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Is constantly turning a light switch on and off bad?

  1. Aug 13, 2013 #1
    i was messing with a friend of mine, constantly switching his bed lamp on and off; he said that's how light bulbs get burnt out. I don't understand the physics behind this. All that's happening is an alternating on and off AC power going through the light bulb. is it the arc at the switch site? or is there just nothing valid to what he's saying?
     
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  3. Aug 13, 2013 #2

    phinds

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    The arc in the switch will reduce the life of the switch and the change in filament size as the current starts and stops will reduce the life of the bulb. Ever notice how incandescent bulbs pretty much always burn out right when you switch them on?
     
  4. Aug 13, 2013 #3
    Filament bulbs have a lower resistance when they are cold than when they are hot. This means that when they are switched on there is a momentary current which is larger than the rated current.
     
  5. Aug 13, 2013 #4

    sophiecentaur

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    At switch on, because of the high power dissipated (mentioned above), there is rapid expansion of the tungsten, which stresses it. Tungsten is not, I believe, a good 'structural' material (it's talent is in its high melting point) and constant stressing will cause it to shed its surface and become thinner and weaker.
    Low voltage filaments (standard car headlamps and flasher bulbs, for instance) are shorter and fatter and are much stronger. They will survive much more thermal abuse (on off on off) than your average mains filament and last longer. Stage lighting is turned up and down very frequently but a dimmer is used, which slows up the temperature cycling stress and they can also take many more on off cycles.
     
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