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Inductance and AC Source: Why current?

  1. Sep 15, 2011 #1
    Hello. I'm am studying topics related to inductances and Faraday's law and I'm having hard time at PICTURING situations. Mental representations.
    So I propose an example of something I don't understand and we may go on from there.

    Consider circuit consisting of an inductance and an AC source.
    In texts it is said that voltage across the inductance must be opposite to voltage from the source due to Kirchoff's Law.
    1) If that is so, why is there any current at all?

    Second point is understanding causally in time an inductor.
    Suppose a steady current in an inductor, and suddenly we increase it an infinitesimal di ( or a finite value if you wish), Faraday's Law saws there will be an induced current to oppose the change, supose -di. But now current has changed again, and so on.
    I don't understand even what I don't understand.

    Thanks to everyone in advance.
  2. jcsd
  3. Sep 15, 2011 #2


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    Gold Member

    I know what you mean.

    But then, in a simple resistance circuit, the voltage across the resistance is equal to the power supply, so one could ask the same question as to why is there any current at all. Maybe that helps you overcome the difficulty.
  4. Sep 15, 2011 #3
    Since there is no ohmic resistance in the circuit, an infinitesimal difference in voltages is capable of creating arbitrary large currents. The inductor produces the voltage which is just right for the given amount of current to flow.

    With inductor, you cannot suddenly change the current. If you try, for example by breaking the circuit, you will get this huge voltage spike across the inductor. You can instantaneously change the voltage to whatever you like and the current will gradually build up.

    (with capacitors, the situation is opposite: you cannot instantaneously change the voltage of a capacitor. If you try you'll get arbitrary large currents. But you can control the current and the voltage will follow).

    A good mental picture is to imagine electrons in the inductor having significant inertia. Mechanical analog of inductance is a flywheel.
  5. Oct 20, 2011 #4
    Hello! Two months later I re read your answer and at least I'm having a better feeling.
    However, maybe it is a stupid question:
    I can accept from Faraday's law that if flux changes from zero to some value in time t, by time t there will be a voltage induced.
    But what is the physical reason that implies current will not change immediately too. A potential difference has been applied and if the loop has resistance R then V=IR, I see no transitory.

    Thank you!
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