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When do inductors act like a short circuit and when they dont?

  1. Oct 16, 2009 #1
    -My professor said that if i is constant through an inductor that V=0 through the formula V=L(dv/dt) . So if an inductor is in series with a voltage source it will act like a short circuit. But if the inductor is in series with a resistor and the voltage source the inductor will not act like a short circuit.

    -My question is why doesnt the inductor act like a short circuit if the inductor is in series with a resistor and a voltage source. Wouldnt the current through the circuit be constant? Is the current through the inductor different when it is in series with the resistor? Please explain this to me because im totaly confuse.

    Attached is the example:
    inductor-1.jpg
     
  2. jcsd
  3. Oct 17, 2009 #2
    Under DC, a constant current, the inductor acts as if it weren't there. So all you have left is the resistor... so to speak.

    However, if the current is changing, like in AC, then voltage will be induced across the inductor.
     
  4. Oct 17, 2009 #3
    Of course, you meant to write, V = L(di/dt).

    A voltage source can be changing, like an AC voltage source, but I think your professor is talking about a constant DC voltage source.

    Let's take 'short circuit' to mean that the component acts like a ideal conductor. This would mean that any current through the component would result in zero voltage across the component.

    A constant voltage applied across an ideal inductor will result in a constantly increasing current. This is not a short circuit.

    You need to talk to your professor and work it out.
     
  5. Oct 19, 2009 #4
    sorry i forgot to say this situation is for DC source. So can you explain these two examples. The one with a resistor and the one without. Wouldnt the inductor act like a short circuit in both of them since the current through the circuit is constant? Is the current different from the current entering the inductor and the current leaving the inductor? Please explain . Im so confused.
     
  6. Oct 19, 2009 #5
    Sure, the theoretical inductor is a short... but no such inductor exists in the real world. Every inductor really has an inductive component and a resistive component. The resistive component and the inductive component are separated in the analysis for the purposes of simplifying the calculation. Every inductor really has resistance, and what I think your professor means to say is that in a DC cct the inductor behaves like a resistance only. The zero ohm inductor is a theoretical nonesuch, it's just that the inductance plays no part in determining the value of the DC current.

    Hard to get your head around this? Write back, we've all been there.
     
  7. Oct 28, 2009 #6
    thank for everyones help.
     
  8. Oct 30, 2009 #7
    The inductor will act like a short circuit in both cases for a DC voltage source.

    edit: I guess your question has been answered already.
     
  9. Oct 30, 2009 #8

    Redbelly98

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    Without a resistor, for a hypothetical constant-voltage source, no the inductor does not act like a short:
    With a resistor present, the inductor acts like a short (0 voltage, with non-zero current) only after some time has passed.
     
  10. Oct 31, 2009 #9

    vk6kro

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    Meanswing has another thread :
    https://www.physicsforums.com/showthread.php?t=349614

    which makes me wonder if he is missing the point here.

    These are questions about what happens to an inductor when power is applied or removed.

    Especially when it is removed, the inductor generates a large positive going spike.
    It does eventually settle to being purely resistive (due to the resistance in the wire of the coil) but for a short period when power is applied or removed, the inductor is nothing like a short circuit. A low value resistor directly across the inductor will limit the magnitude of this spike.

    If DC power is applied directly across an inductor, there may not be much of a spike because the power source might have very low internal resistance and the supply continues to supply much the same voltage regardless of changing current in the inductor.
    The current will increase until it is limited by the series resistance of the inductor.
     
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