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Homework Help: RC parallel circuit, find the value of R and C

  1. Mar 28, 2014 #1
    1. The problem statement, all variables and given/known data

    The circuit is a Parallel RC

    Z= 105KΩ∠-27 and Frequency is 50 Hz.

    I was given the following information and hints.

    1/z = 1/R + jwc

    I need to find the common denominator of 1/R + jwc, Invert it to find z and simplify Z into one real and one imaginary part.

    2. Relevant equations

    Need to find the value of R and C

    3. The attempt at a solution

    I started with finding the common denominator,

    = 1/R +(jwc*R)/R

    1/z = (1+jwRC)/R

    To find z i simplified the equation above,

    R = z(1+jwCR)

    z = R/(1+jwCR)

    Im not sure if i did this right as i cant find R and C. Any help will be much appreciated.
  2. jcsd
  3. Mar 28, 2014 #2


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    Staff: Mentor

    Why not start by calculating 1/Z, since you're given a numerical (complex) value for Z?
  4. Mar 28, 2014 #3
    If i do 1/z i would get 8.485x10-6 +j 4.323x10-4. Im not sure on how i would get and RC from 1/z = (1+jwRC)/R.
  5. Mar 28, 2014 #4


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    Staff: Mentor

    You don't need RC. You can pick out the admittances of the two components directly from the complex value of 1/z.

    1/z is the admittance of the parallel circuit. How do admittances in parallel add?

    (The resistor analogy is two conductances in parallel).
  6. Mar 28, 2014 #5
    I worked out R= 118K ohm and C = 1.37x10-8. I checked by putting the values back into the equation and got the correct z. I guess i dont understand what my lecturer is going on about finding the common denominator etc.
  7. Mar 28, 2014 #6


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    Staff: Mentor

    I suppose your lecturer had envisioned a different path to solution. But this one is pretty straightforward, no?
  8. Mar 28, 2014 #7
    I agree that the way you have explained is much simpler, ill ask my lecturer on what his method is. Thanks for your help.
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