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How do inductors store energy as a magnetic field?

  1. Apr 5, 2016 #1
    My last lecture that I have had, was in inductors. My professor said that they resist changes in current, because they store energy inside the inductor as a magnetic field. I know that as soon as you shut off the energy between the inductor, it starts to dissipate, slowly, until it is completely depleted.

    My question is this, how does an inductor actually store energy in it as a magnetic field, and why does the magnetic field instantly dissipate the second the current is shut off across it?
     
  2. jcsd
  3. Apr 5, 2016 #2
    This is an excellent question.
    A good discussion can be found in Feynman's Lectures part 2, chapter 27. See the link below.
    The discussion is about a capacitor storing energy in the E-field, but a similar story can be made for an inductor and the magnetic field.
    Feynman clearly states this part of the theory with a certain amount of dissatisfaction. I quote:
    "So our “crazy” theory says that the electrons are getting their energy to generate heat because of the energy flowing into the wire from the field outside. Intuition would seem to tell us that the electrons get their energy from being pushed along the wire, so the energy should be flowing down (or up) along the wire. But the theory says that the electrons are really being pushed by an electric field, which has come from some charges very far away, and that the electrons get their energy for generating heat from these fields. The energy somehow flows from the distant charges into a wide area of space and then inward to the wire."
    "Finally, in order to really convince you that this theory is obviously nuts, ..."
    http://www.feynmanlectures.caltech.edu/II_27.html
     
  4. Apr 5, 2016 #3

    anorlunda

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    Magnetic? Yes.

    Instantly? No.

    You just said that it resists changes. That rules out instant changes in current.

    The voltage across an inductor is ##L\frac{dI}{dt}##. It can become very large when you try to stop I. That is how we generate sparks for the spark plugs in cars.
     
  5. Apr 5, 2016 #4
    I realize that it releases it gradually, sorry if I didn't make that clear, what I don't know is why and how it store energy and slowly releases it, instead of just instantly discharging
     
  6. Apr 5, 2016 #5
    Ok, so how does that particularly apply to the question I asked, thank you for the nice response time.
     
  7. Apr 5, 2016 #6
    Also, if you shut off the current to a inductor, wouldn't the change in current be some arbitrary number over zero time? So wouldn't the voltage be infinite across its leads? Why does that not instantly disapate all energy in the inductor?
     
  8. Apr 5, 2016 #7
    You asked "how does an inductor actually store energy in it as a magnetic field".
    Feynman answers to the very similar question "how does a capacitor actually store energy in it as an electric field".
     
  9. Apr 5, 2016 #8

    anorlunda

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    We don't get infinities in real life. When the voltage gets high, a spark jumps across the contacts of the switch and the current decreases in finite time.

    A common error is to mix up ideal components with real life. The clue to that mix up, is when you get infinity for any answer. In real life, no R, L, C, V, I, or switch is purely ideal.
     
  10. Apr 5, 2016 #9
    An inductor is analogous to a flywheel, having lots of inertia. It is hard to get the current flowing and hard to stop it. When you try to suddenly switch off, it is akin to applying a brake to a flywheel. It tries not to respond, but eventually the energy is dissipated as heat in whatever brake you use.
     
  11. Apr 5, 2016 #10
    I understand what it actually does, I just dont understand why? I dont see why the second you shut off the current across an inductor, how it is still able to maintain a magnetic field inside itself?
     
  12. Apr 5, 2016 #11
    If you think about a heavy flywheel spinning, it cannot maintain any momentum once you apply enough braking force to stop it. But it tries not to stop. Similarly, the inductor tries to stop you from switching off, by generating a high voltage across the switch gap. This is like the mechanical reaction against braking (F=-MA). But once you force the current down to zero, actually be applying a reverse voltage with the switch, the magnetic field has gone.
     
  13. Apr 5, 2016 #12
    where does it get the energy to create that strong of a opposite voltage, if it is not connected to any source. My main issue is I dont see how it physically stores energy, rather, i see it as it allows energy to go through it, and that energy self inducts itself, and opposes itself, thus opposing current change. The issue with that way of thinking (which is probably totally wrong), is that the second the current stops flowing through it, the field should sease to exist, and thus the self induction instantly stops (i know this isnt what really happens now, but I dont know why at all). I would like to know the reasoning behind why it does what it does, instead of just know what it does. Thank you all very much for all of your help, you all are very kind to be helping me
     
  14. Apr 5, 2016 #13
    When a magnetic field is built, it is a store of energy. Maxwell originally used a mechanical analogy to develop his theories and it used spinning cells having inertia and momentum. The magnetic field will not collapse to nothing until you physically force the current down to zero. The inductor will not let you do this without a struggle.
     
  15. Apr 5, 2016 #14
    why will it not let you force the current down to zero, and why is a electromagnetic field stored energy?
     
  16. Apr 5, 2016 #15

    meBigGuy

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    Let's say you are passing 1 amp through an inductor with a parallel 1 ohm resistor and remove the supply.

    The inductor has a magnetic field, and wants to maintain the 1 amp flow, so it will then create 1 volt to "send" the 1 amp through the 1 ohm resistor, reducing the magnetic field as it flows . The 1 amp reduces exponentially in time because a change in current is required to create the voltage across the resistor. This continues until the field is depleted.

    If you do it with a 10 ohm resistor, it will initially generate 10 times the voltage to produce the initial 1 amp.
     
  17. Apr 6, 2016 #16

    NascentOxygen

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    It will, but you have to cause its current to reach zero before all its energy has been drained.

    A magnetic field around a solenoid represents stored energy, because you can use it to power an electric or electronic circuit after all other voltage sources have been removed.
     
  18. Apr 6, 2016 #17
    You are now ready to read the Feynman Lecture and see that even he did not understand it !
     
  19. Apr 6, 2016 #18
    How does the magnetic field not instantly shut off as soon as you pull the switch? Like how does it physically store the energu
     
  20. Apr 6, 2016 #19
    If he does not know the answer, does anyone? Is their an actual concrete answer? And if he didn't know it, then what is the lecture on? And how will it help me?
     
  21. Apr 6, 2016 #20
    So does it have yl do with maxwell theories on electromagnetic waves? Does it propionate because it turned into a em wave?
     
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