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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.

    -Dan
     
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