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Capacitors lag!

  1. Oct 22, 2008 #1
    hello Forum,

    why is the voltage on a capacitor not instantaneously following the change in the current (which follows the change in the generator source voltage)?
    Where does the delay come from (90 degree phase retardation)?
    WE can think of the capacitor as having some inertia.
    The same story seems to exists for the inductor.
    Using the mechanical oscillator analogy, I always thought it this way: the inertia (like a mass) is the inductance L. The spring constant k is the 1/C, (the inverse of the capacitance).

    Does this have to do with the finite speed of light?

  2. jcsd
  3. Oct 22, 2008 #2


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    The voltage on a capacitor V=Q/C. It takes some time for Q to increase to its final value.
  4. Oct 22, 2008 #3
    sure, but while the voltage at the source changes, charge is not actually traveling from the source to the capacitor.
    In the field view of electric circuits, fields rule. There is a delay in the update of the fields at the capacitor end that does not occur in a resistor.
  5. Oct 22, 2008 #4


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    Charge is actually traveling from the source to the capacitor.
    The E field in the capacitor is proportional to the charge on the capacitor.
  6. Oct 23, 2008 #5
    Good question....
    q= it means the current (i) requires some finite time (t) to build up charge q. (referncing earlier posts)....but why are current and voltage are 90 degrees out of phase....what's the physical reason??


    Wikipedia shows some helpful math, but not an underlying physical explanation:


    Anybody know??
  7. Oct 23, 2008 #6
    Energy. It takes work to change the cap voltage, but not the current. Thus current can change abruptly, whereas voltage changes gradually. The energy stored by a cap is C*(V^2)/2. All real caps have a little inductance. It takes work to change the current as well, but not nearly as much.

    An analogy exists for the inductor. To change the current, work needs to be done, but not when changing the voltage. In reality, inductors possess a small capacitance so that a little work is done changing the voltage.

    It's all about work.
  8. Oct 23, 2008 #7


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    In principle, current cannot flow across a capacitor because there is a gap between its plates. However, if you connect an uncharged capacitor to a dc battery, the battery will charge the capacitor, and current will appear to flow across the gap. As the capacitor is charged to its final constant voltage, it becomes harder and harder to put more charge on the plates, because identical charges don't like being squeezed together. This will cause the current to stop eventually.

    So the final constant voltage corresponds to 0 current. And at the start, when current first starts to flow, it is easiest to put charge on the capacitor to change its voltage. So the peak current occurs not at the maximum voltage, but at the maximum change in voltage.
  9. Oct 23, 2008 #8
    capacator voltage peaks to the source voltage immediately?...That makes sense..then as the first electron arrives it will repel the second, then two will oppose the third and so on....slowing current flow....what keeps them bunching up on one plate of the capacitor is the driving force of the source electrical potential....in turn, like charges on the other plate are repelled, effectively creating a temporary current flow...likewise, it must tail off...

    Good show, atyy!!!
  10. Oct 23, 2008 #9


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    capacitor voltage peaks when charge peaks when there is no current - capacitor *rate of change in voltage* peaks immediately, just like the current which is the *rate of change in charge*
    Last edited: Oct 23, 2008
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