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

Impedance and Admittance

  1. Feb 29, 2012 #1
    I have somehow worked myself into a mental loop that I need a push to break out of.

    Consider an inductor in series with a resistor. In sinusoidal steady state, the combination has an impedance Z = R + jωL. The admittance is given by (1/Z) = (R-jωL)/(R2+(ωL)2), and if R is zero, it is simply -j/(ωL) as expected. But the admittance can be broken into conductance and suseptance, so the calculated admittance Y = G+jB = 0 - j/(ωL).
    But I was expecting G = infinity!

    Clearly I am confusing exactly how and when these quantities are defined. Any help would be appreciated.

  2. jcsd
  3. Feb 29, 2012 #2
    That's a good one. But think of if R=0, you only have an inductor

    [tex]Z_L=jωL\;\Rightarrow Y_L=\frac 1 {jωL}=-\frac j {ωL}[/tex]

    There should be no G. You don't even have R in the equation!!!
  4. Mar 1, 2012 #3
    So is a G only defined if you have an R, or is an R only defined when you have a finite G?

    I guess I would like a clear and unambiguous mathematical definition of how and when R,X,G, and B are defined.

    (I have read statements before such as one cannot always define an impedance and admittance matrix simultaneously, for example, a short circuit has no well defined admittance matrix, so I believe my question is a simpler case of this)
  5. Mar 1, 2012 #4


    User Avatar

    Staff: Mentor

    Let's see whether you can get me just as confused.

    Admittances in parallel simply add. So G+jB is a resistor in parallel with a reactance.
    If the resistor is of infinite Ohms (i.e., a perfect insulator) then G=0.
    If G=infinity then you would be talking about a short circuit across the inductor.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook