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Homework Help: Electromagnetic Induction repulsion

  1. Apr 25, 2017 #1
    1. The problem statement, all variables and given/known data

    What happens when you close W? What happens when you open W again at Q? If there is a ring of metal at Q, on the coil.

    2. Relevant equations

    There is no relevant equations in this case.

    3. The attempt at a solution
    North will be on top and south on the bottom when there is a current. The magnetic field will be poiting upwards, since thats how it works in coils. This will make the ring of metal make create an induced magnetic field according to Lenz law it will be pointing downwards and thus the top of the metal ring will be N and the bottom will be S. But what will happen, will it try to leave the coil or not? I do know South repel South etc.. But now i have difficulites saying what will happen to the metal ring.
  2. jcsd
  3. Apr 25, 2017 #2


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    A current will be induced in the ring while the magnetic field through the ring is changing. The field will change right after closing, and right after opening. Also, if the ring moves, then that will be a change in the field (through it). Hopefully that gives you some ideas on how to proceed.
  4. Apr 25, 2017 #3


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    You are on the right track. Take a look at this and you will understand better what's going on.
  5. Apr 26, 2017 #4
    Thanks for the replies guys, this was what i was thinking. That it might jump when u turn it on. But not when u turn it off. I wrote my explanations. But how should i explain it using "South repel South" and "north repel north" etc..?
  6. Apr 26, 2017 #5

    So I think that when the current is turned on. There will be a induced magnetic field created due to chane in magnetic flux over a short amount of time. Thus according to Lenz law it will be the opposite direction of what is created when the current is turned on ("in" the metal ring, drawn in orange in this picture)

    And since its not "in the coil" the magnetic field for the metal ring will go from North to south. Thus the top of the metal ring will be north and the bottom will be south. Thus the ring will not jump in this set up when it is turned on!
    But I dont know. Will it JUMP or not? Because I mean the south is on the bottom on a "north pole" but still the top of the metal ring is north so there might be a repulsion? I do need help here.

    Scenario 2: When it is turned off. The top of the metal ring will become south and the bottom North due to Lenz law. However. Now there will be no current. So is there even a chance for it to jump? Or does the North pole and the South pole from the current be there a few milliseconds after u turn the current off or does it go extinct instantly? Because if it stays a few milliseconds now there will be North versus North and thus it should jump a bit right? Or How should i think?
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