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: Irms, Vrms and power calculations

  1. Apr 12, 2012 #1
    1. The problem statement, all variables and given/known data

    Basically, I need to understand how to calculate Irms and I*rms given a Vrms and an impedance Z.

    Example question:
    Given Vrms = 120 V at an angle of 30° and an impedance of Z = 10 Ω, find Irms and I*rms.

    2. Relevant equations

    How exactly do I solve for Irms and I*rms?

    3. The attempt at a solution

    I figure that Irms = Vrms / Z. However I am a bit confused as to the I*rms.

    From my notes, I would imply that I just flip the sign of the angle, but this does not seem right.

    My attempt:

    Irms = Vrms/Z = 120angle30/10angle0 = 12angle30
    And for I*rms, just flip the sign of the angle: 12angle-30
  2. jcsd
  3. Apr 12, 2012 #2


    User Avatar

    Staff: Mentor

    Yup. basically that's it. I* represents the complex conjugate, which for a complex number means that the sign of the imaginary component of the number is inverted. This has the effect of inverting the sign of the angle in polar form.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook