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Homework Help: Finding the parallel impedance in a circuit

  1. Aug 24, 2012 #1
    1. The problem statement, all variables and given/known data
    I need to find the total impedance in a circuit that has:
    j60 ohms, 30 ohms, -j60 ohms and 20 ohms in parallel

    2. Relevant equations
    Ztotal=1/(Z1+ Z2+ Zn)

    3. The attempt at a solution
    I've tried doing this, but what I think is the -j60 and the j60 should cancel therefore leaving only the 30 ohms and 20 ohms in parallel, which would equal 1/50, but the answer says it is 12 ohms.

    Attached is an image of the circuit and answer.

    Attached Files:

  2. jcsd
  3. Aug 24, 2012 #2


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    Staff: Mentor

    So then, 1/30 + 1/20 = 1/50 ? Check your math.
  4. Aug 24, 2012 #3
    Parallel impedance is:

    Ztotal=[1/(Z1+ Z2+ Zn)]^-1

    Also remember when adding fractions you must have a common denominator.

    For example 1/a + 1/b = (a+b)/ab
  5. Aug 24, 2012 #4
    but how does that j5 get up there?
  6. Aug 24, 2012 #5
    To be honest i think that's a mistake. The negative and positive j components cancel out. That should say 5/20 not j5/j20 even though those two ratios are equivalent.

    If you're having trouble adding fractions try this page:
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