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Homework Help: Difference in Brightness between Two Bulbs

  1. Mar 31, 2016 #1
    1. The problem statement, all variables and given/known data
    Two lightbulbs of resistance R1 and R2 (R2 > R1) are connected in series. Which is brighter? What if they are connected in parallel? Explain.

    2. Relevant equations

    3. The attempt at a solution
    I know the solution.

    In series, P1=I^2R1 < P2=I^2R2, so R2 is brighter.

    In parallel, P1=V^2/R1 > P2=V^2/R2; since R2 > R1, R1 is brighter here.

    But my question is this: How can I know which equation to use for series and which for parallel? (The text says nothing.)

    Why couldn't I have argued in the following way?

    In paralle, P1=I^2R1 < P2=I^2R2, so, even in parallel, R2 is brighter. Why does this line of reasoning fail?

    My best answer is this:

    I is different in each case because they are connected in parallel. So, we can't use P=I^2R for this part; we have to find an equation for which another variable is the same. Best bet is P=V^2/R. Even if that's right, I still don't think I really have grasped the relevant issues here.

  2. jcsd
  3. Mar 31, 2016 #2

    Buzz Bloom

    User Avatar
    Gold Member

    Hi crastinus:

    In series what is the same for both R1 and R2?
    In parallel what is the same for both R1 and R2?

    Hope this helps.

  4. Mar 31, 2016 #3
    Thanks, Buzz! That helped!
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