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Homework Help: Understanding phase relationships for AC capacitor/inductor circuits

  1. May 31, 2013 #1
    Hi guys!
    So i've just recently started learning about phase relationships for inductors and capacitors in an AC circuit. But a few things keep bugging me:

    Say we have a RL circuit (a resistor + inductor + voltage source all in SERIES), and we we're asked to draw a phasor diagram for the voltages and currents.

    when they say the current of an inductor lags the voltage for an inductor, does this mean:

    1. The inductor causes the current through the whole circuit to lag (as all components are in series)?

    2. I've always imagined the voltage source being kind of..'independent' of the circuit, so I've always thought that the Vsupply phasor and the Icircuit phasor should always be in phase no matter what the circuit components are. From what i've learnt so far, this doesn't seem to be the case. E.g If a capacitor,resistor and voltage source were all in parallel, then..does the capacitor cause the supply voltage to 'lag' the circuit current? -> shouldn't the supply voltage and circuit current be in phase?
    can anyone shed some light on this?

    thanks guys :shy:
    Last edited: May 31, 2013
  2. jcsd
  3. Jun 1, 2013 #2


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    No, that is not true. The voltage source is independent, and the same current flows through each element. The voltage across them has a phase shift with respect to the current. You can draw the vector of the supposed current along the x axis, and draw a vector for each voltage, and add them to get the resultant. If you get that the voltage leads the current with some angle θ, the current lags with respect to the voltage with the same angle.

    Connecting an AC voltage source to the parallel connected capacitor, inductor and resistor, the current has different phase on the different circuit elements with respect to the voltage. The net current is the sum of all currents, it also has some phase difference.

    Have you learned about complex impedances?

  4. Jun 1, 2013 #3
    @ehild We've touched on it, but not alot

    since the inductor causes the circuit current to lag, then for a capacitor in parallel with a resistor and also in parallel with a voltage source, does the capacitor cause the circuit voltage to lag?
  5. Jun 1, 2013 #4


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    If you connect a DC current source across a capacitor the capacitor gets charged. The voltage is proportional on the charge of the capacitor:U=Q/C. Initially there is no charge and the voltage is zero. Later the charge gradually increases and so is the voltage. You can say that the voltage lags behind the current or the current leads the voltage.

    With an AC voltage source the voltage across the capacitor is pi/2 phase angle behind the current. That is equivalent to say that the current leads the voltage by pi/2 phase.

    In case of parallel R, L, C across a voltage source, the voltage is given. The current flowing through the resistor is in phase with the voltage. The capacitor current leads the voltage, the inductor current lags behind the voltage.


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  6. Jun 2, 2013 #5
    ah! thanks so much!!
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