the current through the filament is too large. The resistance is too low. So the temperature is too high. Is that correct? What actually happens to the filament, does it break? Why would a high temperature/ high current make the light bulb fail?
     
  5. Jun 11, 2008 #4

    mgb_phys

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    The filament is a thin metal wire - rather like a fuse.
    When the filament is cold the resistance is lower and so the currentis higher.

    The current heats the wire which expands which puts a mechanical strain on it.
    Also when the light is first turned on is the greatest temperature difference and so the greatest strain - this is probablyy more important than the resistance in reality
     
  6. Jun 11, 2008 #5
    thanks alot for that, I'm pleased and my teacher should be pleased aswell
     
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