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Homework Help: Parallel and Series, Lightbulb Problem

  1. Jul 19, 2007 #1
    1. The problem statement, all variables and given/known data

    4 identical light bulbs are connected either in series (circuit 1) or parallel (circuit 2) to a constant voltage battery with negligible internal resistance as shown.

    http://img413.imageshack.us/img413/7250/seriesoz2.th.jpg [Broken]

    http://img413.imageshack.us/img413/327/parallelfe8.th.jpg [Broken]

    (Sorry about the sloppy diagram but that's pretty much the gist of it)

    Compared to the individual bulbs in circuit 1, the individual bulbs in circuit 2 are:

    A) 1/4 as bright
    B) less than 1/4 as bright
    C) 4 times brighter
    D) the same brightness
    E) more than 4 times brighter

    3. The attempt at a solution

    The resistance in Circuit 1 would be 4R while the resistance in Circuit 2 would 1/4 R. Because the parallel has a lower resistance, it has a greater current therefore there is more power in the parallel configuration. I thought the right answer was C) 4 times brighter but it is wrong and I can't understand why that is. What am I missing here?
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Jul 19, 2007 #2
    Brightness, in that problem, probably corresponds to the power output of the lightbulbs. Remember that power = current * voltage drop
  4. Jul 19, 2007 #3


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    Staff: Mentor

    Think in terms of power, not in terms of voltage. What is the equation for the power dissipated by a resistive component, in terms of the voltage across it? Hint -- it's not a linear relationship.
  5. Jul 19, 2007 #4


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    Staff: Mentor

    Rats. EFuzzy was too quick for me!

    BTW, remember EFuzzy that on homework help, we should not give out complete answers. We just give out hints and ask the original poster (OP) to do the final work.

    Thanks for chiming in, though. More help is always appreciated.
  6. Jul 19, 2007 #5
    I think I got it now. Thank you!
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