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Homework Help: Why are Vc and Vl larger than Source current at resonance in an RLC circuit?

  1. Nov 6, 2011 #1
    I'm looking at this question and I can't seem to come up with an answer, dont the VL (inductive voltage) and Vc (capacitor voltage) just cancel each other out leaving you with Vr (resistance voltage) as the source voltage, but I don't see why VL and Vc are much higher than the source voltage

    Sorry title meant source VOLTAGE, not current
    Last edited: Nov 6, 2011
  2. jcsd
  3. Nov 6, 2011 #2
    I don't get it...what does the circuit looks like? any initial conditions?
  4. Nov 6, 2011 #3
    We had a 22 ohm resistor, 0.1 uC capacitor and 30 mH inductor in series, and as we increased the frequency up to the theoretical resonance, we found VL p-p and Vc p-p to be much higher values than Vi, which was the source voltage. This was using an oscilloscope
  5. Nov 6, 2011 #4
    At resonance Vl and Vc are equal and 180degrees out of phase. You could say that they cancel out.
    This means that the supply voltage is across R (22ohms) use this to calculate the CURRENT
    The voltage across L and C is then given by current x Reactance of L (or C).
    You calculate reactance from the value of L and C and the resonant frequency.
    It can be a great surprise !!! The voltage across L or C can be many times greater than the supply voltage (They 'cancel out.... but individually they are still there)..... you can get a shock when you don't expect it.
    I hope this helps, later I will do some calculations using your numbers and get back to you.
  6. Nov 6, 2011 #5
    Is V_resistance allowed to be higher than the source voltage as well?
  7. Nov 7, 2011 #6
    No. The voltage across the resistance has its MAX value at resonance. This means that the current will be a maximum and this is what is meant by RESONANCE.
    The maximum voltage across the resistance will be the supply/source voltage.
    Have you met 'vector diagrams' showing Vl, Vc and Vr ? These are a great help to show how these voltages vary with frequency and how they combine.
    If you need any more details I would try to put something together and attach a Word document.
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