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Homework Help: Ranking brightness of lightbulbs in a circuit

  1. Mar 4, 2017 #1
    1. The problem statement, all variables and given/known data

    In the circuit shown, all the light bulbs are the same. Why is the following statement true? :

    A is the brightest. B and C are equally bright, but are dimmer than A.

    https://pbs.twimg.com/media/C6GllsQVMAA4chz.jpg:large [Broken]
    https://twitter.com/FauxNews101/status/838133087464640513 [Broken]

    2. Relevant equations

    P = I * V

    3. The attempt at a solution

    There is more of a voltage drop across B to C than A, so the power is less for both B and C than A since power is proportional to I and V? Since B and C are the same then they should be the same brightness because they each have the same resistance, in series?
    Last edited by a moderator: May 8, 2017
  2. jcsd
  3. Mar 4, 2017 #2


    User Avatar

    Staff: Mentor

    No. lightbulb A is in parallel with B+C. So the voltage drop across B is half of that across A, and the drop across C is half that across A.

    Try again? :smile:
  4. Mar 4, 2017 #3
    The voltage drop of B+C equals the voltage drop of A. This is so because light bulb A equals the voltage drop across the equivalent of B+C since light bulbs in parallel have the same voltage. So B or C has half the voltage that A does; and because of that, the power for B and C is equal, but B or C is less power than A.
  5. Mar 4, 2017 #4


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    Staff: Mentor

    Yes, good. :smile:
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