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: LCR series circuit

  1. Mar 5, 2006 #1
    the question is :
    suppose the resonance frequency of the LCR series circuit is fr. Will the current in the circuit lag behind the voltage or lead the voltage in the following cases :
    (i) when the applied AC frequency f > fr
    (ii) when f < fr

    we can say that the current lags behind the voltage when the circuit in inductive. The phase angle can be calculated by [tex]\tan\phi=\frac{X_{L}-X_{C}}{R}[/tex] . But how can we conclude whether XL - XC is postive or not when we are given the applied frequency.

    I was able to figure out one thing. When the applied frequency is same as the resonance frequency, XL = XC , which means [tex]\tan\phi=0[/tex]. so the current and voltage are in phase.
    Last edited: Mar 5, 2006
  2. jcsd
  3. Mar 5, 2006 #2
    What you need to do here is sketch a quick phasor diagram of the circuit. The details don't matter, just as long as you have something to work with. Now, both [tex]X_C[/tex] and [tex]X_L[/tex] depend on the frequency of the current in the circuit. So what happens to [tex]X_C[/tex] as we increase the frequency from the resonance frequency? What happens to [tex]X_C[/tex] as we increase the frequency from resonance frequency? What do these changes do to your phasor diagram? From this you can figure out what happens to your phase angle.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook