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RLC circuits

  1. Dec 15, 2007 #1
    1. The problem statement, all variables and given/known data
    What is the current supplied by the emf when the frequency is very large and very small?

    | |
    | ------------------------
    | | |
    | 100ohm 50ohm
    10Vrms | |
    | 50mH 20*10^-6F
    | | |
    | ------------------------
    | |
    2. Relevant equations
    Vrms =v/2^(1/2), Z=(R^2 + (XL - Xc)^2)^(1/2), VR =IR, VL =IXL, Vc=IXc

    3. The attempt at a solution

    I don't know where to start, since they didn't give the frequency.
    Please help
  2. jcsd
  3. Dec 15, 2007 #2
    the picture didn't shows up very good, it's a 100ohm and a 50mH in series and parrell with a 50ohm and a 20*10^-6F in series
  4. Dec 15, 2007 #3


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    Gold Member

    HTML ignores consecutive whitespace.
    Using the quote feature, I was able to get your circuit.
    Surround by
    Code (Text):
     [/ code] (no space in the tag)


    |                                 |
    |                  ------------------------
    |                  |                               |
    |             100ohm                         50ohm
    10Vrms          |                               |
    |              50mH                             20*10^-6F
    |                  |                               |
    |                  ------------------------
    |                                   |
  5. Dec 15, 2007 #4


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    Forget equations. Think about how you can replace the capacitor and inductor at high enough and low enough frequencies (ie the limits as f->0 and f->infinity)
    Last edited: Dec 15, 2007
  6. Dec 15, 2007 #5
    If the frequency is very high, the supplied voltage is rapidly changing. How does the inductor behave in response to this rapidly changing emf?

    If the frequency is very low, the supplied voltage can be approximated as nearly constant over a finite interval of time. How does the inductor behave as a result of this nearly constant source?

    Now go back and consider what the capacitor is doing in both cases.
  7. Dec 16, 2007 #6


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    Whether the 50 ohm resistance is connected in series with the capacitor or is it the capacitive reactance of the capacitor?
  8. Dec 16, 2007 #7
    I believe they are in series, as the capacitive reactance depends on the frequency of the source. By giving the resistance as a definitive 50 ohms, it seems to be implied that it does not depend on the frequency of the emf.
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