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Alternating currents circuits

  1. Sep 28, 2014 #1
    1. The problem statement, all variables and given/known data
    part 1 Consider the circuit of figure G, where R1=20 ohms R2=10 ohms X1=37.7 ohms X2=-53.1 ohms. Compute the current and the phase angle between the current and the applied voltage. In the figure R1 is in series with X1 and R2 and X2 are in series. R1 and X1 are in parallel with R2 and X2. X1 is an inductor and X2 is a capacitor.
    part 2 determine the values of L and C from the first part.
    2. Relevant equations
    phase angle= arctan((XC-XC)/R)
    v=iZ
    XL=2Pi*f*L
    XC=1/(2Pi*f*C)
    Z= R+jX where j is the square root of -1

    3. The attempt at a solution.
    I feel like the problem is missing something like voltage or frequency but maybe I am just forgetting how to get this from the information given.
    I think but am not sure that the phase angle would be 85.8 degrees because of arc((37.7--53.1)/(6.66))=85.8 degrees. (1/10ohms+1/20)^-1=6.66 ohms
     
    Last edited by a moderator: Sep 28, 2014
  2. jcsd
  3. Sep 28, 2014 #2

    gneill

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    Staff: Mentor

    Hi Fyr554, Welcome to Physics Forums.

    Can you give some details about your calculation of the phase angle? Why did you choose that particular expression? From your description of the circuit the resistors don't seem to be in series. parallel.

    Does the following look anything like your mysterious figure G? Did your figure give any hints as to the frequency of the source?
    Fig1.gif

    <<edit: changed "series" to "parallel" above (based on the figure 6.66 used for resistance in the phase angle expression) >>
     
    Last edited: Sep 29, 2014
  4. Sep 28, 2014 #3
    That is how the figure in the text book looked but I didn't have any way of adding that picture to the thread. and no it didn't give anything on the frequency. I didn't know if there was a way to figure that out from the information provided. I emailed my professor in hopes she would be able to help me but o haven't gotten answer. The arctan equation for the phase angle is from the book and I am unsure if that is what I am suppose to use or is even the correct answer.
     
  5. Sep 29, 2014 #4

    gneill

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    Staff: Mentor

    Okay, well for part 1 I would suggest reducing the circuit to its equivalent impedance first. It'll be a single complex number. Then note that by Ohm's law, I = V/R. From that you should be able to deduce what the phase angle of the current should be with respect to the angle of the impedance (how's your basic complex arithmetic?).

    For part 2 you need some other information about the circuit in operation, such as frequency or a pair of voltage and current values.
     
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