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Inductor behavior connected directly to DC (no resistor)?

  1. Oct 10, 2015 #1
    If an ideal inductor is connected directly (no resistor present) to a DC voltage source, why does it behave like a short-circuit at time t=0? If voltage across an inductor is related to the change in current (i.e. di/dt), then shouldn't the voltage across the inductor be very large at time t=0 because the current goes from zero to some value instantaneously (assume no resistor is series). This would imply that the inductor should act like an open-circuit at time t=0. Wouldn't the inductor oppose the initial current spike and prevent the initial current from flowing? I know that this is not true; however, I am trying to determine why not.

    Thanks for your help.
     
  2. jcsd
  3. Oct 10, 2015 #2

    berkeman

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    Staff: Mentor

    It is neither a short nor an open. Can you write the equation that relates the current through the inductor to the voltage across it?
     
  4. Oct 10, 2015 #3
    berkeman - The equation is I(t) = Vc/R * (1-e^(-t*L/R)). Using the equation, I understand that the current becomes infinity (or indeterminate) when R=0. However, I am trying to understand what physically is happening. If R=0, does a magnetic field still form when current flows through an ideal conductor? Thanks.
     
  5. Oct 10, 2015 #4

    berkeman

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    I had a more fundamental equation in mind:

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

    That's the way I think about inductors. What happens when you apply a step voltage across an inductor?

    BTW -- the current through the ideal inductor only becomes infinite in infinite time. It is a linear ramp from t=0 to whenever. And with real inductors, the current is limited by the inductor's series resistance (check the datasheets for this number) and the source voltage.
     
  6. Oct 11, 2015 #5

    meBigGuy

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    Gold Member

    It is true and your thoughts to the contrary are incorrect. Why do you think the ideal inductor conducts at t=0? It starts with 0 current and ramps linearly while building a magnetic field.

    What caused your mixup?
     
    Last edited by a moderator: Oct 12, 2015
  7. Oct 12, 2015 #6

    NascentOxygen

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    Staff: Mentor

    Note to participants in this thread: please don't over-complicate the issue. The focus here is on an ideal inductor.
     
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