Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Response of RLC circuit i'm lost

  1. Sep 8, 2010 #1
    the problem statement is attached.

    i've begun by converting everything into phasor notation but i'm not quite sure where to go from here. when i try to add the C and the L phasors, i get zero. this doesn't seem right.
    another problem is i have no idea what to do when i get the impedances of the components added together. do i just add that to the input voltage for the answer?

    any help appreciated.

    Attached Files:

  2. jcsd
  3. Sep 8, 2010 #2
    so i've converted the circuit to it's current source equivalent ( i can't remember if thats called thevenin or norton) and am able to add the impedances.

    what to do from here?

    i think i'm supposed to add the component impedances to the voltage input signal impedance and plug into v=ir for the voltage. but maybe not... i'm lost like i said. am i on the right track or far from it?
  4. Sep 8, 2010 #3
    Yes, you have to go into the frequency domain. From here, you can treat all the components like they're resistors with their respective impedances

    So, yeah. For your source, you have 2 at an angle -pi/4. Impedance of Res, cap and inductor are 1, -j, and j, respectively (i.e., Zc = -j/(wC))

    Now you just have to do circuit analysis from here. How can you get y(t)? Since voltages are the same in parallel, what if we combine the inductor and cap?

    -j // j = ([-j]*[j])/(j-j) = undefined.

    Well that's weird. I'm not sure what that means physically. I was going to combine those two components and then use voltage division across the equivalent impedance there. Ask your teacher about this. Maybe try to do a node equation?
  5. Sep 9, 2010 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Education Advisor

    What you're finding is that when ω=1000, the LC combination has infinite impedance because that's the resonance frequency of the circuit. No current can flow, so the entire input voltage appears at the output.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook