1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    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!

Phasor voltage

  1. Apr 8, 2005 #1
    now we are in phasors and I am missing something in this problem:
    given a circuit of indep current source (5cos(wt)), R, L (with vL(t)) all in series, no values for them,
    although voltage across inductor vL(t) = 3cos(wt + theta), we need to find theta.

    So, by using impedance for L: Z = jwL, vL(t) = Z*I,
    so I got vL(t) = jwL5 /_ 0 which is also = 3/_theta
    And now what? The answer is 90 degrees, but how do I get it?

    Thanks a lot.
    Last edited: Apr 8, 2005
  2. jcsd
  3. Apr 9, 2005 #2
    The reason its 90 degrees is simple... the voltage is indeed ZI, and I has a phase of zero--- but you would do well to convince yourself that the impedance Z is purely imaginary, so you have a phase for your impedance equivalent to the arctan of Im[Z]/Re[Z] where Re[Z] = 0, and thus you have an arctan of infinity, which is undoubtedly equal to 90 degrees.

    Therefore you have V = ZI which yields a phase of 0 + 90, the angular frequency and the amplitudes of Z and I are immaterial for this problem.
  4. Apr 9, 2005 #3
    By the by, do you intro EE students still happen to make use of Nilsson and Riedel by any chance?
  5. Apr 9, 2005 #4
    I guess, not.

    Thanks for reply!
  6. Apr 9, 2005 #5
    Oh sorry, it's just a textbook that I found really great for my intro EE courses.
  7. Apr 11, 2005 #6
    Thanks, I might check it out as complementary source. We are using DeCarlo and Lin, since DeCarlo is actually a professor here, I doubt they will change our texbook any time soon :rolleyes:
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?