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Req of this series/parallel circuit?

  1. Oct 27, 2012 #1
    1. The problem statement, all variables and given/known data
    What is the Req of this circuit?
    The answer should be 150 ohms, but I can't get that.

    2. Relevant equations
    Req = R1 + R2
    1/Req = 1/R1 + 1/R2

    3. The attempt at a solution
    If I treat R1 and R3 as series, I get R13= 200 ohms.
    Then if I treat R13 and R2 and parallel I get R123 = 66.7 ohms.
    Then if I treat R123 and R4 as series, I get Req = 266.7 ohms.

    Also, since the current is 0 (open switch), then the potential difference between points P and Q would also be 0, right?
     
    Last edited: Oct 27, 2012
  2. jcsd
  3. Oct 27, 2012 #2

    CWatters

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    A hint..

    There appears to be a wire (grey) connected from one end of R3 to the other end. What does that mean for the voltage across R3 or R3 in general?
     
  4. Oct 27, 2012 #3
    No clue. It's the same as the source's voltage?

    Also, did you manage to get Req?
     
  5. Oct 28, 2012 #4

    CWatters

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    The grey wire has a very low resistance, lets assume it's zero Ohms. You therefore have R3 in parallel with a zero Ohm resistor..

    For that bit of the circuit..

    I/R = 1/R3 + 1/zero

    but 1/zero = ∞

    so R3 in parallel with zero Ohms = zero Ohms.

    The circuit reduces to..
     

    Attached Files:

  6. Oct 28, 2012 #5

    CWatters

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    I'll let you have another go at calculating Req.
     
  7. Oct 30, 2012 #6
    Smart, thanks.
     
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