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What happens when a piece of iron is put into an inductor?

  1. May 16, 2014 #1
    1. The problem statement, all variables and given/known data

    You have a circuit with current I running through a solenoid. You insert a small piece of iron into the coil; what happens?

    2. Relevant equations

    d(Magnetic Flux)/dt = -EMF

    3. The attempt at a solution

    Iron is diamagnetic, so inserting it will decrease the magnetic field inside the inductor. Faraday's Law and Lenz's Law tell us that a changing flux induces an EMF to create a field opposing this change in flux. Therefore, my thought is that the current increases, then decreases back to its original value. Does that seem right?
  2. jcsd
  3. May 17, 2014 #2


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    Is iron diamagnetic????????

  4. May 17, 2014 #3

    rude man

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    I agree with ehild!
    The answer to your question is no. What does Faraday say? Is flux affected if anything is added into the coil?
    (Constant applied emf assumed).
  5. May 19, 2014 #4
    So the answer should be nothing, since normal iron is non-magnetic?
  6. May 19, 2014 #5

    rude man

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    This problem needs to be better defined. Is the current sinusoidal? DC? If sinusoidal, is it constant-voltage or constant-current drive? Part of a circuit with series resistance?

    Iron is very magnetic. Ferro-magnetic. "Ferro" stands for "iron".
  7. May 20, 2014 #6
    Where did you get the idea that iron is diamagnetic? You probably don't understand the difference between magnetic, ferromagnetic, diamagnetic, and paramagnetic.
  8. May 20, 2014 #7
    The current is constant DC.

    Ferromagnets are permanent magnets. This is named after iron, commonly a permanent magnet.

    Diamagnetic materials, when placed inside a magnetic field, create an opposing field.

    Paramagnetic materials create a parallel field.

    Isn't this correct? I don't understand how we can answer the question, as written, however. If the iron were magnetized, we would need to know its orientation, in order to calculate the changing flux...

    You are right that my understanding of this is lacking.
  9. May 20, 2014 #8

    rude man

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    Iron is not 'commonly' a permanent magnet, although basic iron can be permanently magnetized to some extent.

    OK, so the current is constant DC. So obviously there is no change in current.

    What about the flux thru the coil? Is it increased or decreased or stays constant when you shove the iron into the coil? Defend your answer (use Ampere's law).

    Then use Faraday's law to determine if the voltage across the inductor changes.
  10. May 20, 2014 #9
    If the iron is not permanently magnetized, it won't change the flux, and the current won't change. If it is permanently magnetized, the change in flux depends on its orientation. The current can do whatever as the flux changes, then return to its original value.
  11. May 21, 2014 #10

    rude man

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    Afraid that's wrong.

    First, permanent magnetization has nothing to do here. The iron is characterized by having high permeability, not magnetic retention.

    Second, the flux changes greatly wih the entry of the ciron core into the inductor coil.

    You just said the current is dc which means it's constant. If it isn't constant then you must describe the entire circuit.

    You need to do a lot of revision of basic electricity and magnetism.
  12. May 21, 2014 #11
    Ahhhh. Ok. I think I understand. H is essentially B - M. We can use Ampere's Law to get H, and then add on M to get B, which will be much larger than it was before. From there, the result follows directly that the current decreases as the core is inserted, then returns to its original value.
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