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Homework Help: Find the Current and potential difference in a parallel/series circuit

  1. Apr 30, 2013 #1
    1. The problem statement, all variables and given/known data

    Okay. I got two different answers for these and I don't know which one is right so I could continue from there. If you will please help by letting me know which answer is right and why, I'd very much appreciate it.

    This is the figure:
    where R = 20ohm

    1. Find the current in R (20ohm resistor)
    2. Find the potential difference between points a and b

    2. Relevant equations


    3. The attempt at a solution

    Answers I got:

    One says that the first 3 resistors on the top are all in parallel, and then the total of that is in series with the other two that have 20 and 5 ohm, so the total is 27.5

    The other answer is that in the resistor at the very top, the current goes through the wire then meets a junction and separates into say, i1, i2, i3 and then they all meet again at the junction near point a. So the bottom 2 resistors are in series, but then the total of that and the two resistors above it are all in parallel, then in series with the very top one.
    So we have 1/25+1/10+1/5
    then r= 2.94
    added to 10
    so 12.94

    If you could please let me know which one is on the right track, I'd appreciate it.
  2. jcsd
  3. Apr 30, 2013 #2


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    Staff: Mentor

  4. Apr 30, 2013 #3


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    Staff: Mentor

    Your second version is on the right track, and the value you obtained for the total equivalent resistance is good (although units are not specified. Always specify units!).
  5. Apr 30, 2013 #4
    Okay. Thank you so much. I'll continue it now. I thought I was making stuff up at some point!
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