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Finding capacitance

  1. Jan 26, 2014 #1
    I have done no. 1, but I don't how to do no. 2. Someone, please help.

    Power source voltage e(t) = sqrt(2)*20sin2t [V],
    current i(t) = 5sin(2t - pi/4) [A].

    1. Find the values of resistance R, and inductance L.
    2. Let V be the phasor of voltage v(t). Find the capacitance value of the capacitor C that maximizes the magnitude of V.
     

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  2. jcsd
  3. Jan 27, 2014 #2

    rude man

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    You know R and L so you can come up with the voltage V1 between L and R.

    You can also write an expression for the voltage V2 between the 25 ohm resistor and C. This will of course be a function of E and C.

    Then maximize (V2 - V1) with respect to C. (The value of C will not be a function of E).
    All V are phasors. E is the phasor of e(t).
     
  4. Jan 27, 2014 #3
    How do I write an expression for the voltage V2 between the 25 ohm resistor and C? I don't know the current across it.
     
  5. Jan 27, 2014 #4

    rude man

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    The current phasor through (not "across") C and the 25 ohm is E/Z where Z = 25 -j/ωC.
     
  6. Jan 27, 2014 #5
    So now I get V1-V2. Next, do I write the function of d(v1-v2)/dC = 0?
     
  7. Jan 27, 2014 #6
    Can you check this, please? Is this correct?
     

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    Last edited: Jan 27, 2014
  8. Jan 27, 2014 #7

    rude man

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    Absolutely right! You might want to make sure it's a maximum, not a minimum or inflection point).
     
  9. Jan 27, 2014 #8

    rude man

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    Sorry, that would go a bit beyond what we're supposed to do.
     
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