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Homework Help: Lightbulb Brightness

  1. Jun 29, 2010 #1
    1. The problem statement, all variables and given/known data
    Two light bulbs of resistance R1=10 ohms and R2 = 15ohms are connected in series with a 25V battery. Which bulb glows more brightly?

    Now the bulbs are connected in parallel with the battery. Which bulb now glows more brightly?

    2. Relevant equations

    3. The attempt at a solution
    ...This seems like an easy question, but I just don't understand it! I know if they were the same resistance the brightness of each bulb would be the same, but don't really know why. I thought the amount of current passing through each bulb determined brightness, but then I read that from the Junction rule, the current through each component is the same!
  2. jcsd
  3. Jun 29, 2010 #2
    Hey, the brightness depends on the power consumed by the bulb. When bulbs are in series,yes, as you said current flow is same. You calculate power consumed by each bulb now using relation
    [tex] P= I^2 R[/tex]
    from which you can find which bulb is more brighter.
    When they are in parallel, surely, bulb with higher current through it (or which has lower resistance) will be brighter, since voltage here is same on two bulbs.
  4. Jun 29, 2010 #3


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    Homework Helper

    In series combination the same current flows through both the resistances. Brightness depends on the heating effect. And it is given by either I^2*R, or V^2/R or V*I. So in series combination larger resistance is brighter.

    In parallel combination...........?
  5. Jun 29, 2010 #4
    Ok, thanks heaps!
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