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What is the rms potential difference across the resistor?

  1. Dec 8, 2005 #1
    Hello every9one i'm stuck on finding the first part:
    A series RLC circuit is driven by an alternating source at a frequency of 380 Hz and an emf amplitude of 90.0 V. The resistance is 20.0 , the capacitance is 12.1 µF, and the inductance is 24.2 mH.

    What is the rms potential difference across the resistor?
    my first answer was wrong,
    Am i t hinking this right?
    Vab = Vmax sin(wt)
    Vab = iR
    i = 90/20 = 4.5, but thisn't isn't i max is it?
    do i have to take i/sqrt(2) to get i max?
    then with i max can i find the rms across the ressitor?
    I found a formula that syas:
    Vab = Vmax*sin(wt) but time isn't given at all, netiehr isVmax! :bugeye:
  2. jcsd
  3. Dec 8, 2005 #2


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    You must set up an equation for the current involving ALL of the circuit components. You cannot get the answer by considering the resistance alone.
  4. Dec 8, 2005 #3


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    This is not correct {i = 90/20 = 4.5 A}.

    As Integral indicated, the 90 volts applies to the R + L + C in series.

    R is a non-reactive component so the voltage drop will be in phase with the applied volatage. L and C are reactive components so the voltage drops will be out of phase with the applied voltage.

    So VR has same shape, but different amplitude, than V.
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