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Homework Help: Removing a light bulb

  1. Feb 22, 2017 #1
    1. The problem statement, all variables and given/known data
    Circuit quesiton.PNG

    2. Relevant equations

    Ohm's Law, Equations for parallel and series combinations of resistors

    3. The attempt at a solution

    Working this problem I obtain a solution that doesn't make sense, which is that the bulbs are dimmer.

    If I assign 10 ohms to each bulb, I believe the total resistance would be:


    After removing A, it would just be 30 ohms.

    Before: 25 ohms
    After: 30 ohms

    So now there's more resistance and less current?
  2. jcsd
  3. Feb 22, 2017 #2


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    Gold Member

    That's an unhelpfully vague statement. WHERE would there be more or less of something and what would the effect be?
  4. Feb 22, 2017 #3
    Less overall current - true. So figure out each bulb individually to see how much current - before and after.
  5. Feb 22, 2017 #4
    Right, and I still find that they are dimmer. If there's less current after removing the bulb A, then the current to the bulbs next to the battery are going to be less (since they should have the same current as the overall current going over them) and therefore they should be dimmer? I'm told they should be brighter.

    I was referring to overall current and resistance.
  6. Feb 22, 2017 #5
    I think they should be dimmer if they have less current. What about the other bulb not next to the battery?
  7. Feb 22, 2017 #6
    So you said that before removing a bulb,

    Rbefore = 10 + 10 + 5 = 25 ohms

    After removing one,

    Rafter = 10 + 10 + 10 = 30 ohms

    What if the battery was 100 V. What would the current be through each bulb?
    I'm particularly interested in the Before condition.
  8. Feb 22, 2017 #7


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    Gold Member

    Yes, I assumed that your were, and I was making the point that that is not really all that helpful. What you need to do is be completely specific, and I see that you STILL are not doing that, just talking in generalities when you could in fact be solving for exact numerical values for everything in the circuit by assigning arbitrary values to the elements the way you started out doing and which you should have followed through with.
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