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Solve for Equivalent Resistance - Resistor is shorted?

  1. Sep 25, 2016 #1
    1. The problem statement, all variables and given/known data
    I'm studying for a test and since this is technically a textbook problem I figured I'd post here to prevent being flagged.
    upload_2016-9-25_14-6-36.png
    I have the circuit redrawn.
    upload_2016-9-25_14-3-32.png

    2. Relevant equations
    Series: Req = R1+R2...
    Parallel: 1/Req= 1/R1 + 1/R2...

    3. The attempt at a solution
    I want to understand why when solving this circuit for Req it is (3 || 30) || 7.5 = 2ohms.
    According to the solution I'm looking at the 75 ohm resistor is considered a short and no current flows across this? I don't understand why this is the case.

    Also I was originally trying to be lazy and solve and just add things up without redrawing the circuit and ended up with 2ohms by doing:
    ((5||20)+2)||3 = 2ohms
    Was this just coincidence?
     
  2. jcsd
  3. Sep 25, 2016 #2
    That is because the 75 ohm "resistor" is connected in parallel with a zero resistance piece of wire.

    Nope, its not a coincidence - if you deform the circuit, your consideration is correct.
     
  4. Sep 25, 2016 #3
    You da man. That makes a lot of sense.
     
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