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A question about inductors

  1. Dec 28, 2015 #1
    I understand why slowing the current, causing a decrease in the flux, momentarily increases the current (stabilizing). I can use the right hand rule and Lenz's law to picture what is happening in that case.

    But why does increasing the current add resistance? The induced magnetic field is dependent on the current, so adding current should just add flux in the existing direction, shouldn't it? Or is it that an inductor requires a ferromagnetic core, and this develops its own magnetic field, separate from the magnetic field produced by the current, and it is the core's field that wants to remain stable? Will a solenoid without a ferromagnetic core behave like an inductor? Even marginally?
     
  2. jcsd
  3. Dec 28, 2015 #2

    tech99

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    If you try to increase the current in a conductor, it has to build a bigger magnetic field, which will contain more stored energy, and you have to supply this energy. The energy is supplied because you have to push harder (increase the voltage) to increase the current. When you try to reduce the current, energy stored in the magnetic field is given back to you as electrical energy.
    In answer to your last question, a solenoid without an iron core has a small inductance, as does a straight wire, but iron cored solenoids have very large inductance.
     
  4. Dec 28, 2015 #3
    Why does increasing the current add resistance?
    You are correct the strength of the magnetic field increases with the current. But since the current is sinusoidal the magnetic field is not static. Therefore the inductor is now subjected to a strengthened and moving magnetic field which therefore induces a potential which opposes the current that produced the magnetic field. The resistance (the correct word is impedance) is therefore increased, due to higher induced potential, opposing the current.

    Now the real question is therefore why does the magnetic field induces a potential which opposes the current? Now this is pure conservation of energy. Imagine if the resultant potential was actually aiding the current it would produce a stronger magnetic field and that in turn would produce more potential and more current and still stronger magnetic field. Essentially we could create a perpetual machine.
    HTH
     
  5. Dec 28, 2015 #4
    As far as ferromagnetic core is concerned, magnetic field conduction is better in such a medium than air. Therefore the ferromagnetic core causes higher opposing potential.
     
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