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Homework Help: Frequency and RLC circuits

  1. Apr 2, 2013 #1
    1. The problem statement, all variables and given/known data

    This was a part question and I got everything else but one part (#4!):

    1) A 60-Hz generator with an RMS potential of 240 V is connected in series with a 3350 Ohm resistor and a 1.5 microFarad capacitor. What is the RMS current in the circuit?
    IRMS = 6.34 * 10-2 A

    2) In the previous question, what is the phase angle between the current and total voltage (in degrees)?
    [itex]\phi[/itex] = 27.8 deg

    3) What would be the average (RMS) power consumed in the circuit in the previous question?
    Pavg = 13.5 W

    4) At what frequency would the circuit in the previous problems have to be operated in order to have the current and total circuit voltage be in phase?

    2. Relevant equations

    z = sqrt(R^2 + XC^2) <-- not sure if I had to use this one
    XC = 1/ωC
    ω = 2πf

    3. The attempt at a solution
    I don't really know how to solve this (or if I'm even using the right equations to solve for it) but I tried solving for f in the formula and I got 72.3 as the answer and it was wrong in the homework assignment. I think phase difference is pi/2 so if it's in phase, do I need to add pi/2 to my final answer...? I'm very lost with this question as you can tell.
  2. jcsd
  3. Apr 2, 2013 #2

    rude man

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    Well, for one thing, it's a trick question.

    What is the expression for phase angle between total applied voltage and current for a series R-C circuit?

    Or: go back to part 2 and change the frequency until you get in-phase between total applied voltage and current.
  4. Apr 3, 2013 #3
    I think it's cos[itex]\phi[/itex] = R/Z, Z = sqrt(R^2 + XC^2)... I don't know if I thinking about this correctly but does in phase mean that the angle = 0?
  5. Apr 3, 2013 #4

    rude man

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    That formula is correct.
    Yes, "in phase" means zero phase angle between V and i.

    So now what does XC have to be to make the phase angle zero?
  6. Apr 3, 2013 #5
    Hmm... well if cos-1(1) = 0, then my answer has to equal 1 somehow... so wouldn't XC need to be 0 because:

    cos[itex]\phi[/itex] = 3500 [itex]\Omega[/itex] / sqrt((3500 [itex]\Omega[/itex])^2 + 0^2) = 1
    [itex]\phi[/itex] = cos-1(1) = 0 deg
  7. Apr 3, 2013 #6
    Hi physics 126 classmate here. I'm also stuck on this question. I propose to get I and V in phase, the frequency has to be infinity.
  8. Apr 3, 2013 #7
    Yeah, that's what I was thinking cause if the XC is 0 then we would have to be dividing over 0 to get f since XC = 1/(2πf*C)... is that what you did?
  9. Apr 3, 2013 #8
    yup just yolo'd and typed infinity into the answer slot. It is correct :) Also the last question, the answer is The current will be the same at very low frequencies and at very high frequencies only.
  10. Apr 3, 2013 #9
    Say what! Haha awesome.

    Ah, I got that one! Thanks! :P
  11. Apr 3, 2013 #10
    Enjoy the free 100% for anyone googling answers
  12. Apr 3, 2013 #11

    rude man

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    Told yu it was a trick question!

    But - what's this about the current being the same at very low & very high frequencies???
  13. Apr 4, 2013 #12
    It's a separate question altogether not included in the original post
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