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Current/voltage leading and lagging

  1. Apr 24, 2008 #1
    Hi, i was wondering why is it that for an inductor the current lags voltage and for capacitor the current leads voltage? Thank You.
     
  2. jcsd
  3. Apr 24, 2008 #2
  4. Apr 24, 2008 #3
    I'll add the best way to remember what occurs in each device is our good old pal: ELI the ICE man.
     
  5. Apr 24, 2008 #4
    An inductor's energy is proportional to the square of its current. No work needs to be done to change its voltage. But, work must be done to change an inductor's current. In an inductor, voltage can change abruptly, but current can only change gradually. In a physical inductor, some capacitance is present across the turns in the winding, so that a little work is needed to change the voltage.

    The capacitor is the counterpart of the inductor. Its stored energy is proportional to the square of the voltage. In a cap, current may change abruptly, whereas voltage must change gradually. Work needs to be done to change its voltage, but not its current. In a real cap, there is always some inductance, so that some work is done changing the current.

    Off the top of my head, that is a brief overview. More detail can be added, but that is the basic reason.
     
  6. Apr 24, 2008 #5

    mheslep

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    Gold Member

    Mathematically, you can see lag and lead (phase difference) from:

    [tex]
    L\frac{di}{dt}= v
    [/tex]


    [tex]
    C\frac{dv}{dt}= i
    [/tex]

    so that if the current i(t) through an inductor with inductance L is sin(t), then the voltage v(t) across the inductor is cos(t).

    Asfor a physical why, look to the energy uptake and release by the magnetic field of the conductor and similarly with the electric field in the capacitor.
     
    Last edited: Apr 24, 2008
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