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Can a single capacitor circuit have phase shift?

  1. May 9, 2012 #1
    Hi guys,

    Short question here, just wondering if it's possible for a capacitor, alone in a circuit with an AC sinusoidal voltage source to experience phase shift?

    Meaning can the voltage over a capacitor have a different phase to the voltage from the voltage source.

    Thanks!
     
  2. jcsd
  3. May 9, 2012 #2

    sophiecentaur

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    If you have a 'Voltage Source' then its phase cannot be different from what it 'wanted' to be (that's the definition). A capacitor, connected across it must follow that voltage. To get any phase shift, there has to be some resistance in series - either one you soldered in or the 'real' internal resistance of your source. You have to measure phase relative to some reference, of course.
     
  4. May 9, 2012 #3
    Thanks so much for the reply!

    So phase shift only occurs over the element? I'm having some trouble understanding what causes the phase shift, is it because there is another element in the circuit so there is a place for a phase shift to exist without touching the source?
     
  5. May 9, 2012 #4

    sophiecentaur

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    The phase shift is not "over the element" it is the difference in the voltage that appears across the element COMPARED with the voltage that appears across two other points in the cicruit. ergo - you need at least three points in a circuit before you can discuss 'phase shift / phase difference.
    Alternatively, you can discuss the phase difference between the Voltage and Current waveforms associated with just one element. But you need to specify which you are doing before making any definite statement.
    BUT, for a capacitor connected to a voltage source, the current will be infinite all the time (except at max and min) because any charge imbalance will be rectified in zero time if there is no R involved. Such models are not much use to the student, actually!
     
  6. May 9, 2012 #5
    It's true, however I had a lab where I needed to identify an unknown circuit. One of the possible circuits given was a capacitor by itself and I needed to explain why it could not have been a single capacitor as there was phase shift present.

    I just simulated something in pSpice as well, out of curiosity, does the phase difference only happen if the resistor is before the capacitor in the circuit? When I tested it in pSpice a circuit with a resistor after the capacitor had no difference in phase compared to the source.
     
  7. May 9, 2012 #6

    sophiecentaur

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    Why would there be a difference between source and capacitor if there were no resistor in between?
    MAybe you should rely on the logic of the circuit in itself rather than on a simulation which may not be doing what you think it's doing?
    Simulations are great for complex circuit analysis (no-one would bother to do a complex circuit by hand these days) but, for one or two components, does it really help?

    But what is the phase that you are calculating? is it between two voltages of a voltage and a current. If you give the information here that you gave spice, then it would be more clear.
     
  8. May 9, 2012 #7
    t0N5X.png
    Voltage source:
    Voff: 0
    Vamp: 240
    Freq: 50
    TD: 0
    TR: 0
    Phase: 90

    Traces here:
    ux7Vu.png

    Gonna be honest, out of all the subjects this one is the hardest for me, I just don't think in circuit logic. I've spent more hours on this subject than any but it's still fail city haha.

    It's my last circuits unit though, going into Computer Systems engineering.
     
  9. May 9, 2012 #8

    vk6kro

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    Is that capacitor 1 Farad? If so, you won't get much of a phase shift.

    Try 1 μF.

    Measure from ground to a point between the resistor and the capacitor. You should get a clear phase shift compared with the input voltage.
     
  10. May 9, 2012 #9
    Thanks, I tried that, you're absolutely right. It did cause a phase shift. It was confusing me quite a bit how I could have gotten those results.
     
  11. May 9, 2012 #10

    psparky

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    Seeing that the resistance of a capacitor is 1/Jωc.......or 1<-90/(ωc).....

    You're damn skippy that you will get a phase shift between the voltage and current once you plug it into your V=IR!

    Your max phase shift of 90 degrees of the current will occur with the least amount of real resistance. As real resistance goes up in series with your capacitor....the phase shift will get less and less.

    Again...it's all in the math.

    OH sorry....you are talking about voltage vs voltage.....see sophie.
    If your interested in current vs voltage shift...see my speel.
     
  12. May 10, 2012 #11

    sophiecentaur

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    Not just being picky here, but . . . . The word is Impedance. The word Resistance had other connotations and we mustn't confuse the two.
    We then say that the Impedance, Z is R+jX, where X is the Reactance. For a Capacitor, the Reactance is 1/ωC.

    You can't afford to neglect the use of Complex Numbers for this or you can't arrive at a phase shift.
    V=IZ gives you it.
     
  13. May 10, 2012 #12

    psparky

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    Exactly what I was trying to say. How silly of me.
     
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