1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

RCL Circuit - Problem due at 11 PM EST tonight, have been working on it for days!

  1. Mar 3, 2009 #1
    1. The problem statement, all variables and given/known data
    A series RCL circuit contains only a capacitor (C = 8.46 μF), an inductor (L = 6.01 mH), and a generator (peak voltage = 77.4 V, frequency = 2.45 x 103 Hz). When t = 0 s, the instantaneous value of the voltage is zero, and it rises to a maximum one-quarter of a period later. (a) Find the instantaneous value of the voltage across the capacitor/inductor combination when t = 4.32 x 10^-4 s. (b) What is the instantaneous value of the current when t = 4.32 x 10^-4 s?

    2. Relevant equations
    v(t) = V*sin(2pi*ft)
    z= square root(R^2 + (Xl-Xc)^2)

    3. The attempt at a solution
    the instantaneous voltage in part a = 27.77 V. (this is correct)

    my answer to part b is incorrect but I don't know why:

    X of L=2pi(2450)(.00601)=92.5 Ohms
    X of X=1/(2pi(2450)c) = 7.678 Ohms
    Z=square root(0^2 - (92.5 - 7.678)^2) = 84.822
    I0=V0/Z = 77.4/84.822 = 0.91249 A
    I=I0sin(2pi*ft + (pi/2))
    I=(0.91249)sin(2pi(2450)(4.34x10^-4) + (pi/2)) = 0.841 A
  2. jcsd
  3. Mar 3, 2009 #2
    Going through your answer very quickly, all I can spot is your phase angle. Since it's almost purely inductive you should have a minus where the plus now is. Hopefully I'm not too late.
  4. Mar 3, 2009 #3
    Oh wow... I can't believe that was it. It was too late for full credit, but now I know that for my test tomorrow. Thanks! :D
  5. Mar 4, 2009 #4
    To further help you determine the sign of the phase angle. If XL is greater than XC then you need to subtract the angle, if XL is smaller than XC you need to add the angle.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Threads - Circuit Problem tonight Date
RC Circuit Problem Feb 25, 2018
Norton equivalen circuit problem Sep 20, 2017
Wye-Delta and Series-Parallel Circuit Problem Aug 26, 2017
Circuits problem Need help by 10:00 tonight (EST). Apr 29, 2008