1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Circuit Question

  1. May 6, 2007 #1
    1. The problem statement, all variables and given/known data

    [​IMG]

    2. Relevant equations

    Parallell: [tex]\frac{1}{R_{eq}} = \frac{1}{R_{1}} + \frac{1}{R_{2}}[/tex]

    Series: [tex]R_{eq} = R_{1} + R_{2}[/tex]

    3. The attempt at a solution

    Part 1 - When the switch is not closed:

    [tex]R_{eq_2} = R + ((90+10)^{-1} + (90+10)^{-1})^{-1}[/tex]
    [tex]R_{eq_1} = R + 50[/tex]

    Part 2 - When the switch is closed:

    [tex]R_{eq_2} = R + (90^{-1} + 10^{-1})^{-1} + (90^{-1} + 10^{-1})^{-1}[/tex]
    [tex]R_{eq_2} = R + 18[/tex]

    Since [tex]R_{eq_1} = 2R_{eq_2}[/tex], then

    [tex]R + 50 = 2(R + 18)[/tex]
    [tex]R + 50 = 2R + 36[/tex]
    [tex]R = 14[/tex]

    The answer in the back, though, is 24. What have I done wrong?
     
    Last edited: May 6, 2007
  2. jcsd
  3. May 6, 2007 #2
    Good question, it looks to me like you did it right. Are you sure you didn't make a numerical error (this part looks okay too, though I didn't actually calculate)? How trustworthy are the answers in your book?
     
  4. May 6, 2007 #3
    Thanks, mindscrape. I suppose the book could have misprinted the answer. One more thing, though, if you don't mind:

    When the switch is closed, I calculated two parallel circuits. Are these two circuits, in turn, parallel to each other, or in series to each other (like I did above)?
     
    Last edited: May 6, 2007
  5. May 7, 2007 #4
    When the switch closes, it creates a node between the four resistors. The resulting parallel connections will be in series with each other - the way you calculated it. :smile:
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Circuit Question
  1. Circuit +/- question (Replies: 10)

  2. Circuit Question (Replies: 3)

  3. Circuits Question (Replies: 8)

  4. Circuits question (Replies: 3)

  5. Circuits question (Replies: 1)

Loading...