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Filament size and resistance

  1. Feb 8, 2009 #1
    Does anyone have a clear understanding on the relationship between the filament size of a lightbulb and the wattage? I was under the impression that resistance causes the filament to heat up and glow, so the more resistance the brighter the light bulb. A filament with more resistance, then, would have to be smaller. However I have read that a larger filament causes a bulb to grow brighter. Is this true, and why?
  2. jcsd
  3. Feb 9, 2009 #2


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    It is true, because the wall is in essence a constant voltage source. Given a constant voltage and a purely resistive load (a decent approximation), a filament with half the resistance will allow twice the current through. Because power is V*I, for a constant V and twice the I (current), you will have twice the power.
  4. Feb 9, 2009 #3
    Thanks. But are you saying that more current causes the filament to glow brighter? If this is true could you please try and explain it to me. (sorry)
  5. Feb 9, 2009 #4

    Andy Resnick

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    Science Advisor
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    google "resistive heating".
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