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Series and parallel R-L-C turned circuits

  1. Sep 14, 2013 #1
    1. The problem statement, all variables and given/known data

    A coil of inductance 0.20 H, resistance 60 ohms is connected in parallel with a 20 μF capacitor across a 20 V, variable frequency supply.
    Determine:
    1. the resonant frequency
    2. the dynamic resistance
    3. current resonance
    4. Q- factor at resonance

    Is anyone there who could help me to go through with this question please?!

    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Sep 14, 2013 #2

    NascentOxygen

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    Have you drawn a schematic?

    What do you understand by the resonant frequency?
     
  4. Sep 14, 2013 #3

    rude man

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    Show some work! You can at least calculate the current for the parallel network.
     
  5. Sep 14, 2013 #4
    I believe the resonant frequency equation is:

    fr = 1 / 2∏√(LC)

    However, I am struggling to work out the value for C (Capacitance). As in the question I have 20uF capacitor.

    Is 20uF the value for C?
     
  6. Sep 14, 2013 #5

    rude man

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    Yes. fr is also correct.

    For par 2 I don't know what is meant by 'dynamic resistance'. Is it the real component of impedance or is it ∂V/∂I without regard to phase?

    For part 3 you must solve V/I. You will find that the 'current resonance" is not the resonance of part 1.

    Part 4 is messy.

    In fact, this is a very messy problem altogether except for part 1.
     
    Last edited: Sep 14, 2013
  7. Sep 14, 2013 #6
    Can you please help me work out the value of C for Part 1?

    For Part 2. Dynamic resiatance is the same as dynamic impedance.
     
  8. Sep 14, 2013 #7

    rude man

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    You just suggested C = 20 uF and I said "Yes".

    So the answer is dynamic resistance = dV/dI? At resonance or anywhere?

    I have to warn you, this is a bear of a problem.
     
    Last edited: Sep 14, 2013
  9. Sep 14, 2013 #8

    NascentOxygen

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    No, not precisely. That is true for an ideal resonant circuit where R=0. For any practical circuit, with non-zero R, that is only a handy approximation. This problem requires that you determine the precise resonant frequency.

    Yes. The problem specs give you R, L and C. Draw that circuit and write an expression for the impedance of each element, in Ohms.
     
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