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Homework Help: Homework question!

  1. Oct 3, 2007 #1
    Sorry if this is the wrong forum, it's a homework question I'm stuck on, though. :|

    The question is that if a 60 Watt bulb has a hot resistance of 240 ohms when it's connected to a 120 VAC source, how much power is the bulb using?

    Now I'm not exactly sure what equation to use, I thought it would be P = V squared divided by R? That just gives me 60 W, though. What equation do I use to get the power used?
  2. jcsd
  3. Oct 3, 2007 #2


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    looks right to me. power being used is V^2/R = 60W.

    Did you post the question exactly as it is written?
  4. Oct 3, 2007 #3
    Yah, that's pretty much how it's written. Here's the exact question:

    A 60 Watt incandescent bulb has a "hot" resistance of 240 ohms when connected to a 120 VACrms source. How much power is the light bulb using?

    So...the answer is 60 Watts, then? Is it a trick question? o_O
  5. Oct 3, 2007 #4


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    Oh wait... in order to get average power, you need to use rms voltage, not peak voltage.

    [tex]v_{rms} = \frac{v_{peak}}{\sqrt{2}}[/tex]

    then use average power = [tex]\frac{(v_{rms})^2}{r}[/tex]
  6. Oct 3, 2007 #5


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    Oh crap. sorry again... so they give you 120V rms not 120peak...

    So then 60W is the answer. hmmm... strange question...
  7. Oct 3, 2007 #6
    Maybe my teacher is checking to see if we're paying attention. :P
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